The science case for an orbital mission to Uranus: Exploring the origins and evolution of ice giant...

The science case for an orbital mission to Uranus Exploring the originsand evolution of ice giant planets

CS Arridge abn N Achilleos cb J Agarwal d CB Agnor e R Ambrosi f N Andreacute gSV Badman hi K Baines jk D Banfield l M Bartheacuteleacutemymbr MM Bisi n J BlumoT Bocanegra-Bahamon p B Bonfond q C Bracken r P Brandt s C Briand t C Briois uS Brooks j J Castillo-Rogez j T Cavalieacute v B Christophew AJ Coates ab G Collinson xJF Cooper x M Costa-Sitja y R Courtin t IA Daglis z I de Pater aa M Desai ab D Dirkx pMK Dougherty ac RW Ebert ab G Filacchione ad LN Fletcher ae J Fortney af I Gerth pD Grassi ad D Grodent q E Gruumln agah J Gustin q M Hedman ai R Helled aj P Henri uS Hess ak JK Hillier al MH Hofstadter j R Holme am M Horanyi ah G Hospodarsky anS Hsu ah P Irwin ae CM Jackman ao O Karatekin ap S Kempf ah E Khalisi aqK Konstantinidis ar H Kruumlger v WS Kurth an C Labrianidis as V Lainey at LL Lamy tM Laneuville au D Lucchesi ad A Luntzer av J MacArthur b A Maier aw A Masters hacS McKenna-Lawlor ax H Melin f A Milillo ad G Moragas-Klostermeyer aqA Morschhauser ay JI Moses az O Mousis ba N Nettelmann af FM Neubauer bbT Nordheim ab B Noyelles bc GS Orton j M Owens bd R Peron ad C Plainaki adF Postberg aq N Rambaux beat K Retherford ab S Reynaud bf E Roussos v CT Russell bgAM Rymer s R Sallantin g A Saacutenchez-Lavega bh O Santolik bi J Saur bb KM Sayanagi bjP Schenk bk J Schubert bl N Sergis bm EC Sittler x A Smith a F Spahn bn R Srama aqT Stallard bo V Sterken agbq Z Sternovsky ah M Tiscareno l G Tobie bp F Tosi adM Trieloff al D Turrini ad EP Turtle s S Vinatier t R Wilson ah P Zarka t

a Mullard Space Science Laboratory University College London UKb The Centre for Planetary Science at UCLBirkbeck London UKc Department of Physics and Astronomy University College London UKd ESTEC European Space Agency The Netherlandse Queen Mary University of London UKf Space Research Centre University of Leicester UKg IRAP Toulouse Franceh ISAS JAXA Japani Department of Physics Lancaster University UKj NASA Jet Propulsion Laboratory USAk University of Wisconsin-Madison USAl Cornell USAm Univ Grenoble Alpes IPAG F-38000 Grenoble Francen Rutherford Appleton Laboratory STFC UKo Technical University Braunschweig Germanyp Delft University of Technology The Netherlandsq Universiteacute de Liegravege Belgiumr National University of Ireland Maynooth Irelands Johns Hopkins University Applied Physics Laboratory USAt LESIA LrsquoObservatoire de Paris Franceu LPC2E CNRS Universiteacute drsquoOrleacuteans Orleacuteans Francev Max Planck Institute for Solar System Research Goumlttingen Germanyw ONERA Francex NASA Goddard Space Flight Centre USAy ESAC European Space Agency The Netherlandsz Department of Physics University of Athens Greeceaa University of California Berkeley USAab Southwest Research Institute San Antonio TX USAac Department of Physics Imperial College London UK

Contents lists available at ScienceDirect

journal homepage wwwelseviercomlocatepss

Planetary and Space Science

httpdxdoiorg101016jpss2014080090032-0633amp 2014 Published by Elsevier Ltd

Planetary and Space Science 104 (2014) 122ndash140

ad INAF-IAPS Istituto di Astrofisica e Planetologia Spaziali Rome Italyae Department of Physics University of Oxford UKaf University of California Santa Cruz USAag Max Planck Institute for Nuclear Physics Germanyah LASP University of Colorado USAai University of Idaho Moscow ID USAaj Tel Aviv University Tel Aviv Israelak LATMOS Franceal Heidelberg University Germanyam University of Liverpool UKan University of Iowa USAao Department of Physics and Astronomy University of Southampton UKap Royal Observatory of Belgium Belgiumaq University of Stuttgart Germanyar Universitaumlt der Bundeswehr Muumlnchen Germanyas UTesat-Spacecom GmbH Germanyat IMCCE-Observatoire de Paris UMR 8028 du CNRS UPMC Universiteacute Lille 1 77 Av Denfert-Rochereau 75014 Paris Franceau Institut de Physique du Globe de Paris Franceav University of Vienna Austriaaw Space Research Institute Austrian Academy of Science Austriaax Space Technology Ireland National University of Ireland Irelanday DLR Germanyaz Space Science Institute USAba Observatoire de Besanccedilon Francebb University of Cologne Germanybc University of Namur Belgiumbd University of Reading UKbe Universiteacute Pierre et Marie Curie UPMC - Paris 06 Francebf Laboratoire Kastler Brossel CNRS UMPC Francebg Institute of Geophysics and Planetary Physics University of California Los Angeles USAbh University of the Basque Country Spainbi Institute of Atmospheric Physics Prague Czech Republicbj Department of Atmospheric and Planetary Sciences Hampton University Virginia USAbk Lunar and Planetary Institute University of Arizona USAbl Department of Earth Sciences University of California Los Angeles USAbm Office for Space Research and Technology Academy of Athens Greecebn University of Potsdam Germanybo Department of Physics and Astronomy University of Leicester UKbp LPG CNRS ndash Universiteacute de Nantes Francebq International Space Science Institute Bern Switzerlandbr CNRS IPAG F-38000 Grenoble France

a r t i c l e i n f o

Article historyReceived 17 December 2013Received in revised form23 July 2014Accepted 7 August 2014Available online 22 August 2014

KeywordsUranusMagnetosphereAtmosphereNatural satellitesRingsPlanetary interior

a b s t r a c t

Giant planets helped to shape the conditions we see in the Solar System today and they account for morethan 99 of the mass of the Sunrsquos planetary system They can be subdivided into the Ice Giants (Uranusand Neptune) and the Gas Giants (Jupiter and Saturn) which differ from each other in a number offundamental ways Uranus in particular is the most challenging to our understanding of planetaryformation and evolution with its large obliquity low self-luminosity highly asymmetrical internal fieldand puzzling internal structure Uranus also has a rich planetary system consisting of a system of innernatural satellites and complex ring system five major natural icy satellites a system of irregular moonswith varied dynamical histories and a highly asymmetrical magnetosphere Voyager 2 is the onlyspacecraft to have explored Uranus with a flyby in 1986 and no mission is currently planned to thisenigmatic system However a mission to the uranian system would open a new window on the originand evolution of the Solar System and would provide crucial information on a wide variety ofphysicochemical processes in our Solar System These have clear implications for understandingexoplanetary systems In this paper we describe the science case for an orbital mission to Uranus withan atmospheric entry probe to sample the composition and atmospheric physics in Uranusrsquo atmosphereThe characteristics of such an orbiter and a strawman scientific payload are described and we discuss thetechnical challenges for such a mission This paper is based on a white paper submitted to the EuropeanSpace Agencyrsquos call for science themes for its large-class mission programme in 2013

amp 2014 Published by Elsevier Ltd

1 Introduction

Giant planets account for more than 99 of the mass of theSunrsquos planetary system and helped to shape the conditions we seein the Solar System today The Ice Giants (Uranus and Neptune) arefundamentally different from the Gas Giants (Jupiter and Saturn)in a number of ways and Uranus in particular is the mostchallenging to our understanding of planetary formation and

n Corresponding author at Mullard Space Science Laboratory University CollegeLondon UK Tel thorn44 1483 204 150

E-mail address carridgeuclacuk (CS Arridge)

CS Arridge et al Planetary and Space Science 104 (2014) 122ndash140 123

evolution (eg Lissauer 2005 Dodson-Robinson and Bodenhei-mer 2010) Our Solar System provides the only local laboratory inwhich we can perform studies that help us to understand thenature of planetary systems in general The fact that Keplerobservations have shown that UranusNeptune class planets area common class of exoplanet (Fressin et al 2013) makes it all themore timely and compelling to better explore these fascinatingsystems

The Ice Giants are fundamentally different from the Gas Giants(Jupiter and Saturn) in a number of ways and Uranus in particularis the most challenging to our understanding of planetary forma-tion and evolution with its puzzling interior structure unclearenergy balance and internal energy transport mechanisms and itshigh obliquity Yet our exploration of the Ice Giants in our ownSolar System remains incomplete with several fundamental ques-tions unanswered Voyager 2 remains the only spacecraft to havereturned data from the uranian environment see for examplepapers in Science 233(4759) from the Voyager 2 Uranus encounterwith an introduction given by Stone and Miner (1986) and thecurrent authoritative book on the Voyager 2 encounter science(Matthews et al 1991)

A mission to Uranus will provide observations and measure-ments that are vital for understanding the origin and evolution ofUranus as an Ice Giant planet answer the fundamental question ofwhy some giant planets become icy and other so gas rich andprovide a missing link between our Solar System and planetsaround other stars Observations of Uranusrsquo rings and satellitesystem will also bring new perspective on the origin of giantplanet systems and will help validate the models proposed for theorigin and evolution of Jupiterrsquos and Saturnrsquos systems The cruisephase will also offer the possibility of testing the law of gravitationin a dynamic environment still poorly probed and study the outerheliosphere and its connection to the Sun Such a mission to theuranian system would open a new window on the origin andevolution of the Solar System and directly addresses two ofEuropean Space Agencys (ESA) Cosmic Vision themes ldquoWhat arethe conditions for Planet Formation and the Emergence of Liferdquoand ldquoHow Does the Solar System Workrdquo The fundamentalprocesses occurring within the uranian system confirm that theexploration of Uranus is essential in meeting ESAs Cosmic Visiongoals A mission to Uranus is also highlighted in the NASAPlanetary and Heliophysics Decadal Surveys (Squyres et al 2011Baker et al 2013)

In 2013 ESA issued a call for science themes for its large-class(L-class) mission programme This paper represents the whitepaper on the scientific case for the exploration of Uranus that wassubmitted to this call (a compilation of these white papers can befound at httpsciesaintscience-ewwwobjectdoccfmfobjectid=52029) and looks forward to future missions This white paperfollowed the Uranus Pathfinder mission proposal that was sub-mitted to ESAs medium-class (M-class) mission programme in2010 which is described in Arridge et al (2012) In September 2013a Uranus-focused workshop ldquoUranus beyond Voyager 2 fromrecent advances to future missionsrdquo was held at the Observatoryof Paris (Meudon France) and was attended by 90 scientists andengineers from 12 countries interested in the scientific explora-tion of this unique planetary system (the detailed programmeabstracts and lists of participants is available at httpuranussciencesconforg)

In Section 2 of this paper the science case for a Uranus missionis presented and arranged into three key themes (1) Uranus as anIce Giant Planet (2) An Ice Giant Planetary System and (3) UranusAeronomy Aurorae and Highly Asymmetrical Magnetosphere Inaddition a mission to Uranus naturally provides a unique oppor-tunity to study the outer heliosphere fundamental gravitationalphysics and Solar System bodies such as Centaurs near the orbit of

Uranus and so in this paper we also describe the science caseassociated with a cruise phase in the outer Solar System The shortmission concept that was described in the white paper is pre-sented in Section 3 along with a discussion of the critical enablingtechnologies

2 Scientific case

21 Uranus as an ice giant planet The interior and atmosphereof Uranus

Fig 1 indicates the bulk composition for various Solar Systemobjects and shows how different the Ice Giants are from the GasGiants Jupiter is an HHe planet with an ice and rock mass fractionof 4ndash12 as inferred from standard interior models (Saumon andGuillot 2004) Uranus and Neptune seem to consist mostly of icesand rocks but current observations are only able to provide anupper limit of 85 on the ice and rock mass fraction (Fortney andNettelmann 2010) The self-luminosity of Uranus is the lowest ofall the planets in the solar system suggesting that the interior ofUranus either is not fully convective or that it suffered an early lossof internal heat perhaps in a giant impact The internally-generated magnetic field of Uranus is highly complex and unusualwhich suggests some fundamental difference between thedynamo in Uranus interior and those of the Earth and Gas GiantsUnderstanding the internal structure of Uranus (the nearest IceGiant) is indispensable for estimating the bulk composition ofouter planets in particular their ice-to-rock ratio There is cur-rently no interior model for Uranus that agrees with all theobservations representing a significant gap in our understanding

Terrestrialplanets

Jupiter Saturn Uranus Neptune Kuiperbelt

Oortcloud

0

50

100

150

200

250

300

350

Mas

ses

(ME )

Hydrogen and helium

Heavy elements

Fig 1 Composition of various solar system objects split into hydrogen and helium(yellow) and heavy elements (blue) Modified from Guillot and Gautier (2010)(For interpretation of the references to colour in this figure legend the reader isreferred to the web version of this article)

Table 1Physical and orbital parameters of Uranus

Equatorial radius 25559 km (frac141 RU)Mass 145 ME

Sidereal spin period 17 h 12 min 36 s (772 s)Obliquity 97771Semi-major axis 192 AUOrbital period 843 Earth yearsDipole moment 50 ME

Magnetic field Highly complex with a surface field up to 110000 nTDipole tilt 591Natural satellites 27 (9 irregular)

CS Arridge et al Planetary and Space Science 104 (2014) 122ndash140124

of the Solar System Compared to the Gas Giants this differingbulk composition and the internal structure reflects the differentformation environments and evolution of the Ice Giants relative tothe Gas Giants (eg Guillot 2005) providing a window onto theearly Solar System Table 1 lists the gross properties of UranusThe origin of Uranus large obliquity is perhaps one of themost outstanding mysteries of our Solar System A variety ofexplanations have been invoked including a giant impact scenariowhich may also be implicated in Uranus low luminosity and smallheat flux and tidal interactions (Boueacute and Laskar 2010 Morbidelliet al 2012) Examining the interior structure and composition ofUranus and its natural satellites and studying the ring system mayallow us to unravel the origin of this Solar System mystery

The composition of Uranus contains clues to the conditions inthe protosolar cloud and the locations in which it formed Forinstance a subsolar CO ratio could indicate formation at adistance where water (but not CH4) was frozen The commonpicture of gaseous planet formation by first forming a 10 ME coreand then accreting a gaseous envelope is challenged by state-of-the-art interior models which instead predict rock core massesbelow 5 ME (Saumon and Guillot 2004 Fortney and Nettelmann2010) Uranus inclination and low heat loss may point to anothercatastrophic event and provides additional important constraintsfor planetary system formation theory New observations of IceGiants are therefore crucial in order to resolve this and achieveCosmic Vision goals on understanding the formation of planets

Uranus atmosphere is unique in our Solar System in that itreceives a negligible flux of heat from the deep interior andexperiences extremes of seasonal forcing due to the high 981obliquity with each pole spending 42 years in darkness Thisunusual balance between internal and radiative heating meansthat Uranus unique weather is governed principally by seasonalforcing Furthermore the substantial enrichment of some heavyelements (but perhaps not all N being strongly depleted in thetroposphere) and small envelopes of H2ndashHe in the Ice Giants andthe cold atmospheric temperatures relative to the Gas Giants yieldunique physicochemical conditions Uranus therefore provides anextreme test of our understanding of planetary atmosphericdynamics energy and material transport seasonally varyingchemistry and cloud microphysics structure and vertical couplingthroughout giant planet atmospheres At higher altitudes thetemperature in Uranus thermosphere is several hundred degreeshotter than can be explained by solar heating (as is also found forSaturn and Jupiter) and remains a fundamental problem in ourunderstanding of giant planet upper atmospheres in general (egHerbert et al 1987) Even though Earth-based observations ofUranus (Infrared Space Observatory Spitzer Herschel ground-based) have improved dramatically in the decades since Voyager 2many questions about this very poorly explored region of our SolarSystem remain unanswered Certain spectral regions particularlythose obscured by telluric water vapour are inaccessible from theground The overarching atmospheric science objective is toexplore the fundamental differences in origin meteorology andchemistry between the Ice and Gas Giants to reveal the under-lying mechanisms responsible for Uranus unique conditions

211 What is the internal structure and composition of UranusAt present there is no Uranus interior model that is consistent

with all of the physical constraints such as Uranus gravity fieldluminosity magnetic field and realistic ice-to-rock ratio Fig 2illustrates a model that is consistent with the gravity and magneticfield data but not with the luminosity of the planet Uranus andNeptune are known to have substantial elemental enrichments incarbon and deuterium (Owen and Encrenaz 2006 Feuchtgruberet al 2013) but abundances of other simple elements (N S and O)

their isotopic ratios (12C13C 14N15N 16O17O) and the noble gases(He Ne Ar Xe Kr) have never been adequately constrainedNevertheless Uranus bulk atmospheric composition provides akey diagnostic of planetary formation models To developimproved models of Uranus interior better compositional datamust be obtained (Helled et al 2010)

The mass of the core also places constraints on planetaryformation models For example if HHe is mixed into the deepinterior with only a small central core this could suggest gasaccretion onto a low-mass proto-planetary core or efficientvertical mixing or inclusion of disk-gas into the building planete-simals rather than accretion onto a large ice-rock core of 10 MEFurthermore the predicted large size of Uranus core relative tothe H2ndashHe envelope may make Uranus our best opportunity forstudying the elemental composition and thermochemistry of theouter solar nebula at the earliest stages of planetary formationMeasurements of Uranus bulk atmospheric composition lumin-osity magnetic and gravity fields and normal-mode oscillationswill place new constraints on Uranus interior and on the originsand evolution of Uranus The gravity field can be measured both byradio science and by observing the precession of Uranus ten densenarrow elliptical rings (Jacobson et al 1992 Jacobson 1998 2007)Magnetic field measurements can be used to assess the structureof the dynamo region Measurement of noble gas abundances andisotopic ratios can be obtained with a shallow (1 bar) entry probewhilst some isotopic ratios can be determined by remote sensingA deep (45 bar) atmospheric entry probe would be able toresolve the question of whether the SN ratio is enhanced abovesolar abundance Giant-planet seismology building upon themature fields of helio- and astro-seismology will revolutioniseour ability to probe the interior structure and atmosphericdynamics of giant planets

Improved knowledge of the composition and interior structureof Uranus will also provide deeper insight into the processes thatremixed material in the protoplanetary disk caused for exampleby the formation of Jupiter (Safronov 1972 Turrini et al 2011) ordue to extensive primordial migration of the giant planets (Walshet al 2011)

212 Why does Uranus emit very little heatPlanets are warm inside and cool down as they age Gravita-

tional energy from material accretion was converted to intrinsic

H2He

H+

H2O (l)

Diamond sedimentationHe sedimentation

Phase transition

Superionic water

Core

Ionic water

Magnetic field generation

Molecular water~2000 K

~01 Mbar

~6000 K~5 Mbar

Fig 2 Model of Uranus interior


thermal energy during formation and is steadily radiated awaythrough their tenuous atmospheres as they age Voyager measure-ments suggest that Uranus evolution produced a planet withnegligible self-luminosity smaller than any other planet in ourSolar System (Pearl et al 1990) Thermal evolution modelsprobe the energy reservoir of a planet by predicting its intrinsicluminosity Such models reproduce the observed luminosity ofJupiter and Neptune after 456 Gyr of cooling independent ofdetailed assumptions about their atmosphere albedo and solarirradiation The same models however underestimate it for Saturnand overestimate it for Uranus Indeed Uranuss atmosphereappears so cold (its intrinsic luminosity so low) that accordingto standard thermal evolution theory Uranus should be more than3 Gyr older that it is However the uncertainties on the Voyager-determined energy balance are large enough to substantiallyreduce that discrepancy In particular as the observational uncer-tainty (Pearl et al 1990) in the albedo and the effective tempera-ture (derived from the brightness temperature) are significantUranus could as well cool down adiabatically just as Neptune if itsreal heat loss is close to the observed upper limit

The small self-luminosity combined with the sluggish appear-ance of the atmosphere as viewed by Voyager suggests that theinterior of Uranus is either (a) not fully convective or that (b) itsuffered an early loss of internal heat Case (b) would suggest thatthe interior is colder than in the adiabatic case with crystallinewater deep inside (Hubbard et al 1995) This points to acatastrophic event in Uranus early history that shocked the matterand led to a rapid energy loss In case (a) we would expect theinterior to be warmer with water plasma (eg Redmer et al 2011)implying large-scale inhomogeneities possibly caused by immis-cibility of abundant constituents such as helium and carbon orupward mixing of core material that inhibit efficient heat trans-port However during the last decade ground-based observationshave revealed the appearance of convective cloud features typi-cally at mid-lattiudes suggesting localised convective regions ofadiabatic thermal gradients in the deep troposphere (Sromovskyet al 2007 de Pater et al 2011) Vertical transport of energy andmaterial seems to occur only in localised regions on this enigmaticplanet In fact the inferred size of a non-convective internal regiondepends sensitively on the imposed intrinsic heat flux value amostly stable interior is predicted if the heat flux is close to zerobut a fully convective interior is possible as for Neptune shouldthe upper limit of the observed heat flux value prove true

In order to better constrain Uranus internal heat flux (derivedfrom the measured albedo and brightness temperature) tighterobservational constraints of these quantities are necessary Theseinferences come from a single measurement from the Voyagerflyby at a single point in Uranus seasonal cycle Thus the balancebetween Uranus emission and absorption may be seasonallyvariable and new global measurements of reflected solar andemitted infrared radiation are required to assess the presence orabsence of an internal heat source and its importance as drivingmechanisms for Uranus meteorological activity Atmosphericproperties and profiles measured by an atmospheric entry probeusing a combination of radio science an on-board accelerometerand a nephelometer may also shed light on heat transport in theatmosphere

213 What is the configuration and origin of Uranus highlyasymmetrical internal magnetic field

Understanding the configuration of Uranus internal magneticfield is essential for correctly interpreting the configuration of themagnetosphere its interaction with the rings moons and solarwind and for understanding how dynamo processes in theinterior of Uranus generate the field In contrast to the magnetic

fields of Earth Mercury Jupiter and Saturn which are dominatedby a dipole nearly co-aligned with the rotation axis those ofUranus and Neptune are characterised by a large offset and tiltbetween the dipole and spin axes with strong quadrupole andoctupole contributions to the internal magnetic field The mag-netic field data from Voyager 2 are sufficient to crudely constrainthe internal field of Uranus but more complex and (currently)poorly constrained models are required to fit the data (Holme andBloxham 1996) At the planetary surface the magnetic dipolequadruople and octupole components of the total internal field areof comparable strength but at the top of the dynamo region(075RU) the latter two dominate Fig 3 illustrates the highlyasymmetrical nature of Uranus internal magnetic field using themodel of Herbert (2009) compared with Saturns highly symme-trical internal field using the model of Davis and Smith (1990)

A variety of competing numerical dynamo models (eg Stanleyand Bloxham 2004 2006 Soderlund et al 2013) have beendeveloped which can explain these fields but new magnetic fieldmeasurements are required to allow us to determine which is theclosest to reality The field is also expected to have undergonesecular changes since the Voyager 2 epoch (Christensen andTilgner 2004) Magnetic field measurements at a variety ofplanetocentric latitudes and longitudes will provide a wealth of

Fig 3 Comparison of Uranus highly asymmetrical field with Saturns symmetricalfield using the models of Herbert (2009) and Davis and Smith (1990) respectivelyThe colour scale indicates the magnitude of the radial field at the 1 bar level(a) Saturn field (Davis and Smith 1990) and (b) Uranus field (Herbert 2009)


data from which to test these competing models This will lead tosignificant changes in our understanding of field generation in IceGiant planets and of planetary magnetic field generation ingeneral Models of the internal field can also be greatly improvedby the use of auroral images which provide additional high-latitude constraints Herbert (2009) combined the Voyager obser-vations of the internal field and assumed magnetically conjugatesouthern and northern UV auroral emissions to derive such ahigher order model Better-quality images of auroral emissionsthan are possible from Earth (eg Lamy et al 2012) are paramountfor improving the accuracy of the planetary field model

214 What is the rotation rate of Uranus interiorA correct interpretation of the internal structure of Uranus

relies on an accurate knowledge of the internal rotation rate of theplanet (Nettelmann et al 2013) Modelling of Uranus internalmagnetic field and observations of radio emissions (UranianKilometric Radiation (UKR)) and atmospheric motions all provideindependent estimates of the rotation rate of the planet althoughnot always from the same region of the planet Analyses ofVoyager 2 data have yielded three estimates of the rotation rateof Uranus from 17 h 12 min 36 s (772 s) (Herbert 2009) to 17 h17 min 24 s (736 s) (eg Ness et al 1991) New measurements ofUranus magnetic field and UKR will enable us to significantlyimprove the accuracy on the determination of the planetaryperiod (to a few parts in 105) and check if second order effects(eg Saturn displays different radio periods in both magnetichemispheres each varying with time) are present

215 How is Uranus atmospheric structure and compositioninfluenced by its unique seasons

The potential absence of an internal heat source rendersUranus weather unique among the giant planets Neptune withits powerful self-luminosity provides an important counter-example of a convectively-active weather layer The extreme 98o

obliquity of Uranus subjects the atmosphere to extremes ofseasonal forcing with each pole spending decades in darknessDespite the bland visible appearance of Uranus from Voyagerrecent ground-based observations (eg Sromovsky et al 20072009 2014 de Pater et al 2011 Fry et al 2012) have shown theplanet to be more dynamically active than previously thought(Fig 4)

Bright poles seen in 13-cm images from the Very Large Array(VLA) show that the polar regions of the deep troposphere aredepleted in absorbers relative to the equator thus indicating large-

scale atmospheric motions (Hofstadter et al 2006) The samepattern is seen in the CH4 distribution at higher altitudes(Karkoschka and Tomasko 2009) (Fig 5) Seasonal changes inclouds and dynamics have also been observed in 1986 the sunlitSouth Pole appeared bright due to a polar lsquocaprsquo of stratosphericaerosols The bright South Pole diminished over the ensuing yearsand became a faint polar band of brighter material while a newcollar of bright material became visible in the northern springtimehemisphere High resolution ground-based observations in 2012(Fig 4) reveal what may be convective clouds of CH4 which mayeventually form a polar hood as was seen in the southern polarregions during the Voyager flyby (eg Atreya et al 1991) All ofthese are indicative of the meridional circulation which on thishighly seasonally driven planet is likely to be unique but alsoinstructive about how planets work under more general obliquityinsolation conditions The long temporal baseline of high spatialresolution atmospheric observations will allow us to study thenature frequency distribution and morphology of discrete cloudactivity (eg storms vortices) In particular we aim to understandthe origin lifecycle and drift rates of Uranus dark spots andassociated bright clouds (large anticyclonic vortices eg Hammelet al 2006) for a direct comparison with the lifecycles observedon Neptune Finally the relative importance of wave activityversus moist convection in vertical mixing could be uniquelytested on Uranus given the anticipated low levels of convectiveactivity

216 What processes shape atmospheric chemistry and cloudformation on an ice giant

Reflected sunlight observations can be used to identify thecomposition and distribution of Uranus main condensation clouddecks The brightest white features are thought to be caused byices of CH4 overlying a putative cloud of NH4SH or H2S butprobably not NH3 (de Pater et al 1991) with a deep cloud of waterhypothesised at much higher pressures Thin photochemical hazelayers may exist above the condensate clouds in the upper tropo-sphere and stratosphere leading to oscillations in the temperatureprofiles due to localised radiative heating Indeed stratospherichazes of small particles (likely to be condensed hydrocarbons)were observed in high-phase angle imaging from Voyager 2 (Rageset al 1991) a geometry that can only be provided by a visitingspacecraft These condensed hydrocarbons may sediment down-wards into the troposphere serving as cloud condensation nucleior as coatings for existing particles complicating our capabilitiesfor uniquely identifying the composition of the cloud decks

Fig 4 High contrast imaging from the Keck telescope in 2012 revealed a scalloped wave around Uranus equator discrete clouds at midlatitudes and a mottled chaoticappearance at the poles showing that Uranus is not as bland or sluggish as the Voyager 2 flyby originally suggested Credit L SromovskyP FryH HammelI de PaterUniversity of Wisconsin-MadisonWM Keck Telescope


The optical properties and spatial distributions of these tropo-spheric and stratospheric hazes are poorly known but they maycontribute significantly to the radiative heating of the upperatmosphere and thus our understanding of atmospheric circula-tion in Uranus stably-stratified atmosphere

Below the clouds the atmospheric composition is poorlyknown The altitude of the deep H2O condensation cloud is poorlyunderstood because the bulk water abundance may be enhancedby 10ndash30 times the solar abundance (de Pater and Lissauer 2010)The H2O cloud may exist over extended pressure ranges beneath50ndash80 bar and may even merge with a region of super-critical H2Oin Uranus interior It is not clear what chemical gradients areresponsible for the emergence of dark spots (anti-cyclones) andassociated bright orographic clouds Above the clouds the InfraredSpace Observatory (ISO 1995ndash1998) and Spitzer Space Telescope(2003ndashPresent) showed that stratospheric chemistry initiated byUV-driven photolysis of CH4 powers a rich photochemistry result-ing in a soup of hydrocarbons in the upper atmosphere (Moseset al 2005) This hydrocarbon chemistry differs from the othergiant planets as the sluggish vertical mixing means that CH4 is nottransported to such high altitudes so that hydrocarbon photo-chemistry operates in a very different regime (ie higher pres-sures) than on the other giants Furthermore ISO and Herschel(2009ndash2013) observed oxygenated species in the high atmospherepotentially due to infalling dust and comets (Feuchtgruber et al1997 Cavalieacute et al 2014) It is important to search for previouslyunidentified or unmapped stratospheric species (CO HCN CO2etc) such as those related to coupling between the neutral atmo-sphere and the uranian ringsatellite system

Remote sounding observations are required to place constraintson Uranus bulk inventory vertical distribution composition andoptical properties of Uranus clouds and hazes A deep (45 bar)atmospheric entry probe would enable the measurement of bulkCH4 and H2S abundances as well as the abundances of key noblegases and isotopic ratios to understand the origin of this ice giant

217 What processes govern upper atmospheric structureThe thermosphere and ionosphere form a crucial transition

region between interplanetary space and the planet itself Power-ful currents generated by electric fields imposed by the magneto-sphere can result in large energy inputs to the upper atmospherethe energy from these sources may be tens to hundreds of timesgreater than that due to the absorption of solar (extreme ultra-violet (EUV)) radiation The unique orientations of Uranus mag-netic dipole and spin axis combined with strong seasonal drivingproduce a highly time-dependent and complex interactionbetween the solar wind magnetosphere ionosphere and thermo-sphere Therefore this system provides a unique opportunity to

understand how insolation and particle precipitation from thesolar wind magnetosphere contribute to the energy balance in theupper atmosphere These processes are suspected to be involved inmaintaining a temperature several hundred Kelvin hotter than canbe explained by solar heating alone This requires additionalheating and the apparent partial seasonal control (Melin et al2011 2013) suggests that this is strongly modulated by the way inwhich varying magnetospheric configurations couple with theupper atmosphere to produce time-variable fields and currents

Mapping temperatures electron densities and the distribu-tions of ions and molecules in the ionosphere and thermosphereusing UV and IR remote sensing (in concert with in situ magneto-spheric fields and particles measurements Section 23) will permitan unravelling of the thermospheric heating problem and willprovide evidence for auroral activity in response to varying solaractivity

22 An ice giant planetary system rings and natural satellites

Uranus has a rich planetary system of both dusty and densenarrow rings and regular and irregular natural satellites Thisunique example of a planetary system holds an important key tohelp us unravel the origin and evolution of the Solar System Fig 6illustrates some of the main features of rings and natural satellitesover a wide range of radial distances and Fig 7 shows a zoom ofthe inner region from Uranus to Miranda the innermost of the fivemajor moons From this figure one can see the dense packing ofthe uranian ring and inner satellite system Ground-based obser-vations have found changes in the rings and satellites since theVoyager 2 flyby indicating that fundamental instabilities in thecoupled ringndashmoon system are of clear importance for under-standing the evolution of planetary systems (de Pater et al 2007)Fig 8 shows Voyagers single high-phase image of Uranus ringsystem revealing a plethora of dust structures More recentobservations have revealed an outer ring system (Showalter andLissauer 2006 de Pater et al 2006b 2013) However yet the lackof a near-infrared spectrometer on Voyager 2 means that thecomposition of the rings is almost entirely unknown It is clearfrom the albedo that the ring particle surfaces and possibly theparticles themselves are very different from those in Saturns ringand must include a non-waterndashice component

The five largest moons of Uranus (Miranda Ariel UmbrielTitania Oberon ndash see Fig 9) are comparable in sizes and orbitalconfigurations to the medium-sized moons of Saturn They arehowever characterised by larger mean densities about1500 kg m3 on average and by different insolation patternswith their poles directed towards the Sun during solstice owingto the large axial tilt of the planet Oberon lies outside of themagnetosphere (depending on season during solstice it spends

Visible[Voyager 2] Near IR [HST] Near IR [Keck] Thermal IR [VLA]

Microwave[VLA]

Fig 5 Images of Uranus in a variety of wavelengths After Arridge et al (2012)


periods in the magnetotail) and Titania is sometimes outside themagnetosphere depending on the upstream solar wind conditions(Fig 6) but Miranda Ariel and Umbriel orbit within the magneto-sphere and hence space weathering should have modified their

surface properties causing particles to be ejected from theirsurfaces The observations performed during the flyby of Voyager 2revealed surprising amounts of geological activity on these moonspossibly involving cryovolcanic processes Finally the uranian

Voyager 2 periapsis

Orp

helia

Bia

nca

Cre

ssid

a Des

dem

ona

Julie

t Port

iaR

osal

ind

Cup

idB

elin

daPe

rdita

Puck

Mab

Mira

nda

Cor

delia

1986

U2R

ζ rin

g

6 5

4 ri

ngs

α rin

gβ

ring

η γ

δ ri

ngs

λ rin

g

μ rin

g

ν rin

g

ε rin

g

2 R1 R 3 R 4 R 5 R

Fig 7 Schematic of the inner region of the Uranus system from Miranda inward White arcs indicate narrow rings and dark blue-grey arcs indicate the orbits of naturalsatellites The satellite sizes are to scale but a factor of 10 larger than reality for reasons of visibility The thickness of the narrow rings are also not to scale Periapsis ofVoyager 2 is indicated with an arrow inside the orbit of Puck (For interpretation of the references to colour in this figure legend the reader is referred to the web version ofthis article)

Fig 6 Schematic of the Uranus system showing the major narrow rings (white) dusty rings (grey) embedded natural satellites (white dots) and the major regular satellitesThis diagram is correctly scaled for distance but the moon sizes have been scaled by a factor of 10 and the width of the narrow rings is not to scale The periapsis distance ofVoyager 2 is indicated by a white arrow inside of the orbit of Puck The typical distance to the subsolar magnetopause (Bagenal 1992) is indicated by the vertical red lineImages of the major regular satellites are from Voyager 2 (credit NASAJPL) and have not been photometrically corrected (For interpretation of the references to colour inthis figure legend the reader is referred to the web version of this article)


system is host to a set of irregular moons with evidence fordynamical groupings that may hold keys to understanding theevolution of Uranus in particular the great collision hypothesis forthe obliquity of Uranus

The study of the moons and rings of Uranus ndash in particular theircomposition and dynamical stability their subsurface and deepinterior structure and their geological history and evolution andhow that relates to their formation ndash are important parts of ESAsCosmic Vision goal for understanding how the Solar SystemworksThe possibility that Uranus irregular satellites are capturedCentaurs or comets can also contribute to understanding smallprimitive bodies and may provide lessons for our understanding ofthe origin of life in the Solar System particularly since objectsexposed to the solar wind are subjected to very different spaceweathering processes than those protected from the solar wind(eg Fig 6)

221 What is the composition of the uranian ringsThe composition of the uranian rings is almost entirely

unknown as Voyager 2 did not carry an infrared spectrometercapable of detecting the rings However it is clear from their lowalbedo that at least the surfaces of the ring particles are verydifferent from those in Saturns rings and must have a significantnon-waterndashice component The particle-size distribution ofUranus main rings is also mysterious where the main ringscontain particles between 10 cm and 10 m with a surprising lackof cm-size particles detected by the Voyager 2 radio occultation(French et al 1991) The ring system has also changed significantlysince the Voyager flyby in ways we do not understand (Showalterand Lissauer 2006) and new rings and satellite components havebeen discovered These need to be characterised at close range inorder to understand how their rapid evolution fits into variousparadigms of Solar System evolution

A Uranus orbiter will enable high-resolution near-infrared andvisible observations of the rings and small moons which willconstitute a significant advance in our understanding of the evolu-tion of the uranian system and will provide constraints on planetaryevolution models Observations of the narrow rings are needed tounravel the dynamics of their confinement and to confirm theories ofself-maintenance and of shepherding by moons which are relevantto other disk systems including protoplanetary disks Mapping thespatial variations of both composition and particle size will clarifyphenomena such as contamination and material transport within thesystem Stellar solar and radio occultations will enable the determi-nation of the ice-fraction and size distribution of ring particles A dustdetector can directly determine from in situ measurements thenumber densities as well as the speed and size-distributions of dustyring material Moreover a chemical analyser subsystem can provideunique information on the composition of these grains bearing thepossibility to constrain isotopic ratios of the constituents (Briois et al2013) Because larger ring particles and the uranian satellites are themain sources of the dust dust measurements give direct informationon the composition of these bodies Also of interest are the ringsinteraction with Uranus extended exosphere and their accretiondisruption interplay with the nearby retinue of small moons

Fig 8 Composite image of Uranus main rings in forward-scattered (left) and back-scattered (right) light The left-hand image is the only image of Uranus rings takenat a high phase angle (by Voyager 2 after closest-approach) These images showthat the dense main rings are interleaved with a network of dust structures and thedetails of how these structures work is largely unknown Credit NASAJPL

Fig 9 The five largest moons of Uranus shown to scale and with the correct albedo as imaged by Voyager 2 Miranda was imaged in the most detail but images of Titaniaand Oberon were not of a sufficiently high resolution to resolve details of tectonic structures Credit Paul SchenkNASAJPL


222 How do dense rings behave dynamicallyThe main rings are eccentric and inclined and generally bounded

by sharp edges ndash see reviews by Elliot and Nicholson (1984) andFrench et al (1991) Although theories exist regarding thesecharacteristics including resonant interactions ldquoshepherdingrdquo bynearby satellites and self-maintenance the mechanisms are farfrom understood Our understanding of these mechanisms is highlyrelevant to other disc systems including protoplanetary and debrisdiscs Existing data give preliminary hints that self-gravity wakesand spiral density waves which are important diagnostics as well asdriving phenomena in Saturns rings (eg Cuzzi et al 2010) alsoexist in at least some parts of Uranus rings but much more detailedobservation is needed to characterise them

The rings of Uranus are the best natural laboratory for investigat-ing the dynamics of dense narrow rings an important complementto the dense broad disk exemplified by Saturns rings and diffusiverings at Jupiter and Neptune (Tiscareno 2013) These observationswill undoubtedly reveal many new structures and periodicities andpossibly new moons that play important roles in ring confinementRings can also shed light on the planets gravitational and magneticfields as well as the influx of interplanetary meteoroids (eg Hedmanand Nicholson 2013) High-resolution images of the rings from anumber of orbits and phase angles are needed in order to unraveltheir dynamics

223 How do Uranus dusty rings workThe Cassini mission has taught us that dusty rings are shaped

by solar radiation forces which depend on particle properties(size albedo etc) as well as by the gravitational influence ofsatellites Thus a study of the dynamical structure of dusty ringswill unveil much about the particles currently unknown materialproperties

The post-Voyager discovery of the ν ring is especially intri-guing as this dusty ring lies between the orbits of two closely-packed satellites but does not itself have any apparent source(Showalter and Lissauer 2006) It is quite possible that the ν ring isthe remains of a moon that was disrupted by a collision fairlyrecently The innermost dusty ζ ring appears to have movedseveral thousand kilometres outward between the Voyager 2 flybyand recent Earth-based observations (de Pater et al 2007) butthis changing ring has not been studied closely Ring particlescould be lost to the planet by the drag force from the extendedexosphere of Uranus (Broadfoot et al 1986) and may lead tosimilar effects as the lsquoring rainrsquo at Saturn (OrsquoDonoghue et al 2013)Finally Voyagers single high-phase image of the rings revealed aplethora of otherwise unknown dust structures (Murray andThompson 1990) The bright bands and gaps in this dusty regionare difficult to reconcile with conventional theories

High-resolution images of these dusty rings will allow us todetermine their structure and evolution Detailed observationsmay reveal one or more large source objects for this dusty regionwith possible evidence of accretion among them In situ detectionwith a dust detector together with radio and plasma waveobservations would permit a direct measurement of the localdust density possibly leading to the discovery of new dustpopulations (Kempf et al 2005) and interactions with the mag-netosphere (Hsu et al 2011) A dust detector can also provideinformation on the size-distribution (Spahn et al 2006) and thecomposition of grains (Postberg et al 2011) as well as on theircharge state which might be key (Horaacutenyi 1996) to understandthe individual (Horaacutenyi et al 1992) and collective Hedman et al(2010) dynamics of micron-sized particles Such in situ measure-ments have the potential to reveal the mechanisms behind therapid evolution of the uranian dust rings seen in ground-based

data (de Pater et al 2007) and the intriguing similarities to otherring systems (de Pater et al 2006a)

224 How do the rings and inner satellites interactThe inner moons of Uranus comprise the most densely-packed

known satellite system as can be seen in Fig 7 with 13 known-objects on orbits ranging from 49770 to 97700 km (Cordelia toMab) from the planets centre This crowded system appears to besubject to mutual collisions on timescales as short as 106 yr(Duncan and Lissauer 1997 Showalter and Lissauer 2006 Frenchand Showalter 2012) and several moons show measurable orbitalchanges within a decade or less raising important questionsregarding the origin evolution and long-term stability of theUranus system Lying immediately exterior to Uranus main ringsystem but outside the ldquoRoche limitrdquo so that collisional productsare able to re-accrete into new moons these uranian innersatellites both interact with the rings (as well as with each other)and comprise a parallel system a natural laboratory in which theeffects of collisional disruption and re-accretion can be studiedThe moon Mab lies at the centre of the μ ring which shares withSaturns E ring the unusual characteristic of a blue colour likelydue to a preponderance of monodisperse small particles (de Pateret al 2006b) However while Enceladus creates the E ring bymeans of a fine spray of water crystals escaping from geysers Mabseems much too small ( 50 km across) to plausibly sustain anyinternal activity it is however important to note that the samewas formerly said of Enceladus Mab also exhibits large unex-plained deviations in its orbit (Showalter et al 2008) Closeobservations of the surface of Mab as well as its orbit and itsinteraction with the μ ring are certain to yield significantdiscoveries on the evolution of coupled ringndashsatellite systemsAstrometric imaging of the uranian inner moons would signifi-cantly contribute to understanding this system identifying reso-nant and chaotic interactions that can explain its current workingsand past history

225 What is the origin of the ringinner satellite systemThe close packing of Uranus small moons and its ring system

has suggested that there could be a genetic link between the twoColwell and Esposito (1993) have suggested that Uranus rings maybe the debris of moons destroyed by the meteroid bombardmentover the age of the Solar System The giant impact theory forUranus large obliquity also provides a mechanism for producingthe rings from a disruption of the original satellite system(Coradini et al 2010) More recently it has been suggested thattides themselves may destroy moons and create the rings(Leinhardt et al 2012) These scenarios are similar to recentsuggestions that the satellite systems of Saturn Uranus andNeptune may have resulted from ring evolution (Crida andCharnoz 2012) These scenarios would imply the existence of acycle of material between rings and moons Since Uranus ringmoon system evolves on timescales as short as decades in situtracking of this evolution would be a formidable opportunity tostudy this cycle which may be at work also for Neptune andSaturn but on longer time-scales for these systems By leading acomparative study of spectral characteristics of the rings andmoons we may unveil the origin of both the satellites and ringsby inferring whether they are made of the same material or not

226 What is the composition of the uranian moonsThe five major satellites of Uranus (Miranda Ariel Umbriel

Titania and Oberon) are comparable in orbital configuration andsizes to the medium-sized icy moons of Saturn but with markedlyhigher mean densities (1500 kg m3 on average) Fig 9 showsphotometrically correct and equal-scale images of these five


moons The albedos of the five major satellites of Uranus varyingbetween 021 and 039 are considerably lower than those ofSaturns moons except Phoebe and the dark hemisphere ofIapetus This reveals that water ice which dominates theirsurfaces is mixed in varying proportions to other non-ice visuallydark and spectrally bland material that is possibly carbonaceousin origin (Brown and Cruikshank 1983) Carbon dioxide has beendetected from telescopic observations on Ariel Umbriel andTitania but has not been detected on the furthest regular Uraniansatellite Oberon (Grundy et al 2006) The detected CO2 iceappears to be concentrated on the trailing hemispheres of thesesatellites and it decreases in abundance with increasing semi-major axis (Grundy et al 2006) as opposed to what is observed inthe Saturn system

Due to the absence of a near infrared spectrometer in thepayload of Voyager 2 no detailed information is available on thesurface chemistry of the icy moons Just to give a few examplesthere is no indication about the chemistry of the structuralprovinces identified on the surfaces of Titania and Oberonexhibiting different albedos and different crater density that revealdifferent ages Similarly unknown is the nature of dark material(perhaps rich in organics) that fills the floors of major impactcraters on Oberon as well as the composition of the annulus ofbright material that is enclosed in the large crater Wunda onUmbriel The chemical nature of the flows of viscous materialobserved on Ariel and Titania is also unknown while a clearindication of the presence of ammonia hydrate on the surface ofMiranda suggested by Bauer et al (2002) on the basis of telescopicobservations is lacking The major moons also differ from othermajor satellites around giant planets in that they have verydifferent insolation patterns with their poles directed towardsthe Sun during solstice owing to the large obliquity of the planetAlso Oberon lies outside of the magnetosphere (depending onseason) and Titania is sometimes outside the magnetospheredepending on the upstream solar wind conditions (Fig 6) butMiranda Ariel and Umbriel orbit within the magnetosphere andhence space weathering should have modified their surfaceproperties causing particles to be ejected from their surfaces

The observations performed during the flyby of Voyager 2revealed surprising amounts of geological activity on these moonspossibly involving cryovolcanic processes As can be seen from

Fig 10 Miranda exhibits striking structural geology despite itssmall size (472 km in diameter) with ridges and grooves that maybe the result of internal differentiation processes (Janes andMelosh 1988) or the surface expression of large-scale upwellingplumes (eg Pappalardo et al 1997) Similar internal processespossibly occurred on the comparably-sized Enceladus in thesaturnian system before its intense surface activity and cryovol-canic plumes developed Observations of Miranda thus providea unique opportunity to understand how small moons can becomeso active (Castillo-Rogez and Lunine 2012) Moreover the convexfloors of Ariels graben may provide the only evidence for wide-spread cryovolcanism in the form of viscous extrusive cryolavaflows (Croft and Soderblom 1991 Schenk 1991) a process thathas been elusive in the Solar System with only a few smallexamples documented elsewhere to date for example SipparSulcus on Ganymede (Schenk and Moore 1995) and Sotra Pateraon Titan (Lopes et al 2013) However only very limited observa-tions were possible during Voyager 2s brief encounter at whichtime only the southern hemispheres of the satellites were illumi-nated The diversity of the medium-sized icy satellites at Uranusdemonstrates the complex and varied histories of this class ofobject

Very little is known about the composition of the irregularmoons of Uranus yet they may hold keys for understanding theevolution of the uranian system particularly in relation to thegreat collision hypothesis (eg Parisi et al 2008) PhotometricallySycorax and Caliban are redder than Uranus and its regularsatellites perhaps similar to Kuiper belt objects Centaurs andcomet nuclei (eg Maris et al 2001) suggesting an origin ascaptured objects Although spectrally in the near-IR they are moredifficult to interpret and rotational effects may need to beincluded where the surfaces are spectrally inhomogeneous(Romon et al 2001)

By using an imaging spectrometer in the near infrared rangefrom 08 μm to at least 5 μm it will be possible to unveil thesurface composition of the moons by identifying and mappingvarious chemical species (with particular emphasis on non-waterndashice materials including volatiles and organics) This will ultimatelyenable an unprecedented correlation of surface composition withgeologic units at various spatial scales Spatially resolved chemicalmapping will also help separate the relative contributions ofendogenic subsurface chemistry and exogenic magnetosphere-driven radiolysis across the moons (eg Cartwright et al 2013)the transport of dust in the uranian system (eg Tosi et al 2010)and assess the role of processes that exchanged material betweenthe surface and subsurface

227 What is the origin of Uranus moons and how have theyevolved

As in the jovian and saturnian systems tidal and magneto-spheric interactions are likely to have played key roles in theevolution of the uranian satellite system For instance intense tidalheating during sporadic passages through resonances is expectedto have induced internal melting in some of the icy moons(Tittemore and Wisdom 1990 Tittemore 1990) One such tidallyinduced melting event may have triggered the geological activitythat led to the late resurfacing of Ariel The two largest moonsTitania and Oberon with diameters exceeding 1500 km might stillharbour liquid water oceans between their outer ice shells andinner rocky cores remnants of past melting events (Hussmannet al 2006)

The surfaces of the five major satellites of Uranus exhibitextreme geologic diversity however understanding of their geo-logic evolution and tectonic processes has suffered greatly fromincomplete Voyager image coverage (imaging restricted to theFig 10 Mirandas striking geological features Credit NASAJPL


southern hemispheres) and only medium to low image resolutions(order of several kilometres per pixel except for part of Miranda)which only allow characterisation of the largest geologic units inthe areas that could be imaged (eg Croft and Soderblom 1991)The crater size-frequency distributions of the five satellites usedas a tool for dating surface features and for constraining impactororigin are known only for the southern hemispheres and cratersizes larger than a few kilometres (eg Plescia 1987) There arealso still large uncertainties in the bulk composition of the moons(eg Hussmann et al 2006) which provide fundamental con-straints on their origins

High-resolution images of the satellite surfaces which willprovide key information on the ages and compositions of thesurfaces and will constrain the dynamical and geologic historiesthat led to the observed diversity For example Miranda and Arielexhibit evidence of significant endogenic geological activity High-resolution surface mapping will enable us to determine the degreeto which tectonic and cryovolcanic activity has occurred permit-ting characterisation of the role played by tidal dissipation andunderstanding whether uranian moons have experienced internalactivity similar to that at Enceladus Mapping of the moons willhelp constrain the nature and timescale of this activity andcharacterising the environment in their vicinity may reveal out-gassing if as at Enceladus activity is continuing Collisionalactivity amongst the irregular satellites can produce contamina-tion of the regular satellite surfaces with material from theirregular satellites via dust transport (Schubert et al 2010)High-resolution imagery and spectral data could reveal evidenceof such processes

Accurate astrometric measurements can also be used to quantifythe influence of tidal interactions in the system at present provid-ing fundamental constraints on the dissipation factor of Uranus(Lainey 2008) Gravimetric and magnetic measurements combinedwith global shape data will greatly improve the models of thesatellites interiors bringing fundamental constraints on their bulkcomposition (density) and evolution (mean moment of inertia)Understanding the composition (particularly the ice-to-rock ratio)and the internal structure of the natural satellites will also enable usto understand if Uranus natural satellite system was the originalpopulation of bodies that formed around the planet or if they weresubsequently disrupted potentially via a giant impact that mighthave produced Uranus large obliquity (Coradini et al 2010)

Crater statistics will be crucial in determining the satellitesgeological histories as well as providing critical information aboutthe projectile flux in the outer Solar System Near- and mid-infrared spectroscopy will enable us to understand the surfacecomposition of the moons yielding further information on theirorigin and evolution Occultations will enable us to probe anytenuous atmospheres that may be present and UV spectroscopymay then lead constraints on their chemistry with implications forthe subsurface The dayside magnetopause lies at a distance of 18RU and therefore the major moons (except Oberon and some-times Titania) are embedded within the magnetosphere Thisimplies that their waterndashice surfaces are eroded by magneto-spheric charged particles in addition to photons and micro-meteoroids Measuring the properties of the charged particlesthat these moons can encounter and the energetic neutralparticles released after the ions impact the surface will constrainthe role of plasma bombardment on surface evolution These datawill constitute strong constraints to allow us to understand howsatellite systems form and evolve around Ice Giants The composi-tion of the uranian moons will represent an essential data point inunderstanding the nature and origins of organic and volatilematerial in the outer Solar System

Recent models of icy satellite interiors suggest the largeruranian satellites Titania and Oberon may contain subsurface

oceans (Hussmann et al 2006) and Miranda may be subject torecent or even ongoing activity (Castillo-Rogez and Turtle 2012)The magnetic field induced in Europas subsurface ocean wasreadily detectable by Galileo (eg Khurana et al 1998) and anysuch signatures at Uranus are expected to be strong due to Uranusasymmetrical field

Remote observations of Uranus irregular satellites can be usedto search for potential genetic relationships with the irregularsatellites found in other giant planet systems and thus understandthe evolution of Solar System minor bodies and giant planetnatural satellites Amongst the irregular satellites numericalsimulations and photometry suggest at least two dynamicalgroupings the Caliban group (Caliban Stephano and Francisco)and the Sycorax group (Sycorax Prospero Setebos) with hetero-geneous photometry supporting origins from particular parentbodies and Tinculu Margaret and Ferdinand as single objects witha different origin (Vilas et al 2006 Grav et al 2004 Sheppardet al 2005) However the photometric and spectroscopic obser-vations are not consistent and new observations are required tounderstand the origins of these objects and their relationship toUranus great collision (Maris et al 2007 Parisi et al 2008)

23 Uranus aeronomy aurorae and highly asymmetricalmagnetosphere

The configuration of all the planetary magnetospheres in theSolar System is determined by the relative orientations of theplanets spin axis its magnetic dipole axis and the solar windflow In the general case the angle between the magnetic dipoleaxis and the solar wind flow is a time-dependent quantity andvaries on both diurnal and seasonal timescales Uranus presents aparticularly interesting and poorly understood case because thisangle not only varies seasonally but because of Uranus largeobliquity the extent of diurnal oscillation varies with seasonAt solstice this angle does not vary with time and Uranusmagnetic dipole simply rotates around the solar wind flow Thisis a magnetospheric configuration not found anywhere else in theSolar System Fig 11 illustrates the configuration of Uranusmagnetosphere near solstice as sampled by Voyager 2

Miranda

Ariel

Umbriel

Titania

Oberon

Bow shock

Magnetopause

Magnetotail

Equator

Plasma sheet

Ω

M

Neutral torus

Fig 11 Illustration of Uranus magnetosphere at solstice as sampled by Voyager 2The red vector indicates the magnetic dipole axis and the blue arrow the rotationaxis of Uranus Field lines are in grey and magnetopause and bow shock boundariesin heavy black lines The plasma sheet and inner magnetospheric plasma popula-tions are indicated by the orange and red regions respectively The positions ofnatural satellites are indicated in Uranus equatorial plane and a possible neutraltorus in blue (For interpretation of the references to colour in this figure legendthe reader is referred to the web version of this article)


Because of this unique extreme orientation its magnetosphereis expected to vary from a pole-on to orthogonal configurationduring a uranian year and to change from an ldquoopenrdquo (connected tothe solar wind) to a ldquoclosedrdquo configuration during a uranian daySuch a rapidly reconfiguring magnetosphere with a highly asym-metric internal magnetic field (Section 213) at its core provides achallenge for our theories of how magnetospheres work and willbring new insight in fundamental and universal magnetosphericprocesses Uranus also presents a special case because of its distantlocation in the heliosphere where the properties of the solar windare very different to the near-Earth environment (eg solar windstructures merge by propagating outwards giving rise to succes-sive long perturbations typically lasting 1ndash2 weeks) This providesopportunities to investigate fundamental processes such as mag-netic reconnection under a different parameter regime Along withthe planetary magnetic field the ionosphere of Uranus is theinternal core of the magnetosphere Recent analysis of emissionsfrom Uranus spanning almost 20 years (Melin et al 2011 2013)have revealed a phenomenon that is not seen at the other GasGiants in our Solar System the temperature of the ionosphere is atleast partly controlled by season such that at solstice the upperatmosphere is more than 200 K hotter than at equinox but whereother influences eg from the geometry of Uranus interactionwith the solar wind are also involved

Auroral emissions are also generated at kilometric (radio)wavelengths (1ndash1000 kHz) which cannot be observed from Earthor distant observers As at other planets UKR is thought to begenerated by the Cyclotron Maser Instability However UKRappears to be more complex than similar radio emissions at EarthSaturn or Jupiter and only comparable to Neptunes ones Under-standing the circumstances under which these peculiar radioemissions are generated is of prime importance for the ground-based radio detection of exoplanets with a magnetic field (essen-tial to the development of life) particularly those with highlyinclined magnetic axes with respect to the stellar flow

Because planetary magnetospheres partially shield planetsfrom solar energetic particles and galactic cosmic rays they havea role to play in the development of life In order to further ourunderstanding of how life and the platforms for life exist in thewide variety of magnetic environments in the Universe it is vitalthat we make comprehensive measurements in the widest possi-ble variety of environments These aspects make a study of Uranusmagnetosphere a very important objective for understanding howthe Solar System works and for achieving ESAs Cosmic Visiongoals and those set out in the Planetary Decadal Survey These arenot only relevant for the important question of understanding howasymmetric Ice Giant magnetospheres work but are also highlyrelevant in providing ldquoground-truthrdquo for understanding exoplane-tary magnetospheres

231 What is the overall configuration of the uranianmagnetosphere

Our understanding of the uranian magnetosphere is currentlyessentially limited to data from the Voyager 2 flyby which provided asingle snapshot where the angle of attack between the solar windaxis and the magnetic dipole axis varied between 681 and 521 tosome extent similar to the Earths magnetosphere However the nearalignment of the rotation axis with the planet-Sun line duringsolstice means that plasma motions produced by the rotation ofthe planet and by the solar wind were effectively decoupled(Selesnick and Richardson 1986 Vasyliuntildeas 1986) Therefore incontrast with Jupiter and Saturn solar wind plasma may be able topenetrate deep within the magnetosphere despite the planet being afast oblique rotator although there is evidence for some shielding inthe inner magnetosphere (McNutt et al 1987 Selesnick and McNutt1987 Sittler et al 1987) This may result in short residence times formagnetospheric plasma produced deep within the magnetosphereand may limit the amount of plasma trapping inside the magneto-sphere and consequently the amount of charged particle acceleration(eg Cheng 1987) Proton and electron radiation belts (with energiesup to tens of MeV) albeit slightly less intense than those at Saturnwere also observed in the inner magnetosphere of Uranus (Cheng1991) but their diurnal and seasonal variability is largely unknown

The significant asymmetries in the magnetosphere result in large-scale diurnal reconfigurations of the system on timescales of hoursresulted in a twisted magnetotail topology (Behannon et al 1987Toacuteth et al 2004 Arridge in press) The main plasma sourcestransport modes and loss processes in the uranian magnetosphereand the modes of interaction (pick-up sputtering and chargeexchange) between the magnetospheric plasma and the rings andmoons of Uranus are also largely unknown The configuration anddynamics of the uranian magnetosphere at equinox are entirelyunknown and it is not clear if this will result in a fairly quiescentmagnetosphere such as Neptune or a more rotationally dominatedmagnetosphere like Jupiter or Saturn Recent observations and theo-retical work suggest a limited role for solar wind-driven dynamics atequinox (Lamy et al 2012 Cowley 2013) and solar windndashmagneto-sphere coupling via magnetic reconnection that varies strongly withseason and solar cycle (Masters 2014)

232 What are the characteristics and origins of the uranianaurorae

Aurorae are the most striking diagnosis of the magnetospheredynamics as they can be traced back to the currents generated bythe magnetospheric interactions Several kinds of interactions havebeen characterised at Earth Jupiter and Saturn but the Uranusoptical and radio aurorae as they are known from Voyager 2observations seem to indicate new kinds of interactions Thecharged particles responsible for both optical and radio auroral

Fig 12 (a) Auroral emission map in the H2 band showing auroral intensity in Rayleighs (Herbert 2009) (b) Inferred source regions for the most intense UKR component(Zarka and Lecacheux 1987) From Arridge et al (2012)


emissions and their source regions are also unknown A study of theuranian auroral regions can also lead to information on the thermo-sphere due to atmospheric sputtering produced by auroral particleprecipitation Such sputtered particles can be monitored by aneutral particle detector

There has only been one spatially resolved observation of theUV aurora of Uranus (Herbert 2009) using a mosaic of Voyager 2UV observations mapping emission from H Lyman-α and EUV H2

band emission (Fig 12a) The emission appeared patchy and wasgenerally centred on the magnetic poles with the emission beingthe brightest about midnight magnetic local time There have beensubsequent attempts to observe the aurora in both the farultraviolet using the Hubble Space Telescope (HST) (Ballester1998) and in the IR using ground-based telescopes (eg Traftonet al 1999) Uranus aurorae was recently redetected in the UVusing HST (Lamy et al 2012) and revealed a radically different setof auroral processes controlled by the interaction between themagnetosphere and the solar wind (Cowley 2013) and raisingimportant questions on the generation of planetary auroral emis-sions and possible secular drift of Uranus intrinsic magnetic field

The UKR components which indicate different active regions in themagnetosphere divide into two categories (i) ldquoburstyrdquo (o10min)emissions comparable to that at Earth and Gas Giants and (ii) ldquosmoothemissionsrdquo which are time-stationary emissions (lasting for hours)specific to Ice Giants (Zarka and Lecacheux 1987) These lattercomponents require a continuous source of free energy that has notyet been identified and is apparently maintained in a highly variablemagnetosphere (Fig 12b) New radio observations with a moderninstrumentationwill provide wave properties that were inaccessible toVoyager 2 such as the wave direction and polarisation Continuousremote observations of UKR and in situ measurements within theirvarious source regions will provide essential information to under-stand the origin and characteristics of the variety of known uranianradio components and search for new components

Recent calculations show that new ground-based radio tele-scopes could detect radio emissions from hot Jupiters (Zarka2007) Unlike our Solar System eccentric and complex orbitalcharacteristics appear to be common in other planetary systemsso that the understanding of radio emission produced by Uranuscould have profound importance in interpreting future radiodetections of exoplanets

233 How does solar windndashmagnetospherendashionosphere couplingwork at ice giants

The uranian magnetosphere interacts with a fast magnetosonicMach number and high-beta solar wind which is an importantplasma regime in which to understand magnetic reconnection(eg Masters 2014) however Richardson et al (1988) havereported Voyager 2 observations suggesting the presence of peri-odic reconnection near the magnetopause Evidence of dynamicssimilar to Earth-like substorm activity but possibly internally-driven was also reported at Uranus by Mauk et al (1987) andSittler et al (1987) which indicate that important energy sourcesneed to be quantified including the energy input from the solarwind We do not know how the solar windndashmagnetosphere inter-action is interrupted and modulated by the diurnally changinggeometry Together Uranus ionosphere and internal magnetic fieldact as the inner boundary condition for the magnetosphere Modelsindicate that Uranus ionosphere is dominated by Hthorn at higheraltitudes and H3

thorn lower down (Capone et al 1977 Chandler andWaite 1986 Majeed et al 2004) produced by either energeticparticle precipitation or solar ultraviolet (UV) radiation It seemslikely that a key component of the required additional heating isdriven by particle precipitation andor the way in which varyingmagnetospheric configurations couple with the upper atmosphere

Understanding how the aurorae of Uranus respond to changes inthe solar wind is essential to understanding the Solar Windinteraction with giant planets more generally While theseresponses are well studied for the Earth the situation for the outerplanets is less well understood partly due to the lack of dedicateddeep space solar wind monitors Recent theoretical work (Cowley2013) has argued for distinct differences in magnetotail processesbetween equinox and solstice thus providing a framework for theinterpretation of new auroral images and demonstrating the needfor new in situ measurements The magnetosphere of Uranus wasobserved to be the site of intense plasma-wave activity withremarkably intense whistler mode emissions (Kurth et al 1991)The role of wavendashparticle interactions for the magnetospherendashionosphere coupling and the generation of Uranus auroral emis-sions as well as for the overall energy budget of the magnetosphererequire further consideration

24 Cruise phase science in the outer heliosphere

A mission to Uranus naturally involves a relatively long dura-tion interplanetary transfer (15 years see Section 31) Howeverthis presents an opportunity to undertake studies of the outerheliosphere minor Solar System bodies and fundamental physicsof the gravitational interaction

241 Physics of the interplanetary mediumThe structure of the heliosphere originates in the structure of

the solar magnetic field and is strongly modified by the solarcorona There are a range of important questions on how thisstructure is further modified and processed in the heliosphere andgoes on to modulate the cosmic ray flux in the inner heliosphereon the generation of turbulence and how minor bodies interactwith the heliosphere One of the major issues of the physics ofinterplanetary medium is to understand the mechanisms ofenergy dissipation Injected with large spatial scales by the Sunthe energy is transferred to smaller scales (ionelectron) where itis dissipated as heat Measurements made by the Voyager probeshave revealed variations of the exponents of the power law ofcertain parameters (eg speed magnetic field density) withdistance from the Sun suggesting regime change in the processof energy transfer (Burlaga et al 1997) Few observations of theheliospheric environment beyond 10 AU have been made sincePioneer 10 and 11 Voyagers 1 and 2 and New Horizons with veryfew observations made at solar maximum Energetic particleobservations during cruise out to 192 AU will facilitate furtherstudy of the interaction between the outer heliosphere andinterstellar medium as carried out by Cassini at 95 AU andInterstellar Boundaries Explorer (IBEX) at 1 AU A cruise phase toUranus also allows the characterisation of interplanetary andinterstellar dust with radial distance from the Sun

Interstellar dust penetrates deep into the heliosphere and doesprovide the unique opportunity for an in situ analysis of itsdynamical and compositional information which varies withdistance from the Sun and with the solar cycle The current dataset including composition information of interplanetary andinterstellar grains is very limited Only Cassini carried a spectro-meter and the pointing profile during the cruise phase was notoptimised for interplanetary and interstellar dust measurementsA mission to Uranus would help to close this knowledge gap whichis essential to understand Solar System formation and evolution

242 Fundamental physics and departures from General RelativityGeneral Relativity has been confirmed by all the precision

experiements performed so far But experimental tests leave open


windows for deviations from this theory at short (Antoniadis et al2011) or long (Reynaud and Jaekel 2005) distances General Relativ-ity is also challenged by observations at galactic and cosmic scalesThe rotation curves of galaxies and the relation between redshiftsand luminosities of supernovae deviate from the predictions of thetheory These anomalies are interpreted as revealing the presence ofso-called ldquodark matterrdquo and ldquodark energyrdquo Their nature remainsunknown and despite their prevalence in the energy content theyhave not been detected up to now by other means than gravitationalmeasurements

Given the immense challenge posed by these large scaleobservations in a context dominated by the quest for the natureof dark matter and dark energy it is important to explore everypossible explanation including the hypothesis that General Rela-tivity could not be the correct description of gravitational phe-nomena at large scales (Aguirre et al 2001 Nojiri and Odintsov2007) Extending the range to which gravity is probed is thereforeessential to bridge the gap between experiments in the SolarSystem and astrophysical or cosmological observations (Turyshev2008) In this respect as has been customary for the deep-spacemissions the spacecraft is seen as a test mass (almost) freelyfalling in the Solar System gravitational environment High preci-sion microwave tracking data (as that offered by Ka-band) ndash

besides the standard navigation operations ndash can be analysedsearching for possible deviations from the trajectory predicted byGeneral Relativity Of primary importance in exploiting the infor-mation content of these data will be a proper modelling ofspacecraft dynamics Combining radio-science and accelerationmeasurements not only improves the precision and quality ofspacecraft navigation but also allows us to remove as fully aspossible the systematic effects of non-gravitational forces actingon the spacecraft (Iafolla et al 2010) These scientific goals areintimately connected to the planetary science goals since gravita-tion is directly connected to planetary ephemeris (Fienga et al2010) as well as to the origins of the Solar System (Blanc et al2005)

243 Small icy bodies in the outer heliosphereCentaurs and trans-Neptunian objects (TNOs) are the most

pristine and less-processed remnants of the icy debris that formedthe outer planets and are the most observable Solar Systemanalogues for debris disks observed around other stars Centaursand TNOs are widely thought to be objects scattered from theKuiper belt that may evolve into short-period comets (Cruikshank2005) Surveys of surface properties indicate potential geneticlinks between TNOs Centaurs comets and water-rich asteroidsSome of these objects show evidence of episodic cometary-likebehaviour for example 2060 Chiron (Luu et al 2000) No missionis currently planned to visit a CentaurTNO but the cruise phase fora mission to Uranus provides an opportunity to visit such an objecten route to Uranus As a proof-of-concept we took a nominallaunch date of 2028 and searched for CentaursTNOs that might beaccessible for a flyby en route to Uranus and found that objects2060 Chiron 2010 KG43 330759 and 2007 TB434 could poten-tially be visited en route Comets and asteroids are also naturaltargets for flybys during the cruise phase Naturally furthermission study is required to investigate this in more detail

3 Strawman mission concept

In terms of mission options the primary trade space is betweenan orbiter and a flyby mission Some goals can be partially satisfiedwith a flyby mission but to fully answer the questions laid out inSection 2 requires an orbiting platform to make repeated observa-tions of Uranus and its planetary system There exists an additional

trade space between enhanced remote sensing instrumentationand an entry probe But some science questions (Sections 211215 and 216) can only be answered with an atmospheric entryprobe to a 45 bar depth For the purposes of the Uranus whitepaper the outline mission concept consisted of an orbiter in apolar science orbit with an atmospheric entry probe A specificprime science phase duration was not determined and dependssensitively on a number of factors including the instrumentpayload and science orbits However we note that Arridge et al(2012) and Hubbard (2010) considered 620- and 431-day sciencephase durations respectively In some cases the exploration ofUranus can be seen as easier than Saturn for example particularlyfor the planet itself since a spacecraft can easily inject into a polarorbit This potentially makes the study of moons more difficultthan Cassini-Huygens at Saturn Novel solutions to return sciencedata will be required due to the lower communications rates from19 AU compared to Cassini Table 2 illustrates the strawmaninstrument suite composed of high technology readiness level(TRL) instruments

31 Interplanetary transfers and orbital entry

Interplanetary transfers to Uranus have been studied in anumber of mission analyses (Arridge et al 2012 Hubbard 2010)and demonstrate the feasibility of a mission to Uranus withcurrent technology and including an interplanetary transferbetween 10 and 16 years The mission is feasible with high TRLconventional chemical propulsion and solarndashelectric propulsionemploying ion engines provides potential gains in margins avail-able Δv and platforminstrumentation mass (eg Hubbard 2010)Concepts involving lower TRL technology such as E-sails (egJanhunen 2004 Janhunen et al 2014) would be naturallybeneficial

The range of acceptable periapsis latitudes and radial distancesat Uranus orbit insertion are limited due to the largely unknownring plane hazards This can be mitigated with a high latitudeperiapsis and orbit insertion manoeuver followed by a ring plane

Table 2Strawman scientific payload for a Uranus orbiterentry probe mission

Instrument Heritage

OrbiterMagnetometer CassiniMAG

Solar OrbiterPlasma and particle package RosettaRPC-IES

New HorizonsPEPPSIRadio and plasma wave experiment CassiniRPWS

Bepi-ColomboMMOPWIMicrowave radiometer JunoMWRThermal infrared bolometer LRODiviner

Bepi-Colombo (detectors)Visual and near-infrared mapping spectrometer New HorizonsRALPH

RosettaVIRTISDawnVIR

Ultraviolet imaging spectrometer Bepi-ColomboPHEBUSMars ExpressSPICAM-UV

Visible camera Mars ExpressHRSCNew HorizonsLORRI

Radio science experiment Venus ExpressVeRaRosettaRSI

Accelerometer CHAMPSTARDust detector CassiniCDAProbeMass spectrometer HuygensGCMS

GalileoGPMSNephelometer GalileoNEPRadio science HuygensDWEAccelerometer HuygensHASI


crossing beyond 52000 km inside of which are the main ringplane hazards Although aerocapture is a natural technology to useat orbit insertion the atmosphere of Uranus is poorly understoodand aerocapture is low TRL technology thus representing a high-risk option

Uranus large obliquity permits a range of insertion orbitalinclinations from equatorial to polar The lack of large naturalsatellites does not permit low-fuel inclination changes and so aninitial polar orbit is preferred since these are ideal for studies ofUranus interior atmosphere and magnetic field that are requiredto meet the goals in section two

32 Atmospheric entry probe

An atmospheric entry probe for Uranus has been studied by theESA Concurrent Design Facility (Biesbroek et al 2010) which ledto a 312 kg entry probe (including 20 system margin) using adedicated carrier platform The mission concept we outline wouldinvolve using the Uranus orbiter as a carrier and communicationsrelay The instrumentation for such an entry probe is all availablewithin Europe and is high TRL The key technology developmentrequirement is the thermal protection system for the entry probeHowever such a probe might be provided via internationalcooperation and has been studied by NASA (Agrawal et al 2014)

33 Critical issues

Voyager 2 found that the radiation belts of Uranus were similarto Earth and Saturn in terms of intensity and so the radiationenvironment of Uranus is not judged to be a significant missiondriver Arridge et al (2012) estimated the radiation dose for theUranus Pathfinder mission concept using the SHEILDDOSE-2 soft-ware and found that the largest dose came from the cruise phase(18 krad behind 4 mm of Al) with only 2 krad per science orbitbased on a scaled model of Earths magnetosphere The maincritical issues for a Uranus mission are electrical power (Section331) thermal control (Section 332) telemetry (Section 333)and cruise phase duration (Section 334)

331 Electrical powerThe key technology development requirement for a mission to

Uranus is the provision of sufficient electrical power at 192 AUScaling ESAs Rosetta mission solar arrays out to Uranus weestimate that providing 400 We at Uranus would require 800 m2

solar arrays producing system level issues associated with a largelaunch mass and spacecraft moment of inertia At present anuclear (radioisotope) power source (RPS) is the only viablealternative 241Am is the isotope that has been selected for ESARPS devices that are currently in the developmental stage (seeOrsquoBrien et al (2008) Arridge et al (2012) and Sarsfield et al(2013) for a discussion of issues relating to the use of 241Am)To provide target electrical power of 400 We at Uranus after 14years flight time would require a total RPS system mass of 200 kg(excluding any maturity margin) based on a radioisotope thermo-electric generator (RTG) design with a specific power of 20 Wekgcompared with 29 Wekg (at the beginning of mission) for a NASAmulti-mission RTG using 238Pu (Abelson et al 2005) Although thedevelopment of such technology presents a schedule and cost riskthis is currently under development as part of an ESA develop-ment programme and with sustained investment should reach ahigher TRL in the 2025ndash2035 timeframe In addition to RTGsystems Stirling generator-based nuclear power sources are alsounder development in Europe and specific power values will bedetermined at the end of an active ESA study Stirling-based

solutions could offer an alternative option with a higher specificpower on similar timescales to the RTG programme

332 Thermal controlThermal control is an important driver for every mission

Extreme differences in thermal environment between the innerheliosphere (for trajectories involving Venus gravity assists) andUranus and due to the continuous supply of thermal energy fromRPS units present the most important issues Such thermal controlissues can be adequately managed by modifying existing designsfrom Rosetta and MarsVenus Express Thermal control for aUranus mission was studied using ThermXL based on a spacecraftof a similar size to Mars Express and including waste heatdissipation from the RPS We estimated that electrical heatersconsuming around 50 W would be sufficient to maintain aninternal spacecraft temperature of 30 1C against losses to spaceWaste electrical power from the RPS can be dissipated viaexternally- or internally-mounted shunt resistors but could impacton overall system design Radioisotope heater units based on241Am can offer distributed heating solutions with each unitgenerating between 1 W and 5 W of thermal power This wouldreduce the requirement to distribute waste heat from RTGs ofStirling-based systems and reduce demands for electrical heatingThe use of these heaters or waste heat from RPS solutions shouldform part of a future trade-off study

333 Telemetry ratesTo answer the questions in Section 1 requires significant

volumes of data to be returned over ⪅209 AU Downlink transmis-sions over Ka-band to ESAs Cebreros station using a 4 m (3 m) highgain antenna with a 100 W power input to the transmitter on anorbiter with a pointing accuracy of 0051 (comparable to the Cassiniorbiter) will achieve a downlink rate of 45 kbits (15 kbits) at 101elevation and 72 kbits (24 kbits) at 801 elevation equivalent to170 (60) Mbit per 8 h downlink Using ground station arraysand utilising larger dishes (for example via collaboration with NASAto use the Deep Space Network) will naturally increase these datavolumes These data volumes should be sufficient to achieve theessential science goals

334 Long cruise phase durationTo reduce cruise phase costs a Uranus mission might employ

hibernation modes (similar to those used on New Horizons andRosetta) to minimise operations costs and ground station antennausage A cruise phase science programme as outlined in Section 2will periodically enable the platform and science instruments to beutilised and tested In addition special hibernation modes wouldpermit some instruments to collect low-rate cruise phase sciencedata The use of high TRL technology and minimising the cruisephase operations will reduce demands on spacecraft platformcomponents reduce the mission cost-at-completion and lessendemands on the electrical power system

34 International cooperation

Such a large and significant interplanetary mission wouldnaturally benefit from collaboration with other space agenciesThe white paper had broad support from scientists funded byNASA and JAXA and within Europe Uranus has been named apriority by NASA as recommended by the Planetary DecadalSurvey In the context of international cooperation a partneragency may provide an atmospheric entry probe provide instru-ments for the orbiterentry probe thus lessening the demand onESA member states or may provide a launch vehicle


Acknowledgements

CSA and LNF were supported by Royal Society UniversityResearch Fellowships CSA thanks O Bedworth B Jacobsonand J-P Lebreton for useful discussions and comments on themanuscript

References

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Crida A Charnoz S 2012 Formation of regular satellites from ancient massiverings in the solar system Science 338 (6111) 1196 httpdxdoiorg101126science1226477

Croft SK Soderblom LA 1991 Geology of the Uranian satellites In Bergstrahl JTMiner MS Matthews MS (Eds) Uranus University of Arizona Press Tucsonpp 561ndash628

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Cuzzi JN Burns JA Charnoz S Clark RN Colwell JE Dones L Esposito LWFilacchione G French RG Hedman MM Kempf S Marouf EA Murray CD Nicholson PD Porco CC Schmidt J Showalter MR Spilker LJ Spitale JN Srama R Sremčević M Tiscareno MS Weiss J 2010 An evolving view ofSaturns dynamic rings Science 327 (5972) 1470 httpdxdoiorg101126science1179118

Davis L Smith EJ 1990 New models of Saturns magnetic field using Pioneer 11vector helium magnetometer data J Geophys Res 91 (A2) 1373ndash1380

de Pater I Romani PN Atreya SK 1991 Possible microwave absorption by H2Sgas in Uranus and Neptunes atmospheres Icarus 91 220ndash233 httpdxdoiorg1010160019-1035(91)90020-T

de Pater I Lissauer J 2010 Planetary Sciences Cambridge University PressCambridge UK

de Pater I Gibbard SG Hammel HB 2006a Evolution of the dusty rings ofUranus Icarus 180 (1) 186ndash200 httpdxdoiorg101016jicarus200508011

de Pater I Hammel HB Gibbard SG Showalter M 2006b New dust belts ofUranus one ring two ring red ring blue ring Science 312 (5) 92ndash94

de Pater I Hammel HB Showalter MR van Dam MA 2007 The dark side ofthe rings of Uranus Science 317 (5846) 1888 httpdxdoiorg101126science1148103

de Pater I Sromovsky LA Hammel HB Fry PM LeBeau RP Rages KShowalter MR Matthews K 2011 Post-equinox observations of UranusBergs evolution vertical structure and track towards the equator Icarus 215(1) 332ndash345 httpdxdoiorg101016jicarus201106022

de Pater I Dunn DE Stam DM Showalter MR Hammel HB Min M Hartung MGibbard SG van Dam MA Matthews K 2013 Keck and VLT AO observations andmodels of the uranian rings during the 2007 ring plane crossings Icarus 226 (2)1399ndash1424 httpdxdoiorg101016jicarus201308001

Dodson-Robinson SE Bodenheimer P 2010 The formation of Uranus andNeptune in solid-rich feeding zones connecting chemistry and dynamicsIcarus 207 491ndash498 httpdxdoiorg101016jicarus200911021


Duncan MJ Lissauer JJ 1997 Orbital stability of the uranian satellite systemIcarus 125 (1) 1ndash12 httpdxdoiorg101006icar19965568

Elliot JL Nicholson PD 1984 The rings of Uranus In Brahic A Greenberg R(Eds) Planetary Rings University of Arizona Press Tucson

Feuchtgruber H Lellouch E de Graauw T Beacutezard B Encrenaz T Griffin M1997 External supply of oxygen to the atmospheres of the giant planets Nature389 159ndash162

Feuchtgruber H Lellouch E Orton G de Graauw T Vandenbussche BSwinyard B Moreno R Jarchow C Billebaud F Cavalieacute T Sidher SHartogh P 2013 The DH ratio in the atmospheres of Uranus and Neptunefrom Herschel-PACS observations Astron Astrophys 551 A126 httpdxdoiorg1010510004-6361201220857

Fienga A Laskar J Kuchynka P Le Poncin-Lafitte C Manchel H Gastineau M2010 Gravity tests with INPOP- planetary ephemerides In Klioner SASeidelmann PK Soffel MH (Eds) Relativity in Fundamental AstronomyProceedings of the IAU Symposium 261 pp 159ndash169 httpdxdoiorg101017S1743921309990330

Fortney J Nettelmann N 2010 The interior structure composition and evolutionof giant planets Space Sci Rev 152 (1ndash4) 423ndash447 httpdxdoiorg101007s11214-009-9582-x

French RS Nicholson PD Porco CC Marouf EA 1991 Dynamics and structureof the uranian rings In Bergstrahl JT Miner ED Matthews MS (Eds)Uranus University of Arizona Press Tucson pp 327ndash409

French RS Showalter MS 2012 Cupid is doomed an analysis of the stability ofthe inner uranian satellites Icarus 220 (2) 911ndash921 httpdxdoiorg101016jicarus201206031

Fressin F Torres G Charbonneau D Bryson ST Christiansen J Dressing CDJenkins JM Walkowicz LM Batalha NM 2013 The false positive rate ofKepler and the occurrence of planets Astron Phys J 766 81 httpdxdoiorg1010880004-637X766281

Fry PM Sromovsky LA de Pater I Hammel HB Rages KA 2012 Detectionand tracking of subtle cloud features on Uranus Astron J 143 150 httpdxdoiorg1010880004-62561436150

Grav T Holman MJ Fraser WC 2004 Photometry of irregular satellites ofUranus and Neptune Astron J 613 L77ndashL80

Grundy WM Young LA Spencer JR Johnson RE Young EF Buie MW 2006Distributions of H2O and CO2 ices on Ariel Umbriel TItania and Oberon fromIRTFSpeX observations Icarus 184 (2) 543ndash555 httpdxdoiorg101016jicarus200604016

Guillot T 2005 The interiors of giant planets models and outstanding questionsAnnu Rev Earth Planet Sci 33 493ndash530 httpdxdoiorg101146annurevearth32101802120325

Guillot T Gautier D 2010 Giant planets In Schubert G Spohn T (Eds) Treatiseon Geophysics vol 10 ndash Planets and Moons pp 439ndash464 httpdxdoiorg101016B978-044452748-600165-6

Hammel HB Lynch DK Russell RW Sitko ML Bernstein LS Hewagama T2006 Mid-infrared ethane emission on Neptune and Uranus Astron Phys J644 (2) 1326ndash1333 httpdxdoiorg101086503599

Hedman MM Burt JA Burns JA Tiscareno MS 2010 The shape and dynamicsof a heliotropic dusty ringlet in the Cassini Division Icarus 210 (1) 284ndash297

Hedman MM Nicholson PD 2013 Kronoseismology using density waves inSaturns C ring to probe the planets interior Astron J 146 12 httpdxdoiorg1010880004-6256146112

Helled R Anderson JD Schubert G 2010 Uranus and Neptune shape androtation Icarus 210 (1) 446ndash454 httpdxdoiorg101016jicarus201006037

Herbert F Sandel BR Yelle RV Holberg JB Broadfoot AL Shemansky DEAtreya SK Romani PN 1987 The upper atmosphere of Uranus ndash EUVoccultations observed by Voyager 2 J Geophys Res 92 (A13) 15093ndash15109httpdxdoiorg101029JA092iA13p15093

Herbert F 2009 Aurora and magnetic field of Uranus J Geophys Res 114 A11206httpdxdoiorg1010292009JA014394

Hofstadter MH Butler BJ Gurwell MA 2006 Imaging of Uranus at sub-millimeter to centimeter wavelengths Bull Am Astron Soc 38 488

Holme R Bloxham J 1996 The magnetic fields of Uranus and Neptune methods andmodels J Geophys Res 101 (E1) 2177ndash2200 httpdxdoiorg10102995JE03437

Horaacutenyi M 1996 Charged dust dynamics in the solar system Annu Rev AstronAstrophys 34 383ndash418

Horaacutenyi M Burns JA Hamilton DP 1992 The dynamics of Saturns E ringparticles Icarus 97 (2) 248ndash259

Hsu H-W Postberg F Kempf S Trieloff M Burton M Roy M Moragas-Klostermeyer G Srama R 2011 Stream particles as the probe of the dustndashplasmandashmagnetosphere interaction at Saturn J Geophys Res 116 A09215

Hubbard et al 1995 The interior of Neptune Hubbard Podolak and StevensonNeptune and Triton University of Arizona Press Tucson Arizona USA pp 109ndash138

Hubbard WB 2010 Ice giants decadal study langhttpsitesnationalacademiesorgSSBSSB_059331rang (Retrieved 71010)

Hussmann H Sohl F Spohn T 2006 Subsurface oceans and deep interiors ofmedium-sized outer planet satellites and large trans-neptunian objects Icarus185 (1) 258ndash273 httpdxdoiorg101016jicarus200606005

Iafolla V Fiorenza E Lefevre C Morbidini A Nozzoli S Peron R Persichini MReale A Santoli F 2010 Italian Spring Accelerometer (ISA) a fundamentalsupport to BepiColombo Radio Science Experiments Planet Space Sci 58 (1ndash2)300ndash308 httpdxdoiorg101016jpss200904005

Jacobson RA Campbell JK Taylor AH Synnott SP 1992 The masses of Uranusand its major satellites from Voyager tracking data and Earth-based uraniansatellite data Astron J 103 (6) 2068ndash2078

Jacobson RA 1998 The orbits of the inner uranian satellites from Hubble SpaceTelescope and Voyager 2 observations Astron J 115 (3) 1195ndash1199 httpdxdoiorg101086300263

Jacobson RA 2007 The gravity field of the uranian system and the orbits of theuranian satellites and rings Presented at the American Astronomical SocietyDPS meeting 39 Bull Am Astron Soc 39 453

Janes DM Melosh HJ 1988 Sinker tectonics ndash an approach to the surface ofMiranda J Geophys Res 93 3127ndash3143 httpdxdoiorg101029JB093iB04p03127

Janhunen P 2004 Electric sail for spacecraft propulsion J Propuls Power 20 763ndash764Janhunen P Lebreton J-P Merikallio S Paton M Mengali G Quarta AA 2014

Fast E-sail Uranus entry probe mission Planet Space Sci 104 141ndash146Karkoschka E Tomasko M 2009 The haze and methane distributions on Uranus

from HST-STIS spectroscopy Icarus 202 (1) 287ndash309 httpdxdoiorg101016jicarus200902010

Kempf S Srama R Horaacutenyi M Burton M Helfert S Moragas-Klostermeyer GRoy M Gruumln E 2005 High-velocity streams of dust originating from SaturnNature 433 (7023) 289ndash291

Khurana KK Kivelson MG Stevenson DJ Schubert G Russell CT Walker RJPolanskey C 1998 Induced magnetic fields as evidence for subsurface oceansin Europa and Callisto Nature 395 (6704) 777ndash780 httpdxdoiorg10103827394

Kurth WS Gurnett DA Coroniti FV Scarf FL 1991 Wavendashparticle interactionsin the magnetosphere of Uranus In Bergstrahl JT Miner ED Matthews MS(Eds) Uranus Umiversity of Arizona Press Tucson

Lainey V 2008 A new dynamical model for the uranian satellites Planet SpaceSci 56 (14) 1766ndash1772 httpdxdoiorg101016jpss200802015

Lamy L Prangeacute R Hansen KC Clarke JT Zarka P Cecconi B Aboudarham JAndreacute N Branduardi-Raymont G Gladstone R Bartheacuteleacutemy M Achilleos NGuio P Dougherty MK Melin H Cowley SWH Stallard TS Nichols JDBallester G 2012 Earth-based detection of Uranus aurorae Geophys Res Lett39 L07105 httpdxdoiorg1010292012GL051312

Leinhardt ZM Ogilvie GI Latter HN Kokubo E 2012 Tidal disruption ofsatellites and formation of narrow rings MNRAS 424 (2) 1419ndash1431 httpdxdoiorg101111j1365-2966201221328x

Lissauer JJ 2005 Formation of the outer planets Space Sci Rev 116 11ndash24 httpdxdoiorg101007s11214-005-1945-3

Lopes RMC Kirk RL Mitchell KL Legall A Barnes JW Hayes A Kargel JWye L Radebaugh J Stofan ER Janssen MA Neish CD Wall SD WoodCA Lunine JI Malaska MJ 2013 Cryovolcanism on Titan new results fromCassini RADAR and VIMS J Geophys Res Planets 118 416ndash435 httpdxdoiorg101002jgre20062

Luu JX Jewitt DC Trujillo C 2000 Water ice in 2060 Chiron and its implicationsfor Centaurs and Kuiper belt objects Astron Phys J 531 L151ndash154 httpdxdoiorg101086312536

Majeed T Waite JH Bougher SW Yelle RV Gladstone GR McConnell JCBhardwaj A 2004 The ionospheresndashthermospheres of the giant planets AdvSpace Res 33 (2) 197ndash211 httpdxdoiorg101016jasr200305009

Maris M Carraro G Cremonese G Fulle M 2001 Multicolor photometry of theUranus irregular satellites Sycorax and Caliban Astron J 121 2800ndash2803

Maris M Carraro G Parisi MG 2007 Light curves and colours of the faintUranian irregular satellites Sycorax Prospero Stephano Setebos and TrinculoAampA 472 311ndash319 httpdxdoiorg1010510004-636120066927

Masters A 2014 Magnetic reconnection at Uranus magnetopause J Geophys Res119 httpdxdoiorg1010022014JA020077

Matthews MS Bergstralh JT Miner ED 1991 Uranus University of ArizonaPress Tucson ISBN 978-0-8165-1208-9

McNutt Jr RL Selesnick RS Richardson JD 1987 Low-energy plasma observa-tions in the magnetosphere of uranus J Geophys Res 92 (A5) 4399ndash4410

Melin H Stallard T Miller S Trafton LM Encrenaz Th Geballe TR 2011Seasonal variability in the ionosphere of Uranus Astrophys J 729 134 httpdxdoiorg1010880004-637X7292134

Melin H Stallard T Miller S Geballe TR Trafton LM OrsquoDonoghue J 2013Post-equinoctial observations of the ionosphere of Uranus Icarus 223 (2)741ndash748 httpdxdoiorg101016jicarus201301012

Mauk BH Krimigis SM Keath EP Cheng AF Armstrong TP Lanzerotti LJGloeckler G Hamilton DC 1987 The hot plasma and radiation environmentof the uranian magnetosphere J Geophys Res 92 (A13) 15283ndash15308

Morbidelli A Tsiganis K Batygin K Crida A Gomes R 2012 Explaining whythe uranian satellites have equatorial prograde orbits despite the largeplanetary obliquity Icarus 219 (2) 737ndash740 httpdxdoiorg101016jicarus201203025

Moses JI Fouchet T Beacutezard B Gladstone GR Lellouch E Feuchtgruber H2005 Photochemistry and diffusion in Jupiters stratosphere constraints fromISO observations and comparisons with other giant planets J Geophys Res110 E08001 httpdxdoiorg1010292005JE002411

Murray CD Thompson RP 1990 Orbits of shepherd satellites deduced from thestructure of the rings of Uranus Nature 348 499ndash502 httpdxdoiorg101038348499a0

Ness NF Connerney JEP Lepping RP Schulz M Voigt G-H 1991 The magneticfield and magnetospheric configuration of Uranus In Bergstrahl JT Miner EDMatthews MS (Eds) Uranus University of Arizona Press Tucson pp 739ndash779

Nettelmann N Helled R Fortney JJ Redmer R 2013 New indication for adichotomy in the interior structure of Uranus and Neptune from the applicationof modified shape and rotation data Planet Space Sci 77 143ndash151 httpdxdoiorg101016jpss201206019


Nojiri S Odintsov S 2007 Introduction to modified gravity and gravitationalalternative for dark energy Int J Geom Methods Mod Phys 4 (1) 115ndash145httpdxdoiorg101142S0219887807001928

OrsquoBrien RC Ambrosi RM Bannister NP Howe SD Atkinson HV 2008 Saferadioisotope thermoelectric generators and heat sources for space applicationsJ Nucl Mater 377 (3) 506ndash521

OrsquoDonoghue JT Stallard S Melin H Jones GH Cowley SWH Miller S BainesKH Blake JSD 2013 The domination of Saturns low-latitude ionosphere byring lsquorainrsquo Nature 496 (7444) 193ndash195 httpdxdoiorg101038nature12049

Owen T Encrenaz T 2006 Compositional constraints on giant planet formationPlanet Space Sci 54 (12) 1188ndash1196 httpdxdoiorg101016jpss200605030

Pappalardo RT Reynolds SJ Greeley R 1997 Extensional tilt blocks on Mirandaevidence for an upwelling origin of Arden Corona J Geophys Res Planets 102(E6) 13369ndash13380 httpdxdoiorg10102997JE00802

Parisi MG Carraro G Maris M Brunini A 2008 Constraints to Uranus greatcollision IV the origin of Prospero AampA 482 657ndash664 httpdxdoiorg1010510004-636120078265

Pearl JC Conrath BJ Hanel RA Pirraglia JA 1990 The albedo effectivetemperature and energy balance of Uranus as determined from Voyager IRISdata Icarus 84 12ndash28 httpdxdoiorg1010160019-1035(90)90155-3

Plescia JB 1987 Cratering history of the Uranian satellitesmdashUmbriel Titania andOberon J Geophys Res 92 14918ndash14932 httpdxdoiorg101029JA092iA13p14918

Postberg F Schmidt J Hillier JK Kempf S Srama R 2011 A salt-water reservoiras the source of a compositionally stratified plume on Enceladus Nature 474(7) 620ndash622

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Redmer R Mattsson TR Nettelmann N French M 2011 The phase diagram ofwater and the magnetic fields of Uranus and Neptune Icarus 211 798ndash803httpdxdoiorg101016jicarus201008008

Reynaud S Jaekel MT 2005 Testing the Newton law at long distances Int J ModPhys A 20 2294 httpdxdoiorg101142S021775105024523

Richardson JD Belcher JW Selesnick RS Zhang M Siscoe GL Eviatar A1988 Evidence for periodic reconnection at Uranus Geophys Res Lett 15 (8)733ndash736

Romon J de Bergh C Barucci MA Doressoundiram A Cuby J-G Le Bras ADouteacute S Schmitt D 2001 Photometric and spectroscopic observations ofSycorax satellite of Uranus AampA 376 310ndash315

Safronov 1972 Evolution of the protoplanetary cloud and formation of the earthand planets Translated from Russian Jerusalem (Israel) Israel Program forScientific Translations Keter Publishing House p 212 Saumon amp Guillot(2004) Astron Phys J 609(2) 1170ndash1180

Sarsfield MJ Bell K Maher CJ Carrott MJ Gregson C Brown J WoodheadDA Baker SR Cordingley L Taylor RJ Tinsley TP Rice TG Rhodes CJClough M 2013 Progress on 241Am production for use in radioisotope powersystems In Proceedings of Nuclear and Emerging Technologies for SpaceAlbuquerque NM USA February 25ndash28 2013

Saumon D Guillot T 2004 Shock compression of deuterium and the interiors ofJupiter and Saturn Astron Phys J 609 1170ndash1180

Schenk PM 1991 Fluid volcanism on Miranda and Ariel ndash flow morphology andcomposition J Geophys Res Solid Earth 96 (B2) 1887ndash1906 httpdxdoiorg10102990JB01604

Schenk PM Moore JM 1995 Volcanic constructs on Ganymede and Enceladustopographic evidence from stereo images and photoclinometry J GeophysRes Planets 100 (E9) 19009ndash19022 httpdxdoiorg10102995JE01854

Schubert G Hussmann H Lainey V Matson DL McKinnon WB Sohl F SotinC Tobie G Turrini D van Hoolst T 2010 Evolution of icy satellites Space SciRev 153 (1ndash4) 447ndash484 httpdxdoiorg101007s11214-010-9635-1

Selesnick RS Richardson JD 1986 Plasmasphere formation in arbitrarilyoriented magnetospheres Geophys Res Lett 13 (7) 624ndash627

Selesnick RS McNutt RL 1987 Voyager 2 Plasma ion observations in themagnetosphere of Uranus J Geophys Res 92 (A13) 15249ndash15262

Sheppard SS Jewitt D Kelyna J 2005 An ultradeep survey for irregular satellitesof Uranus limits to completeness Astron J 129 518ndash525

Showalter MR Lissauer JJ French RG Hamilton DP Nicholson PD de Pater IDawson R 2008 HST observations of the Uranian outer ring-moon systemAmerican Astronomical Society DPS meeting 40 2407 Bull Am Astron Soc40 431

Showalter M Lissauer J 2006 The second ringndashmoon system of Uranusdiscovery and dynamics Science 311 (5763) 973ndash977 httpdxdoiorg101126science1122882

Sittler EC Ogilvie KW Selesnick RS 1987 Survey of electrons in the uranianmagnetosphere Voyager 2 observations J Geophys Res 92 (A13)15263ndash15281

Soderlund KM Heimpel MH King EM Aurnou JM 2013 Turbulent models ofice giant internal dynamics dynamos heat transfer and zonal flows Icarus 224(1) 97ndash113 httpdxdoiorg101016jicarus201302014

Spahn F Schmidt J Albers N Houmlrning M Makuch M Seiszlig M Kempf S SramaR Dikarev V Helfert S Moragas-Klostermeyer G Krivov AV Sremčević MTuzzolino AJ Economou T Gruumln E 2006 Cassini dust measurements atEnceladus and implications for the origin of the E ring Science 311 (5)1416ndash1418

Squyres S et al 2011 Vision and Voyages for Planetary Science in the Decade2013ndash2022 Committee on the Planetary Science Decadal Survey NationalResearch Council National Academies Press Washington DC ISBN 0-309-20955-2

Sromovsky LA Fry PM Hammel HB de Pater I Rages KA Showalter MR2007 Dynamics evolution and structure of Uranus brightest cloud featureIcarus 192 (2) 558ndash575

Sromovsky LA Fry PM Hammel HB Ahue WM de Pater I Rages KAShowalter MR van Dam MA 2009 Uranus at equinox cloud morphologyand dynamics Icarus 203 (1) 265ndash286 httpdxdoiorg101016jicarus200904015

Sromovsky LA Karkoschka E Fry PM Hammel HB de Pater I Rages K 2014Methane depletion in both polar regions of Uranus inferred from HSTSTIS andKeckNIRC2 observations Icarus 238 137ndash155 httpdxdoiorg101016jicarus201405016

Stanley S Bloxham J 2004 Convective-region geometry as the cause of Uranusand Neptunes unusual magnetic fields Nature 428 (6979) 151ndash153 httpdxdoiorg101038nature02376

Stanley S Bloxham J 2006 Numerical dynamo models of Uranus and Neptunesmagnetic fields Icarus 184 (2) 556ndash572 httpdxdoiorg101016jicarus200605005

Stone EC Miner ED 1986 The Voyager 2 Encounter with the Uranian SystemScience 233 (4759) 39ndash43 httpdxdoiorg101126science233475939

Tiscareno MS 2013 Planetary rings In Oswalt French and Kalas (Eds) PlanetsStars and Stellar Systems Springer langarXiv11123305rang

Tittemore WC Wisdom J 1990 Tidal evolution of the Uranian satellites IIImdashevolution through the MirandandashUmbriel 31 MirandandashAriel 53 and ArielndashUmbriel 21 mean-motion commensurabilities Icarus 85 394ndash443 httpdxdoiorg1010160019-1035(90)90125-S

Tittemore WC 1990 Tidal heating of Ariel Icarus 87 110ndash139 httpdxdoiorg1010160019-1035(90)90024-4

Tosi F Turrini D Coradini A Filacchione G 2010 Probing the origin of the darkmaterial on Iapetus MNRAS 403 1113ndash1130 httpdxdoiorg101111j1365-2966201016044x

Toacuteth G Kovaacutecs D Hansen KC Gombosi TI 2004 Three-dimensional MHDsimulations of the magnetosphere of Uranus J Geophys Res 109 A11210 httpdxdoiorg1010292004JA010406

Trafton LM Miller S Geballe TR Tennyson J Ballester GE 1999 H2Quadrupole and H3thorn emission from Uranus the Uranian thermosphereionosphere and aurora Astrophys J 524 (2) 1059ndash1083 httpdxdoiorg101086307838

Turrini D Magni G Coradini A 2011 Probing the history of solar system throughthe cratering records on Vesta and Ceres MNRAS 413 (4) 2439ndash2466 httpdxdoiorg101111j1365-2966201118316x

Turyshev SG 2008 Experimental tests of General Relativity Annu Rev Nucl PartSci 58 (1) 207ndash248 httpdxdoiorg101146annurevnucl58020807111839

Vasyliuntildeas VM 1986 The convection-dominated magnetosphere of UranusGeophys Res Lett 13 621ndash623 httpdxdoiorg101029GL013i007p00621

Vilas F Lederer SM Gill SL Jarvis KS Thomas-Osip JE 2006 Aqueousalteration affecting the irregular outer planets satellites evidence from spectralreflectance Icarus 180 453ndash463

Walsh KJ Morbidelli A Raymond SN OrsquoBrien DP Mandell AM 2011 A lowmass for Mars from Jupiters early gas-driven migration Nature 475 (7355)206ndash209 httpdxdoiorg101038nature10201

Zarka P Lecacheux A 1987 Beaming of uranian nightside kilometric radioemission and inferred source location J Geophys Res 92 15177ndash15187 httpdxdoiorg101029JA092iA13p15177

Zarka P 2007 Plasma interactions of exoplanets with their parent star andassociated radio emissions Planet Space Sci 55 (5) 598ndash617 httpdxdoiorg101016jpss200605045


ad INAF-IAPS Istituto di Astrofisica e Planetologia Spaziali Rome Italyae Department of Physics University of Oxford UKaf University of California Santa Cruz USAag Max Planck Institute for Nuclear Physics Germanyah LASP University of Colorado USAai University of Idaho Moscow ID USAaj Tel Aviv University Tel Aviv Israelak LATMOS Franceal Heidelberg University Germanyam University of Liverpool UKan University of Iowa USAao Department of Physics and Astronomy University of Southampton UKap Royal Observatory of Belgium Belgiumaq University of Stuttgart Germanyar Universitaumlt der Bundeswehr Muumlnchen Germanyas UTesat-Spacecom GmbH Germanyat IMCCE-Observatoire de Paris UMR 8028 du CNRS UPMC Universiteacute Lille 1 77 Av Denfert-Rochereau 75014 Paris Franceau Institut de Physique du Globe de Paris Franceav University of Vienna Austriaaw Space Research Institute Austrian Academy of Science Austriaax Space Technology Ireland National University of Ireland Irelanday DLR Germanyaz Space Science Institute USAba Observatoire de Besanccedilon Francebb University of Cologne Germanybc University of Namur Belgiumbd University of Reading UKbe Universiteacute Pierre et Marie Curie UPMC - Paris 06 Francebf Laboratoire Kastler Brossel CNRS UMPC Francebg Institute of Geophysics and Planetary Physics University of California Los Angeles USAbh University of the Basque Country Spainbi Institute of Atmospheric Physics Prague Czech Republicbj Department of Atmospheric and Planetary Sciences Hampton University Virginia USAbk Lunar and Planetary Institute University of Arizona USAbl Department of Earth Sciences University of California Los Angeles USAbm Office for Space Research and Technology Academy of Athens Greecebn University of Potsdam Germanybo Department of Physics and Astronomy University of Leicester UKbp LPG CNRS ndash Universiteacute de Nantes Francebq International Space Science Institute Bern Switzerlandbr CNRS IPAG F-38000 Grenoble France

a r t i c l e i n f o

Article historyReceived 17 December 2013Received in revised form23 July 2014Accepted 7 August 2014Available online 22 August 2014

KeywordsUranusMagnetosphereAtmosphereNatural satellitesRingsPlanetary interior

a b s t r a c t

Giant planets helped to shape the conditions we see in the Solar System today and they account for morethan 99 of the mass of the Sunrsquos planetary system They can be subdivided into the Ice Giants (Uranusand Neptune) and the Gas Giants (Jupiter and Saturn) which differ from each other in a number offundamental ways Uranus in particular is the most challenging to our understanding of planetaryformation and evolution with its large obliquity low self-luminosity highly asymmetrical internal fieldand puzzling internal structure Uranus also has a rich planetary system consisting of a system of innernatural satellites and complex ring system five major natural icy satellites a system of irregular moonswith varied dynamical histories and a highly asymmetrical magnetosphere Voyager 2 is the onlyspacecraft to have explored Uranus with a flyby in 1986 and no mission is currently planned to thisenigmatic system However a mission to the uranian system would open a new window on the originand evolution of the Solar System and would provide crucial information on a wide variety ofphysicochemical processes in our Solar System These have clear implications for understandingexoplanetary systems In this paper we describe the science case for an orbital mission to Uranus withan atmospheric entry probe to sample the composition and atmospheric physics in Uranusrsquo atmosphereThe characteristics of such an orbiter and a strawman scientific payload are described and we discuss thetechnical challenges for such a mission This paper is based on a white paper submitted to the EuropeanSpace Agencyrsquos call for science themes for its large-class mission programme in 2013

amp 2014 Published by Elsevier Ltd

1 Introduction

Giant planets account for more than 99 of the mass of theSunrsquos planetary system and helped to shape the conditions we seein the Solar System today The Ice Giants (Uranus and Neptune) arefundamentally different from the Gas Giants (Jupiter and Saturn)in a number of ways and Uranus in particular is the mostchallenging to our understanding of planetary formation and

n Corresponding author at Mullard Space Science Laboratory University CollegeLondon UK Tel thorn44 1483 204 150

E-mail address carridgeuclacuk (CS Arridge)








2 Scientific case



Terrestrialplanets


Oortcloud

0

50

100

150

200

250

300

350

Mas

ses

(ME )

Hydrogen and helium

Heavy elements

















H2He

H+

H2O (l)


Phase transition

Superionic water

Core

Ionic water



~01 Mbar

~6000 K~5 Mbar



































Microwave[VLA]





Voyager 2 periapsis

Orp

helia

Bia

nca

Cre

ssid

a Des

dem

ona

Julie

t Port

iaR

osal

ind

Cup

idB

elin

daPe

rdita

Puck

Mab

Mira

nda

Cor

delia

1986

U2R

ζ rin

g

6 5

4 ri

ngs

α rin

gβ

ring

η γ

δ ri

ngs

λ rin

g

μ rin

g

ν rin

g

ε rin

g

2 R1 R 3 R 4 R 5 R














































Miranda

Ariel

Umbriel

Titania

Oberon

Bow shock

Magnetopause

Magnetotail

Equator

Plasma sheet

Ω

M

Neutral torus










































Instrument Heritage

















33 Critical issues















Acknowledgements


References

































































































































































2 Scientific case



Terrestrialplanets


Oortcloud

0

50

100

150

200

250

300

350

Mas

ses

(ME )

Hydrogen and helium

Heavy elements

















H2He

H+

H2O (l)


Phase transition

Superionic water

Core

Ionic water



~01 Mbar

~6000 K~5 Mbar



































Microwave[VLA]





Voyager 2 periapsis

Orp

helia

Bia

nca

Cre

ssid

a Des

dem

ona

Julie

t Port

iaR

osal

ind

Cup

idB

elin

daPe

rdita

Puck

Mab

Mira

nda

Cor

delia

1986

U2R

ζ rin

g

6 5

4 ri

ngs

α rin

gβ

ring

η γ

δ ri

ngs

λ rin

g

μ rin

g

ν rin

g

ε rin

g

2 R1 R 3 R 4 R 5 R














































Miranda

Ariel

Umbriel

Titania

Oberon

Bow shock

Magnetopause

Magnetotail

Equator

Plasma sheet

Ω

M

Neutral torus










































Instrument Heritage

















33 Critical issues















Acknowledgements


References





































































































































































H2He

H+

H2O (l)


Phase transition

Superionic water

Core

Ionic water



~01 Mbar

~6000 K~5 Mbar



































Microwave[VLA]





Voyager 2 periapsis

Orp

helia

Bia

nca

Cre

ssid

a Des

dem

ona

Julie

t Port

iaR

osal

ind

Cup

idB

elin

daPe

rdita

Puck

Mab

Mira

nda

Cor

delia

1986

U2R

ζ rin

g

6 5

4 ri

ngs

α rin

gβ

ring

η γ

δ ri

ngs

λ rin

g

μ rin

g

ν rin

g

ε rin

g

2 R1 R 3 R 4 R 5 R














































Miranda

Ariel

Umbriel

Titania

Oberon

Bow shock

Magnetopause

Magnetotail

Equator

Plasma sheet

Ω

M

Neutral torus










































Instrument Heritage

















33 Critical issues















Acknowledgements


References



























































































































































































Microwave[VLA]





Voyager 2 periapsis

Orp

helia

Bia

nca

Cre

ssid

a Des

dem

ona

Julie

t Port

iaR

osal

ind

Cup

idB

elin

daPe

rdita

Puck

Mab

Mira

nda

Cor

delia

1986

U2R

ζ rin

g

6 5

4 ri

ngs

α rin

gβ

ring

η γ

δ ri

ngs

λ rin

g

μ rin

g

ν rin

g

ε rin

g

2 R1 R 3 R 4 R 5 R














































Miranda

Ariel

Umbriel

Titania

Oberon

Bow shock

Magnetopause

Magnetotail

Equator

Plasma sheet

Ω

M

Neutral torus










































Instrument Heritage

















33 Critical issues















Acknowledgements


References





























































































































































Voyager 2 periapsis

Orp

helia

Bia

nca

Cre

ssid

a Des

dem

ona

Julie

t Port

iaR

osal

ind

Cup

idB

elin

daPe

rdita

Puck

Mab

Mira

nda

Cor

delia

1986

U2R

ζ rin

g

6 5

4 ri

ngs

α rin

gβ

ring

η γ

δ ri

ngs

λ rin

g

μ rin

g

ν rin

g

ε rin

g

2 R1 R 3 R 4 R 5 R














































Miranda

Ariel

Umbriel

Titania

Oberon

Bow shock

Magnetopause

Magnetotail

Equator

Plasma sheet

Ω

M

Neutral torus










































Instrument Heritage

















33 Critical issues















Acknowledgements


References





































































































































































































Miranda

Ariel

Umbriel

Titania

Oberon

Bow shock

Magnetopause

Magnetotail

Equator

Plasma sheet

Ω

M

Neutral torus










































Instrument Heritage

















33 Critical issues















Acknowledgements


References


































































































































































































Instrument Heritage

















33 Critical issues















Acknowledgements


References































































































































































33 Critical issues















Acknowledgements


References



























































































































































Acknowledgements


References



























































































































































The science case for an orbital mission to Uranus: Exploring the origins and evolution of ice giant...

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Transcript of The science case for an orbital mission to Uranus: Exploring the origins and evolution of ice giant...