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Icarus 177 (2005) 491–505www.elsevier.com/locate/icaru

Evaluation of the possible presence of clathrate hydrates in Europa’shell or seafloor

Olga Prieto-Ballesterosa,∗, Jeffrey S. Kargelb, Maite Fernández-Sampedroa, Franck Selsisa,d,Eduardo Sebastián Martíneza, David L. Hogenboomc

a Centro de Astrobiología, INTA-CSIC, Carretera de Ajalvir km. 4, Torrejón de Ardoz, 28850 Madrid, Spainb Astrogeology Team, USGS, Flagstaff, AZ, USA

c Lafayette College, Easton, PA, USAd Centre de Recherche Astronomique de Lyon, Ecole Normale Supérieure (CRAL-ENS), 46 allée d’Italie, F-69364 Lyon cedex 7, France

Received 19 July 2004; revised 21 January 2005

Available online 13 June 2005

Abstract

Several substances besides water ice have been detected on the surface of Europa by spectroscopic sensors, including CO2, SO2, andH2S. These substances might occur as pure crystalline ices, as vitreous mixtures, or as clathrate hydrate phases, depending oconditions and the history of the material. Clathrate hydrates are crystalline compounds in which an expanded water ice lattice fothat contain gas molecules. The molecular gases that may constitute Europan clathrate hydrates may have two possible ultimthey might be primordial condensates from the interstellar medium, solar nebula, or jovian subnebula, or they might be secondargenerated as a consequence of the geological evolution and complex chemical processing of the satellite. Primordial ices and volacompounds would be difficult to preserve in pristine form in Europa without further processing because of its active geological hisdissociated volatiles derived from differentiation of a chondritic rock or cometary precursor may have produced secondary clathmay be present now. We have evaluated the current stability of several types of clathrate hydrates in the crust and the oceanThe depth at which the clathrates of SO2, CO2, H2S, and CH4 are stable have been obtained using both the temperatures observedsurface [Spencer, J.R., Tamppari, L.K., Martin, T.Z., Travis, L.D., 1999. Temperatures on Europa from Galileo photopolarimeter–raNighttime thermal anomalies. Science 284, 1514–1516] and thermal models for the crust. In addition, their densities have beenin order to determine their buoyancy in the ocean, obtaining different results depending upon the salinity of the ocean and type ofFor instance, assuming a eutectic composition of the system MgSO4–H2O for the ocean, CO2, H2S, and CH4 clathrates would float but SO2clathrate would sink to the seafloor; an ocean of much lower salinity would allow all these clathrates to sink, except that CH4 clathrate wouldstill float. Many geological processes may be driven or affected by the formation, presence, and destruction of clathrates in Euas explosive cryomagmatic activity [Stevenson, D.J., 1982. Volcanism and igneous processes in small icy satellites. Nature 298,partial differentiation of the crust driven by its clathration, or the local retention of heat within or beneath clathrate-rich layers bethe low thermal conductivity of clathrate hydrates [Ross, R.G., Kargel, J.S., 1998. Thermal conductivity of Solar System ices, witreference to martian polar caps. In: Schmitt, B., De Berg, C., Festou, M. (Eds.), Solar System Ices. Kluwer Academic, Dordrecht, pOn the surface, destabilization of these minerals and compounds, triggered by fracture decompression or heating could result inof chaotic terrain morphologies, a mechanism that also has been proposed for some martian chaotic terrains [Tanaka, K.L., KMacKinnon, D.J., Hare, T.M., Hoffman, N., 2002. Catastrophic erosion of Hellas basin rim on Mars induced by magmatic intrusvolatile-rich rocks. Geophys. Res. Lett. 29 (8); Kargel, J.S., Prieto-Ballesteros, O., Tanaka K.L., 2003. Is clathrate hydrate disresponsible for chaotic terrains on Earth, Mars, Europa, and Triton? Geophys. Res. 5. Abstract 14252]. Models of the evolutionshell of Europa might take into account the presence of clathrate hydrates because if gases are vented from the silicate interior tocean, they first would dissolve in the ocean and then, if the gas concentrations are sufficient, may crystallize. If any methane releas

* Corresponding author. Fax: +34 915201074.E-mail address: prietobo@inta.es(O. Prieto-Ballesteros).

0019-1035/$ – see front matter 2005 Elsevier Inc. All rights reserved.doi:10.1016/j.icarus.2005.02.021

492 O. Prieto-Ballesteros et al. / Icarus 177 (2005) 491–505

pectives,

Europa by hydrothermal or biological activity, they also might form clathrates. Then, from both geological and astrobiological persfuture missions to Europa should carry instrumentation capable of clathrate hydrate detection. 2005 Elsevier Inc. All rights reserved.

Keywords: Ices; Europa; Geological processes; Satellite surfaces; Mineralogy

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1. Introduction

The mineralogy of Europa’s surface has been partlyvealed by reflectance spectroscopy and multispectral iming using available sources of data from space missionsearth-based remote sensing. Observations to date are uconstrained, thus allowing several types of materials. Gchemical modeling has helped constrain the possibilitiesto decide which of the possible observed materials actuare present. Compositional studies are a keystone for ming the interior structure and geologic history of any planbut are especially important for studies of Europa becathere are not many other types of data available. Thetory and geologic processes of Europa are not only parevealed by its composition, but its composition influenthe geologic processes, because properties of the cruschange depending on the materials involved.

It has been known since beforeVoyager that ice is a con-stituent of Europa but that ice is not the only material preson the surface. The non-ice materials cannot be an incoquential trace component of the crust, because many ovations are difficult to explain just from a pure H2O com-position; for instance, the induced magnetic field of Eurorequires an electrically conductive brine ocean(Khurana etal., 1998; Kivelson et al., 1999, 2000), and the geologicallycorrelated color and albedo and other heterogeneous suproperties require crustal dynamics that are coupled in sway to subsurface impurities(Fanale et al., 2001). Earth-based observations have detected Na, K, and O in a tenEuropan atmosphere(Brown and Hill, 1996; Brown, 2001Johnson, 2000; Johnson et al., 2002); these elements are blieved to be sputtered from surface salts and ices(Madey etal., 2002). Sputtering, however, does not release all elemin proportion to their abundances on the surface, and soare left with the recognition that alkali-rich salts and/or icare present and abundant, but we can say little from tobservations about either the molecular/mineralogical foof the host materials or the presence of other substance

Galileo mission data have greatly improved our knoedge about the mineralogy of the surface of Europa(Mc-Cord et al., 1998a, 1998b, 1999, 2001; Carlson et al., 11999, 2002; Carlson, 2004; Clark, 2004), although largegaps and ambiguities remain that limit our understaing of fundamental processes in Europa’s crust and ocThe exact chemical composition of the non-ice componwith low albedo is still under discussion because thefrared spectra can be interpreted as due to three candid(a) salt hydrates, especially sulfate hydrates(Kargel, 1991;Kargel et al., 2000a, McCord et al., 1998a, 1998b), (b) hy-

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drates of sulfuric acid(Carlson et al., 2002); and hydroniumion (Clark, 2004). Each category of material has a southough incomplete theoretical basis. The presence ofdoes not imply the lack of the other. Of course sulfuric acia sulfate, though McCord and colleagues mentioned spically Mg- and Na-sulfates as providing some of the bspectral matches. In fact, hydrates of Mg–Na-sulfates ansulfuric acid could be part of the same cycle of sulfur in Eropa driven either by surface radiolysis(Carlson et al., 2002or internal gas and brine venting(Kargel et al., 2000a); sta-bilization of hydronium ion(Clark, 2004)can be explainedin a strongly acidic sulfuric acid-bearing system. Wherechemical balance actually would be, and the time requirereach a steady-state balance on Europa are not entirelytain from either observational or theoretical standpoints,the weight of evidence has shifted toward a strongly acsystem.

In addition to hydrated sulfate salts and/or acids, C2,SO2, and H2S have been detected by spectroscopy in sevranges of the spectrum on the surface of Europa, Ganymand Callisto(Carlson et al., 1996; McCord et al., 1998Hibbitts et al., 2000). The spectral signal of CO2 and SO2produce stronger spectral absorption bands on Callistoin Europa; on Callisto these bands have been attributethese substances existing as confined molecules withattached to a host material(McCord et al., 1998b; Hibbittset al., 2000); these volatiles might exist as clathrate hdrates instead of ordinary pure ices or disordered inclusof solitary molecules. These molecules are consistenta radiolytically driven surface chemistry, but they are aconsistent with the volatiles that might be expected from ogassing of a chondritic rocky material or from rock that hco-accreted with comet-like materials. A sulfur- and carbrich crust and ocean could have its chemistry initiateddegassing of primitive nebula-related compositions, but mhave been subsequently altered by various redox proceand other reactions operating in the mantle, seafloor, anshell. Thus, as much as planetologists would like to lespecifics of Europa’s evolution from observations, wenot yet there in our understanding. However, variousof self-consistent theoretical chemical evolution modelssuccessful in reproducing the types of materials obseon Europa(Carlson et al., 1999, 2002; Kargel et al., 200McKinnon and Zolensky, 2003; Carlson, 2004).

In the next sections we will determine the available gumolecules that may form clathrate hydrates in Europa,subsequently the formation scenarios and both the thermand buoyantly stable regimes for these phases will be elished. The possible stability of clathrate hydrates within

Clathrate hydrates in Europa 493

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optical surface layer will be examined to test if they migbe observable by remote sensing. After that, the inhibitioclathrate formation by salts in Europa’s ocean will be testheoretically. Finally some geological implications will bdiscussed.

2. Theoretical constraints: Forms, sources, andinventories of clathrate-forming volatiles

Clathrate hydrates might be abundant in the Solar Stem, as some authors have pointed out since the 19(Miller, 1961, 1985; Lewis, 1971; Stevenson, 1982; Lunand Stevenson, 1985; Kargel and Lunine, 1998; Kargeal., 2000a), but their volatility at low pressure usually restricts their present occurrence to below the surface forjects as warm as Earth, Mars, and Europa or as metassurface condensates in the cold fringes of the Solartem. Clathrate hydrates might be formed initially by volattrapping and adsorption during ice condensation fromlar nebula(Iro et al., 2003; Hersant et al., 2004; Owand Bar-Nun, 1995); amorphous water ice containing iclusions of clathrate-forming molecules subsequently mhave been incorporated into comets and planets and cirplanetary nebulae of the gas giants, where dirty amorphor cryptocrystalline mixtures may have been incorporainto planetesimals and primordial icy satellites; anneain those bodies then may have yielded clathrates. Any sprimordial clathrates could bear a relationship to Europvolatile crust composition if primordial comet-like condesates co-accreted with rock and then outgassed from adominated precursor, or if Europa formed by heterogeneaccretion of comet-like volatiles following formation ofrock core.

We note that primordial, juvenile gases (such as socomponents of noble gases, water, and carbon dioxidestill migrating through and venting from Earth’s manand mantle-derived magmas(Allègre et al., 1987; Caffee eal., 1999; Kunz, 1999; Trieloff et al., 2000), complement-ing other volatiles that are chemically or physically heily processed and recycled in the dynamic Earth sys(Symonds et al., 2003). In the case of Europa, however, jvenile volatiles would be trapped by the ocean or floatice shell. Clathrate hydrates would be an expected produe to the reaction of the gases with an ice-rich crust.ropa’s volatile inventories and clathrate products may rea good memory of nebula and accretion processes, evthere has been considerable disturbance of the originalula condensates. Alternatively, a far more complex anddirect formation of clathrates, with complex gas-phasegas-solid chemical interactions, may relate to Europa’s cplex evolution, with no memory of primordial conditions avolatile compositions, as for instance pertains to terresmethane in the sea floor and permafrost environments.

SO2- and H2SO4-fuming sulfate salt brines occur oEarth in Yellowstone National Park hydrothermal ve

e

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(Kargel et al., 1999), and similar brines may occur on Eropa(Kargel et al., 2000a). On Earth such brines are heavprocessed from precursory materials; these sulfurous bhave little to do with primordial aqueous chemistry. Likwise, if we think of seawater or an evaporative lake bras a possible analog of Europa’s ocean, we can imaginehow far that ocean may have evolved from any aqueoustract from the primordial Earth. Similarly, carbon dioxidand most other volatiles (even water) have had a comhistory in Earth. Unless the terrestrial planets’ and mostteoritic volatiles are analyzed isotopically by stepped heaor other component analysis, it is difficult to discern toriginal nebula processes in the gases. Nevertheless,remains some memory in evolved planetary and meteparent bodies of primordial volatile assemblages, despitebillion years of chemical transformations. It would therefoseem likely that the atmospheric and surface volatile mrials observed on evolved and primitive objects acrossSolar System do have some relevance to what is likely tvented from Europa’s rocky interior into its ocean andshell.

Whether Europa is comparatively heavily proces(McKinnon and Zolensky, 2003)or slightly processed(Kargel, 1991; Kargel et al., 2000a)from its original precur-sory composition is not known. In any case, consideratiocometary and various chondritic volatile assemblages isan implausible starting position, as end members. In ailar spirit, it is not implausible to consider that Earth coube a reasonable chemical analog of Europa’s rocky coit experienced some of the same processes as have affEarth’s volatiles.

Kargel (1991)andKargel et al. (2000a)evaluated CI orCM chondrites as a possible primordial analog of EuroWith nearly 6% by mass sulfur (almost half in sulfate foraccording to sources reviewed byKargel, 1991), and aroundhalf that much carbon (mainly organic and carbonates),possibilities for a sulfur- and carbon-rich ocean are vast.let us consider a wider range of objects in the Solar SysPossible volatile inventories for Europa can be calculafrom the inventories on these other objects on a masscentage basis. This is theoretically simplistic, as it doesconsider how Europa’s specific formation and evolution mdiffer from those other objects, a point successfully mby McKinnon and Zolensky (2003)in critiquing the spe-cific evolutionary scenarios advocated byKargel (1991)andKargel et al. (2000a). However, given the rich variety oplanetary characteristics and histories, in the final assment, such an empirical approach is probably the broaand most reliable means to constrain the likely inventorieclathrate-forming volatiles in Europa, unless Europa sohow falls outside the envelope of what has elapsed elsewacross the Solar System. It is possibly best to temperapproach with an emphasis on insights we may gain fEuropa’s sister satellites, Io, Ganymede, and Callisto, wprobably bear a closer relationship to Europa than otherjects.

494 O. Prieto-Ballesteros et al. / Icarus 177 (2005) 491–505

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Consider first CO2. It has been detected on Venus, EaMars, Ceres, Europa, Ganymede, Callisto, Triton, Pluto,many comets. It is a major condensed ice on TritonPluto, probably forming a cryospheric equivalent of crusbedrock. Although it is not predicted in thermodynammodels of nebula chemistry, CO2 is thought to be a producof pre-solar irradiation of low-temperature interstellar codensates, as well as a product of recent ionizing irradiatioplanetary surface ices. CO2 also is produced in planetary interiors by chemical processing under oxidizing conditionis generally believed that most of Earth’s crustal and macarbon was inherited from carbonates and organic carbochondrite-like precursors or from CO2 and organics and hydrocarbons in comets. The key to understanding the CO2 andcarbon abundance of Europa is in taking the correct chof precursors, and then knowing how much carbon has breduced and sequestered in the core, how much is in theof carbonates and heavy organics and sequestered in sedeposits, and how much remains as CO2. We do not knowthese things.

Europa most likely contains at least as much CO2 (in-cluding carbonate CO2) gram for gram of rock mass, aEarth and Venus, and probably no more than “solar” elemtal abundances could maximally permit. Comets andbonaceous chondrites give numbers that approach thelar” ratios. This simple statement avoids questions of hmuch CO2 has been reduced to graphite or carbon monide, whether the trace of CO2 observed on Europa was implanted from or altered by meteoritic infall or the magntosphere, whether CO2 originated in the interstellar mediumor by planetary processes, whether Earth’s or Europa’s2came from comets or by adsorption and gas trapping incates in the solar nebula. All these are interesting quesand may substantially change any estimate of CO2 inven-tory from a more complete model of Europa’s compositand evolution, but they are not entertained in this estimaof Europa’s likely CO2 inventory. If use of terrestrial, chondritic, and cometary CO2 inventories to bracket Europa’snot somehow misleading, it implies that whatever elapon Europa to affect its CO2 inventory, the cumulative effecof all those processes is intermediate between the effecCO2-relevant processes in comets or chondrites vs. thoEarth from the time of nebula condensation and accretiothe present time. There is a further highly relevant ques(not answered here) of how much of Europa’s CO2 is in theform of free CO2 or clathrate-caged CO2, versus how muchis sequestered in the form of carbonate minerals.

Consider first a Mars/Earth/Venus model for a plauslower limit on Europa’s CO2. Venus’ atmosphere is domnated by 89 bars of free CO2, which constitutes about 0.01%of the planet’s mass, about the same as Earth’s inventoCO2 (which mostly exists as carbonates)(Kargel and Lewis,1993). The same percentage of Europa’s rocky mass wyield about 0.1% CO2 in the ocean if most of the satelliteCO2 has been vented into the ocean—not enough to satthe ocean unless it was almost entirely frozen and the2

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concentrated into the last one or two per cent that remaunfrozen (or was last to freeze). Oxidized forms of su(mainly occurring as sulfate salts) total a factor of seveless than carbonates in Earth’s crust; but on Mars, indtions are that sulfate salts are far more abundant thanare on Earth (probably due to a less intense history ofdrothermal brine-basalt interactions on Mars), but the oof magnitude is apt to be similar to that of CO2.

Europa likely contains more than an Earth-equivalfraction of CO2 and other major clathrate-forming volatileand it may be as much as in carbonaceous chondrites.carbon, including organic carbon, in the most volatileriched, primitive carbonaceous chondrites (about 3% C)oxidized by some event, and all the volatiles of the chond(including water of course) were vented to form a volaEuropan crust, a theoretical maximum CO2:(H2O + CO2)fraction of ∼10% could be attained; this quantity exceeclathrate saturation even for very high pressures at Euroseafloor. In this extreme model, one third of the massEuropa’s volatile shell could be made of CO2 clathrate.This seems an unlikely upper limit, as some fraction ofcarbon—and maybe most of it—would be in other checal forms, such as carbonates sequestered on the seagraphite in the deeper crust or mantle, or iron carbide incore.

Comets offer yet another possible analog, if somehEuropa’s volatile crust unexpectedly formed by a procmore closely resembling idealized heterogeneous accrof first rock and then a volatile coating. Hale–Bopp and otcomets typically contain about 2% CO2 (4% CO2 in the icyfraction; Biver et al., 2002a, 2002b). If Europa’s icy shellresembles the icy fraction of a comet (and if the remader of the comet’s carbon fraction were sequestered inrocky mantle or core), then it could attain clathrate satution if CO2 was partitioned mainly in the liquid water phaand roughly 50% of the ocean mass was frozen and 50%mained a briny, carbonated liquid. However, the whole ocdoes not need to be saturated in order for CO2 clathrate toexist in the icy shell. Imagine a CO2-undersaturated oceafilling an extensional fracture with brine; the brine cools agradually crystallizes, and the last fraction of freezing liqfinally reaches saturation, thus creating a clathrate-beadike.

In sum, we calculate 0.1–10% CO2 in Europa’s volatilecrust; the range is believed to span rough lower and ulimits. The bigger point is that there is potentially a lotCO2 in Europa. Almost certainly there is enough to costitute geologically significant quantities of clathrate andCO2 dissolved in the ocean.

CO2 is not the only clathrate-forming volatile preseCO, also a clathrate forming gas, is typically several timmore abundant than CO2 in comets(Biver et al., 2002b).In addition, CH4, N2, Ar, H2S, and/or SO2 are likely to beimportant clathrate-forming gases. Whether the ocean isurated or not, Europan clathrates are apt to contain mixtof some of these gases.

Clathrate hydrates in Europa 495

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Consider Io as another possible crude analog of Eurorocky interior. Assume that Io’s current loss rate of S andhas continued over the age of the Solar System and resents primarily loss of SO2. Scaling the amount of SO2 lostfrom Io to Europa’s smaller rock mass, and assumingall the SO2 has vented from Europa’s rocky interior into tvolatile crust,Kargel et al. (2000a)calculated that a few percent of Europa’s volatile crustal mass could be made of S2.This translates to something on the order of 10% by mSO2 clathrate, if the outgassed SO2 is mainly in clathrateform (alternatives include free solid or liquid SO2 or chem-ical reaction to sulfuric acid, metal sulfate salts, metal sfides, H2S, and elemental sulfur).

Considering that the mass fraction of clathrates infrozen gas–water mixture is anywhere from 2.6 to 7.4 timthe mass fraction of the gas (depending on the gas),easily seen that clathrates may constitute anywhereroughly one percent of the Europan volatile crust (floing shell+ ocean) in an “Earth” model, to possibly halfmore of the volatile crust in a “comet” or “chondrite” modeThese amounts assume that the volatile crust is mostlylidified; if the ocean is mostly liquid, the clathrate-formingases may occur in excess of clathrate saturation orless than saturation abundances and thus may be mainlsolved in the ocean. In any event, clathrates should besidered as possible major constituents of Europa’s icy sand seafloor deposits.

3. Observational constraints

If we neglect the possibility of heterogeneous accretthe source of Europa’s sulfates (salts or acid) and clathforming gases should be endogenic, arising from the aation and differentiation of the rocky layers of the satel(Crawford and Stevenson, 1988; Stevenson, 1982; HeadPappalardo, 1999; Head et al., 1999; Fagents et al., 2Spaun and Head, 2001). Once the salts and ices are eplaced on the surface by processes such as cryomagmand diapirism, the action of radiation over the icy mateshould drive the alteration of sulfate salts to sulfuric aciddrates(Carlson et al., 1999)and thus generate hydroniuion by acid–water hydrolysis(Clark, 2004). This model ex-plains important aspects of Europa’s reflectance spectThe red and yellow hues of Europa’s non-ice terrains cobe explained by short-chain sulfur polymers predicted bymodel (Carlson et al., 1999)and other sulfurous and chacophile impurities.

Despite multiple lines of evidence and theoretical armentation pointing toward a sulfate chemistry of Europocean and crust, and thus an oxidizing system that walso favor the stability of SO2 and CO2, the matter is not soeasily resolved with any certainty. Recent theoretical meling has raised important questions about the likely primdial brine composition and subsequent chemical proces(McKinnon and Zolensky, 2003). Key issues concern th

-

-

;

.

original oxidation state of Europa’s sulfur and possible sfate reduction in a basaltic suboceanic crust. It may bebillions of years of sulfate reduction would have progrsively driven all the sulfur into the core or isolated itmetal sulfides somewhere in the suboceanic crust or maA somewhat similar uncertainty regards the fate of carbTheoretical resolution of these questions will require knoedge about Europa that we are not close to having; ourreal hope of answering questions and resolving ambiguwill be to sample Europa’s crust and eventually its oceWhat remains fairly certain is that Europa’s icy shell aocean are impure and a variety of sulfates can explain whas been observed and are consistent with simple chemmodels, which may or may not be accurate. An approof multiple working hypotheses, based on multiple startpremises, seems a valid way of establishing a range ofsible chemistries and histories and processes.

Anyway, CO2 has been detected on Jupiter’s satellbased on a subtle feature near 4.25 µm. On Callisto, wthe signal is stronger, the band shape has been refinedporting the confined state of CO2 molecules within or at-tached to a host material(McCord et al., 1998b; Hibbitts eal., 2000). Neither pure CO2 ice nor the clathrate is stable othe surface of Europa or Callisto. However, if formed insubsurface and exposed on the surface CO2 clathrate hydratecan survive for a period of time before degassing and reving to any solid ice phase; when it dissociates, it may fothe tenuous atmosphere as detected in Callisto(Carlson etal., 1999).

There are two possible origins for CO2, SO2 and suchsubstances in Europa: (a) endogenic sources and therdriven degassing of the satellite which implies geolocal differentiation of the interior, or (b) exogenic, as tresult of the irradiation of the surface, including the mteoritic/cometary carbonaceous materials. Recent stubased on Galileo data have determined that the distrtion of CO2 over Europa’s surface is correlated with tH2O2 distribution(Carlson et al., 2002; Carlson, 2004); bothmaterials are concentrated near the equator on the leahemisphere. As a result of this observation,Carlson (2004)proposed that the CO2 is the radiolytic product of exogenoumeteoritic carbonaceous material. If this is the real sourcthe volatiles, then they cannot be incorporated into the cto form clathrates, unless there is an effective processburies radiolytically altered surface deposits. However,sublimation and sputtering of Europa’s crust may erodeicy crust and reveal subsurface materials preferentiallparticular places on the satellite, and so it is difficult to dcern whether the CO2 is produced by radiolytic processinor is exposed by it. If the source of CO2 is internal, it will in-teract with the water in any of its phases and form clathrif the gas abundance reaches the point of clathrate formaThus, the observed signal of CO2 might be either exogenior endogenic or both. Planetary chemistry would suggthat both contribute. If these gases are partly from the irior, therefore, they would be involved in the evolution of t

496 O. Prieto-Ballesteros et al. / Icarus 177 (2005) 491–505

Table 1Properties of water ice Ih and structure-I clathrate hydrate compared. Adapted fromSloan (1998)

Properties H2O ice sI-clathrate

Mechanical P wave velocity (m s−1) 3845d 3650d

S wave velocity (m s−1) 1957d 1890d

Poisson’s ratio 0.325a 0.317a

Shear modulus at 272 K 3.5a 3.2a

Young modulus at 268 K (109 Pa) 9.3a 8.5aDensity (g cm−3) 0.917 See text,Fig. 3

Thermal Heat capacity (J g−1 K−1) 2.1b 2.0b

Enthalpy (J g−1) 334b 437c

Linear thermal expansion (10−6 K−1) 56a 104a

Thermal conductivity at 273 K (W m−1 K−1) 2.23a 0.49± 0.002a

a From(Sloan, 1998).b From(Handa et al., 1984).c From(Handa, 1986).d From(Waite et al., 2000).

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thrate

ocean and the icy crust. Degassing has occurred durinevolution of the terrestrial planets, Io, Triton, comets, asome evolved asteroids. Some of these, including EarthIo, continue to outgas. It would not be surprising if Eurosimilarly outgases into its ocean and icy crust, as thoughan ice- and brine-covered Io-like object. However, due toocean, the fate of gases vented from Europa’s deep intwould be different from the situation on Io and clathralikely would form.

Sulfur-bearing substances such as SO2 and H2S were al-ready reported on the surface of Europa before the Gamission(Lane et al., 1981; Noll et al., 1997; Domingue aLane, 1998; Calvin et al., 1995). Galileo’s ultraviolet sensodetected the 0.28 µm peak related to SO2 (Moore and Hud-son, 2000). Data from NIMS show a feature at 4.05 µm dto SO2 and a feature at 3.88 µm interpreted as H2S. The lackof SO2 on the trailing side of Callisto suggests that the ogin of the molecule in this satellite is endogenic(Hibbitts etal., 2000). These authors also interpret the absorption bshape as due to confined molecules.

By comparison with Io’s surface composition, the prence of SO2 and H2S in Europa is also supported(Salamaet al., 1990; McEwen et al., 1988; Nash and Howell, 198.SO2 would seem to be especially relevant to Europa, if Iplumes and surface deposits may be any guide. Io’s cumtive history of global SO2 loss (assuming current observrates have prevailed over time), scaled to Europa’s romass, would suggest that up to several mass percent oicy shell and ocean may consist of this volatile. Unlikewhere SO2 is prone to loss by sublimation, surface andmospheric sputtering, plume ejection, and magnetosphsweeping, SO2 erupted from the rocky interior of Europwould first be trapped in the ocean and icy shell. Carvolatiles, too (such as CO, CO2, CH4, or CS2, clathrates ofthese gases, and carbonate minerals), may constitute spercent or even much more, according to common Solartem ratios of C/S. These gases and their possible amoare of interest with respect to gas saturation and clathhydrate formation. Chemical modeling, and evidence of p

e

al

s

sible explosive cryovolcanic deposits(Crawford and Stevenson, 1988; Fagents et al., 2000; Kargel, 1993, 1996; Kaet al., 2000a)also suggest the presence of these clathrforming gases.

It is important to note that some physical chemical prerties of the clathrates are substantially different from wice (Table 1), so they could locally determine the final staof the crust and the ocean. Parameters such as thermaductivity, density and the low melting points of these mterials are useful to incorporate in the physical geologmodels of Europa.

While clathrate-forming molecules have been obseron Europa, they are not abundant on the visible surfPlanetary subsurface gas hydrates may be abundant atlow depths but seemingly hidden from the spacecraft ssors. Spectral reflectance of clathrate hydrates is expeto be different from water ice at some level of spectral rolution because the bond lengths are changed from wice; furthermore, the guest molecule will be spectrosccally different than the pure condensed guest phase becit will vibrate or rotate differently, and will interact withH2O molecules instead of other guest molecules. Howeso far, spectroscopy has shown clathrate to look very mlike water ice, so it is effectively masked. Results reporfrom the far infrared range show that ice and sI clathrhydrates have similar spectra due to the same strengthe H bonds for both substances(Bertie et al., 1975; Bertieand Devlin, 1983). The spectral response of clathratesto be slightly different from ice, so we should encourageneeded studies by the planetary science and materialsence communities.

4. Formation of clathrates in Europa

The main conditions required to have clathrate hydrain Europa are to have both the materials that form thcompounds and regimes at pressures and temperatuwhich they are stable. As has already been mentioned, ssubstances that are suitable to be guest species in cla

Clathrate hydrates in Europa 497

suc-

si-

theo

in-thesig-e-opi-justtersu-

, so

on-andratelileornal

,ain-

ved

fer-te

im-llowcon-f thehowbil-

hydrates have been detected on the surface of Europaas CO2 and SO2. Other likely clathrate-forming guest molecules in Europa’s ocean and icy shell may include N2 andperhaps O2 (the latter from radiolytic processes), and posbly CO and CH4.

4.1. P –T stable regimes for clathrates in Europa

The stability of clathrate hydrates in the crust andpossible ocean of Europa(Carr et al., 1998; Pappalardet al., 1999; Spohn and Schubert, 2003)depends on theP–T regimes derived from the thermal gradient in theterior of the satellite, and hence on the thickness ofice crust. However, the geothermal gradient should notnificantly affect clathrate stability in the upper micromters/millimeters (the range of depths that is spectrosccally observable). Geothermal gradient has a big effecton crustal temperature at depths on the scale of kilomeWhat will affect that temperature of stability in the subsperficial environment is the diurnal temperature range

h

.

it is possibly the daytime high temperature that might ctrol the minimum depth where clathrates are stable,the mean annual temperature that may control clathoccurrence at depths greater than several meters. GaPhotopolarimeter–Radiometer data have shown that diutemperatures are in the range of 86 to 132 K(Spencer et al.1999), and a mean surface temperature of 106 K (constring the albedo to 0.62± 0.14).

The controversy of the crust thickness is not yet resolso, in order to trace the stable zones for CO2, CH4, SO2,and H2S clathrates within the crust we have assumed difent thermal gradients, one for a thick crust(Pappalardo eal., 1998; Ruiz and Tejero, 2003)and another for a thin on(Greenberg et al., 1999; O’Brian et al., 2002). Consideringthe resulting stagnant lid from the thick crust models, silar values have been obtained for both models for shadepths if critical parameters such as the heat flux arestrained. We used different values of the temperature osurface in the conductive thermal gradient to determinethis parameter controls the beginning of the clathrate staity zone (CSZ) in Europa.

the st

Fig. 1. Phase diagrams for clathrate hydrates with several guest species: (A) CO2, (B) SO2, (C) CH4, (D) H2S. H= clathrate hydrate, I= water ice, Lw= liquidwater, Lguest= liquid guest specie, Q1 and Q2 are the quadruple points of each system. Data source for (A), (C), and (D) isSloan (1998), and for (B) areRoozeboom (1884), Von Tammann and Krige (1925), Davidson (1973), andVan Berkum and Diepen (1979). Note that only the CO2–H2O phase diagram iscomplete, but all of them indicate the key dissociation curve where the hydrate phase is in equilibrium with the gas phase, as would be relevant toabilitylimit and dissociation of clathrate near Europa’s surface. Dashed lines are from Lunine and Stevenson (1982) theoretical data.

498 O. Prieto-Ballesteros et al. / Icarus 177 (2005) 491–505

e indicated

Table 2Depth at which CSZ begins for several clathrate hydrates assuming that the thermal gradient in the conductive part of the icy shell takes thtemperatures in the first column(Spencer et al., 1999)as the surface temperature

Temperature ofthe surface (K)

Stability depth for clathrate hydrates with various guest molecules (mm)

SO2 CO2 H2S CH4

86 – 0.015 0.09 0.55100 10−6 0.065 0.4 3128 6 200 80 3.5× 103

132 7 220 93 1.4× 104

beenthal or

aken

)

nur-

eenype0 K

ients to

y ofl surcovesta-iednessousrys-uresw-

der-

atureing

howrial

ratermhy-

ireds, i

on-stal.

es

ienher-

oflesatede

ther-

nto

-a,

ll

con-ingules

rate

he

illtioncmorp-p

rs ofd

-

ulddelustof a

s and

Methane has not been observed in Europa, but it hastaken into account in the models under the assumptionthis gas might have originated inside due to hydrothermabiological processes.

Dissociation curves of several clathrates have been tfrom Sloan (1998)for CO2, CH4, and H2S clathrates, andfrom Roozeboom (1884), Von Tammann and Krige (1925,Davidson (1973), andVan Berkum and Diepen (1979), forSO2. Using the theoretical data ofLunine and Stevenso(1985)to complete the experimental data to 80 K, the sface regime of Europa was approximated (Fig. 1).

By definition, the CSZ is located below the cross betwthe thermal gradient and the dissociation curve for any tof clathrate. Dissociation pressures are very low at 10(ranged from 1 Pa for CH4 clathrates to 10−7 Pa for SO2clathrate), so taking the mean diurnal temperature (∼100 K)as the temperature of the surface in the thermal gradmodels, the ice covering buried clathrate hydrates needbe no greater than a few millimeters to enable the stabilitclathrates for temperatures near Europa’s mean annuaface temperature. This estimate assumes that the icethickness provides a lithostatic pressure, and the lithotic pressure is also hydrostatic, thus implying that burgas molecules are confined. The actual ice cover thickrequired may be greater if the surface material is porand permeable, or less if the elastic strength of ice ctals produces a confining pressure. At higher temperatcorresponding to daytime highs at low latitudes and in loalbedo regions, the minimum depths of burial are consiably greater, amounting to a centimeter to a meter (Table 2),depending on the gas species and the exact temperHowever, again depending on how the strength of confincrystals might contribute to the confining pressure, andradiolysis may or may not unseal these crystals, the budepth could be much smaller. Below this depth the clathhydrates are stable all along the icy shell and could fogiven a supply of gases. At shallower depths the clathratedrates would not be retained, but, if produced at the requpressure but then erosionally unburied by some procesmight survive metastably or exist stably in inclusions ctained in ice crystals due to the elastic strength of the cryFluid inclusions in terrestrial silicates and salts sometimremain at pressures in the kilobar range(Roedder, 1984)de-spite being in crystals that may be depressurized to amblaboratory conditions. If the ocean is convective and isotmal, clathrates would be stable down to the seafloor.

t

-r

.

t

t

4.2. Buried but exposed SO2-clathrates

In order to check the hypothesis that the absorptionSO2 in Europa might be from subsurface gas molecutrapped within the ice or encaged in clathrates, we estimthe concentration of SO2 within the ice required to produca detectable absorption feature and we compared it tomaximum concentration of SO2 that can be trapped in suface clathrate hydrate.

First, the penetration depth of the incoming radiation ithe ice can be approximated by the depthd(λ) of an icelayer having an extinction optical depth of 1 (extinction=absorption+scattering). The column density of SO2 trappedalongd(λ) is d(λ) × [SO2], where [SO2] is the concentration of SO2 in the ice. At the surface conditions of Europthe highest possible concentration of SO2 in clathrates is[SO2]max= 3.6× 1021 cm−3, if we assume that the unit ceparameter for the sI structure of the SO2 clathrates at 100 Kis 11.85 Å(Tse, 1987; Tse et al., 1987; Uchida, 1997)andthat while the small cages are empty due to the thermaltraction, the effect of the pressure over the fractional fillof the cages and the relative big size of the gas molec(Davidson, 1973; Davidson et al., 1986), all the larger onesare full (subsequently, the hydration number of this clathwould be 7.7). One can see inFig. 2 that this is enough toproduce an SO2 signature in the reflection spectrum. In tnear-infrared, the penetration depth is only∼30 µm but at[SO2]max, the SO2 contained in this thin upper layer stabsorbs more than 10% of the radiation in its absorpband. In the UV, the penetration depth is much higher (5in snow, 25 cm in coarse-grained ice) and, as the abstion coefficient of SO2 is very high around 280 nm, a deeSO2 feature is produced even at a concentration 4 ordemagnitude lower than [SO2]max. Considering the calculatetransparency of ice at the absorption wavelengths of SO2 andthe stability ranges (Table 2), it is conceivable that the observed signatures belong to the clathrate phases.

4.3. Buoyancy of clathrates in Europa’s crust and ocean

Once a clathrate hydrate is formed in the CSZ, it cobe gravitationally unstable at its original depth. To mowhether the clathrates will tend to sink or float in the crand the ocean, density data are needed. The densityclathrate hydrate depends on both the lattice parameter

Clathrate hydrates in Europa 499

ionor

of 1d ofnm.me

ne

ste

andbe

r oflcu-

ation

ityturethe

uld

be-that

andhe text

e ofuch

)ith

O

per-pan-

sure

cyer atureswer

sinkn inualtec-

Fig. 2. Absorption by SO2 inside the ice. The curves give the transmissof a column densityρ of SO2 along the typical penetration depth of ice (snow).ρ = [SO2] × d(λ), where [SO2] is the concentration of SO2 mole-cules and d is the depth of ice (or snow) having an extinction opacityat the wavelengthλ. The dashed curve is given for the near infrared banSO2 around 4 µm while the solid lines are for the UV band around 280In the plotted UV range,d(λ) is of the order of 5 cm in the snow and 25 cin the ice. In the NIR range,d(λ) is only 30 µm in the ice. Roughly, threflection spectrum of ice containing SO2 (at a given concentration [SO2])is the reflection spectrum of SO2-free ice multiplied by the transmissiogiven here for the corresponding [SO2]. SO2 absorption coefficients arfrom Rufus et al. (2003)for the UV and Hitran database(Rothman et al.,2001)for the NIR. Ice and snow extinction coefficients are fromPerovichand Govoni (1991)andPerovich (1993)for the UV and fromWarren (1984)for the NIR.

the composition, and may be calculated as follows:

(1)ρ =Nw

NAva[MWH2O + ∑C

j=1∑N

i=1 yijυiMWj ]Vcell

,

whereNw is number of water molecules per unit cell,NAvais the Avogadro’s number (6.023× 1023 molecules/mol),MWj is molecular weight of componentj , yij is the frac-tional occupation of cavityi by componentj , υi is numberof typei cavities per water molecule in unit cell,Vcell is vol-ume of the unit cell,N is the number of each cavity typein unit cell, andC the number of components in the hydraphase(Sloan, 1998).

All the studied clathrates (CO2, SO2, H2S, and CH4)have sI crystallographic structure, with 6 larger cavities2 smaller per unit cell. Some of these cavities couldempty, reducing the density. The final hydration numbethe clathrate depends on the filled cells and can be calated for each temperature or pressure over the dissocicurve by the CSMHYD program(Sloan, 1998). The occu-pancy of the crystal lattice is critical for the final densvalue, and it is also dependent on pressure and temperaA clathrate with sI structure and no guest molecules incages has a density of 0.8 g cm−1 (Uchida, 1997)(a conceptonly, because with no guest molecules this structure wocollapse).

Composition of the guest species affects the densitycause of the size and the molecular weight. Moleculesfit better produce larger fractional filling of the cages.

.

Fig. 3. Density of several clathrates, which guest is indicated, at 273 Kdifferent pressures that may be reached in the Europan ocean. See tfor details.

Temperature is also an important parameter becausthe strong thermal expansion of these compounds, mgreater than for regular water ice.Tse and White (1988found that the usual 12 Å lattice of sI structure varies wtemperature as follows:

a(T )/Å = 11.835+ 2.2173× 10−5 · T(2)+ 2.2415× 10−6 · T 2.

Udachin et al. (2001)have measured the change for the C2clathrate cell specifically, obtaining

(3)

a(T )/Å = 11.81945− (9.08711× 10−5 · T )

+ (4.59676× 10−6 · T 2)

− (8.35548× 10−9 · T 3).

Cell lattice parameters for the CO2, H2S, and CH4clathrates at 273 K have been calculated, using this temature as an approximation to the temperature of the Euroocean. Specific values for the SO2 clathrate lattice parameter and the hydration number at 273 K are given byByk andFomina (1968)andCady (1983), respectively.

On the other hand, experimental studies of the presdependence of the gas occupancy of sI cages(Cady, 1983;Suzuki et al., 2001)result in an increase of the occupanat higher pressures. Therefore, density would be greathigher pressure while clathrates formed at lower pressin the upper crust would have more empty cavities and lodensity.

In any case, the clathrate hydrate could either float orin the ocean depending on its composition, as is showFig. 3. Assuming a low-salinity water ocean of density eqto 1.0 g cm−3, CO2, SO2, and H2S clathrates would sink buCH4 clathrates would float. But if the ocean were an euttic brine of MgSO4–H2O system (density of 1.19 g cm−3)

500 O. Prieto-Ballesteros et al. / Icarus 177 (2005) 491–505

e,

eenand6;

rted

d

atenss:as-ns

-aveallyany

de-on-ore

72;

the-midtn,rate

asolu-fe

ethe

min a

tingts

boum-

sso-

hellty of

al-eeplye as-

thelongtheheyencur

di-g

lay-ringinto

ttom) of

re-eselu-

re-ial

thege-lowindi-gel,yigh

ty ofyingsm

atesra-

aterratene

rned,the

intolargeaterof

or NaCl–H2O (ρ = 1.16 g cm−3) as the other extreme casthen CO2, H2S, and CH4 clathrates would float, but not SO2clathrate.

4.4. Clathrate hydrate formation from Brines

The composition of water reservoirs in Europa has bproposed to be salty from magnetic data analysisgeochemistry modeling(Kargel and Consolmagno, 199Khurana et al., 1998; Kivelson et al., 1999). Sulfate-enrichedbrines for Europa’s water reservoirs have been suppoby some studies as the more probable composition(Kargel,1991; Kargel et al., 2000a), although this point is not assure(McKinnon and Zolensky, 2003).

Electrolytes in solution are usually taken as clathrhydrate formation inhibitors under Earth conditions. Ioin solution affect the formation of clathrates in two way(a) by decreasing the activity of water, and (b) by decreing the solubility of the gas molecules in saline solutio(for 6H2O·H2O, Ksp = aCO2 · a6

H2O). The effects of somechlorides such as NaCl or CaCl2 on clathrate hydrate formation are already quantitatively known. These systems hbeen extensively studied experimentally and theoreticbecause they are common in Earth seawater and in mfluid inclusions of terrestrial rocks. These chloride saltscrease the dissociation temperature by roughly 5 K for ccentrations around 10% (weight) of solution, and by mthan 10 K near the eutectic solution composition(Dholabhaiet al., 1991; De Roo et al., 1983; Vlahakis et al., 19Bond and Russell, 1949; Sloan, 1998).

We calculate the effect of magnesium sulfate onformation of SO2, CO2, H2S, and CH4 clathrates at constant pressure using a modification of the Hammerschequation [Eq.(4)] (Dickens and Quinby-Hunt, 1997; Sloa1998)as an example of how these salts may affect clathstability in Europa

(4)

(1

T 0d

− 1

Td

)= n · �HFUS(I)

�HDIS

(1

T 0f

− 1

Tf

),

where T 0d and Td are the temperatures at which the g

clathrate hydrate dissociates in pure water and in the stion, respectively,�HDIS is the enthalpy of dissociation oclathrate hydrate,n is the number of water molecules in thhydrate formula,�HFUS(I) is the enthalpy of fusion for purice andT 0

f andTf are the melting temperatures of ice andelectrolyte solution.

The result (Fig. 4) indicates that dissolved magnesiusulfate decreases the crystallization point of the clathratesimilar manner to the way the salt itself reduces the melpoint of water ice. The inhibition of formation only amounto about 2 K at the eutectic proportions of MgSO4 (17%).Since the salt depresses the freezing point of ice by a4 K, the eutectic salt system exhibits a slightly greater teperature difference between ice melting and clathrate diciation temperatures.

t

5. Discussion: Geological implications

Considering the stability ranges throughout the icy sand the ocean of Europa, clathrates may occur in a varieenvironments and structures:

(1) Clathrates may form within hydrous places of the shlow suboceanic crust as the mantle degasses and dburied salts undergo metamorphic degassing and thcending gases react with water.

(2) In the ocean, clathrate hydrates could sequestergases vented from the interior. Dense clathrates, awith salts, may form local mounds or chimneys onseafloor where local venting of gases occurs, or tmay precipitate from within the water column and thsettle uniformly onto the bottom of the ocean as ocin the terrestrial oceans(Roberts, 2001).

(3) Floating clathrate hydrates should rise onto or formrectly on the bottom of the solid floating shell. A coolinocean and thickening ice shell may thus incorporateers of water ice and clathrate hydrates, with the layedependent on the history of cooling and gas ventingthe ocean.

(4) Clathrates may also be concentrated at the top or bo(depending on clathrate type and brine compositionbrine magma chambers within the floating shell.

(5) Clathrates also may form intracrustal veins due toaction of ascending gases with ice in fractures; thclathrate veins may be associated with diapirs or ptons.

(6) Trace amounts of clathrates may form within broadgional layers of buried, radiolytically altered surficmaterial.

If hydrates are present anywhere in large amounts inicy crust, they should produce significant effects on theology of Europa. Both clathrate and salt hydrates havethermal conductivities, as some experimental analysescate(Prieto-Ballesteros and Kargel, 2005; Ross and Kar1998; Tse and White, 1988). If they are present in the iccrust, they would be expected to produce zones of hthermal gradient and perhaps enhance geological activiseveral types, such as warming and softening of underlice, with possible inducement of partial melting or diapiri(Prieto-Ballesteros and Kargel, 2005).

As has been theoretically predicted, the studied clathrmay crystallize from salty water reservoirs at lower tempetures than from pure water, just as the crystallization of wice is affected. The effects of the salt on ice and clathstability are not great, just a few degrees Kelvin. On ohand, this means that insofar as clathrates are concethe sulfates have a smaller effect than chloride salts. Onother hand, the introduction of clathrate-forming gasesa salt-saturated or undersaturated ocean may have aeffect on the salts. Formation of clathrates removes wfrom the solution, so there will be a higher concentration

Clathrate hydrates in Europa 501

s

Fig. 4. Effect of the MgSO4 addition to the dissociation clathrate temperature for (A) CO2, (B) SO2, (C) CH4, (D) H2S. For all the studied clathratetemperature decrease about 2 K for eutectic concentration of the brine. See text for details.

hacesEu-ticate

byby

enda-uid

tingonayce acomusta

mi-ningt the

nge

ason,ayrinesnism

pro-on ofsolidFrac-. It

frac-soci-ettedter-ess,

ions as soon as the clathrates are formed. This propertybeen used to desalinate terrestrial seawater. If this prooccurs in an aqueous magmatic chamber in the crust ofropa, clathration could result in a new type of magmadifferentiation. The formation of clathrates would separthe crystals from the more concentrated brine magmadensity. If the destruction of the clathrate layer occurredany movement or fracturation, clean water ice could ascthrough the brine to higher levels. Such migration of wter would probably also be attended by release of a flphase. If the released water is in solid form, gas venwould occur; if clathrate destruction occurs with formatiof a liquid water phase, cryoclastic brine volcanism moccur instead. Segregation of phases could help produheterogeneous, structured crust; if the processes go topletion, phase segregation may produce large-scale crlayering.

If the cage occupancy ratio of clathrates can vary dynacally with pressure, then ascending crustal diapirs contaisome clathrate might release gases. If clathrates form a

ss

-l

base of the floating shell but then sink, they may scavedissolved gases on the way down.

It is possible that intense shear deformation, suchmight occur along a fault zone, in an ascending icy plutor in the wall rock through which the pluton ascends, mdestabilize clathrate structures and release gases. If bare present, escaping gases may drive cryoclastic volcain association with faults or diapirs.

Destruction of clathrate layers near the surface couldduce catastrophic processes because of the fast liberatigases, and the large negative volume change of thephases upon dissociation and loss of the free gas phase.turing and gravitational collapse of terrain could ensuecould be an autocatalytic, even catastrophic process, ifturing and depressurization caused more clathrate to disate and especially if gas-saturated brine from the ocean jthrough fractures and erosionally widened them. Chaoticrain on Europa could conceivably be related to this procas it may also be on Mars, Earth, and Triton(Kargel et al.,2003).

502 O. Prieto-Ballesteros et al. / Icarus 177 (2005) 491–505

dis-

ar-

nds

ithol-

1;

andateonss an;n-

and

t th

ar-gic

haveation

u et

rge-s, wona-

withces

as-d onible

ceanthis

ice-henand

ctedtheilar

onsave

00;

ratee isedse).

f

lsoringEu-es ablageing

ur in

. En-anicex-float-ould

erEu-the

irsratesdis-ternsity

It has been learned from Earth that when clathratessociateen masse into gas± solid ice or water vapor± liquidwater, large-scale blocky terrains, perhaps similar to mtian and Europan chaos, are formed(Dillon et al., 2001).Kilometer-size domes or broad-based permafrost mouin Alaska might involve clathration of ice(Kargel and Lu-nine, 1998). Other major landforms believed produced winvolvement of clathrates include giant landslides, mud vcanoes, mud diapirs, and crater or pock-mark fields(Dickenset al., 1997; Dillon et al., 2001; Paull and Dillon, 200Bouriak et al., 2000; Maslin et al., 2004). The potentialfor rapid and even catastrophic clathrate destabilizationgas venting on Earth is indicated by the abrupt climchanges, carbon isotopic shifts, and biological extinctiassociated with some of these major methane releasecrustal disturbances in Earth history(Bains et al., 1999Dickens et al., 1997; Dillon et al., 2001; Dickens, 2003; Kenett et al., 2000, 2002; Thomas et al., 2002; SchmidtShindell, 2003; Zachos et al., 2003). Europa would not havea climate analog to these processes on Earth or Mars, bugeologic manifestations could be partly similar.

A widespread stability of clathrate hydrates in the mtian crust has been modeled and a link to critical geoloprocesses has been suggested(Kargel et al., 2000a). For in-stance, martian chaotic terrains and outflow channelsbeen interpreted as possibly due to the abrupt destabilizof clathrate layers underground(Milton, 1974; Dobrovolskiand Ingersoll, 1975; Kargel et al., 2000a, 2000b; Komatsal., 2000; Hoffman, 2000; Tanaka et al., 2001, 2002). Seeingthat Earth and probably Mars have histories of sudden lascale destabilization of clathrates and methane releasemay expect that similar events take place on Europa. Cmara Chaos on Europa(Spaun et al., 1998)could be anexample of the massive clathrate destruction associatedthe depressurization via fractures initiated by disturbanrelated to diapirism or melting.

While the sudden disruption of clathrate layers andsociated crustal dynamics have not yet been considereany planet in much detail, we have evaluated the possexistence of these compounds in Europa’s crust and oand analyzed some possible geological implications forsatellite.

If crustal breakup is so considerable that stopping orberg calving of crust into the underlying ocean occurs, tclathrates that had been stabilized at low temperatureslow pressures in the upper crust might suddenly be subjeto higher temperatures in the ocean, thus furtheringdestabilization of clathrates and release of gases. A simprocess acting on perhaps a smaller scale could be respble for explosive cryovolcanic events, as some authors halready pointed out(Stevenson, 1982; Fagents et al., 20Kargel et al., 2003).

The transition from a condensed phase (such as clathto a multiphase system containing a free vapor phasalways endothermic if chemical reactivity is not involv(as with clathrate dissociation forming a free vapor pha

d

e

e

i-

)

Fig. 5. Conamara chaos region (10◦ N, 271◦ W) as a possible example ocatastrophic destruction of endogenous clathrates in Europa.

Therefore, cooling and crystallization of ice (possibly asalts if the system is already salt-saturated) will occur dudissociation of ice-equilibrated clathrate hydrates (asropa’s are expected to be). However, the system includnegative volume change of the condensed phase assemif the free vapor phase is vented; therefore, a continufracturing process and collapse of the system can occa runaway process of chaotic terrain formation (Fig. 5), asfor instance could have happened in Conamara Chaosergy for the process can be supplied by effervescing ocebrine gushing through and widening the fractures. In antreme case of a supersaturated ocean and clathrate-riching shell, catastrophic degassing and crustal disruption cpropagate to a global scale.

6. Conclusions

SO2, CO2, H2S, and CH4, clathrates are stable undconditions that exist in most of the crust and the ocean ofropa, but their occurrence depends on the free gas frominterior of the satellite. Clathrates might be chief reservoof these gases in Europa’s subsurface. Clathrate hydmay be heterogeneously distributed and structured increte bodies due to densities which differ from pure waphases. If the clathrate hydrates push the global crust de

Clathrate hydrates in Europa 503

d oue

intes

fi-ateslay

trastsureith

ht beruste-t thcan

gh,mis-

thein-

icyb it,

er.gía,

formantle.

rm-

pec-s of

ma-ture

g oflan-

F.,om-gths.

rma-79,

andøring

at-

190–

469–

,

.C.,ntle:5–

MSto 5Geo-

ide,ell:

ectraleo’s

Eu-

uian,lfur

n Eu-

ro-t and

d the

ter:

and. 90,

Oc-ns of

ssure.

qui-xed

299,

ter.

theation

n,h gastabil-uralsical

C,

e as

ria-

L.M.,tion

to higher values, as some authors have already pointe(Crawford and Stevenson, 1988), water-ice diapirs or sombrine magmas could rise through it.

Cryostatic pressure from the icy shell would resultthe complete filling of the sI structural cavities of clathraformed, and thus in higher densities. Depending on thenal concentration of the salts in the ocean, clathrate hydrshould sink or float in the ocean forming layers that can pin some geological processes liberating gases. In conin the upper levels of the icy shell, a decrease in prescauses a fraction of the cavities to be empty, starting wthe smaller ones. Consequently, clathrate hydrates migbuoyant in these levels of the crust, especially if the cis salty. If they have SO2 guests they might be seen by rmote sensing spectrometry because they are stable auppermost part of the crust, where different wavelengthspenetrate the ice.

If any of the interior clathrate layers are large enouthey could be detected by sensing detectors in futuresions. Seismic geophones(Lee et al., 2003), for instance,have been proposed for inclusion in a future mission toJupiter satellites. Seismic reflections could then show theternal structure of Europa, helping to determine if theshell is homogeneous or if any heterogeneities perturthus enriching its geological composition.

Acknowledgments

We thank to Dr. Giles Marion and anonymous reviewThe work has been supported by Centro de AstrobioloSpain.

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