Icarus 205 (2010) 540–558

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Distribution and interplay of geologic processes on Titan from Cassini radar data

R.M.C. Lopes a,*, E.R. Stofan b, R. Peckyno c, J. Radebaugh d, K.L. Mitchell a, G. Mitri a, C.A. Wood e, R.L. Kirk f,S.D. Wall a, J.I. Lunine g, A. Hayes h, R. Lorenz i, T. Farr a, L. Wye j, J. Craig a, R.J. Ollerenshaw a, M. Janssen a,A. LeGall a, F. Paganelli b, R. West a, B. Stiles a, P. Callahan a, Y. Anderson a, P. Valora d, L. Soderblom f,the Cassini RADAR Teama Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, United Statesb Proxemy Research, Bowie, MD 20715, United Statesc Oregon State University, Dept of Geosciences, Corvallis, OR, United Statesd Department of Geological Sciences, Brigham Young University, Provo, UT 84602, United Statese Wheeling Jesuit University, Wheeling, WV 26003, United Statesf US Geological Survey, Branch of Astrogeology, Flagstaff, AZ 86001, United Statesg Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, United Statesh California Institute of Technology, Pasadena, CA, United Statesi Johns Hopkins Applied Physics Laboratory, Laurel, MD 20723, United Statesj Department of Geophysics and Electrical Engineering, Stanford University, Stanford, CA 94305, United States

a r t i c l e i n f o

Article history:Received 19 March 2009Revised 30 July 2009Accepted 17 August 2009Available online 23 August 2009

Keywords:TitanGeological processesRadar observations

0019-1035/$ - see front matter � 2009 Elsevier Inc. Adoi:10.1016/j.icarus.2009.08.010

* Corresponding author.E-mail address: Rosaly.M.Lopes@jpl.nasa.gov (R.M

a b s t r a c t

The Cassini Titan Radar Mapper is providing an unprecedented view of Titan’s surface geology. Here weuse Synthetic Aperture Radar (SAR) image swaths (Ta–T30) obtained from October 2004 to December2007 to infer the geologic processes that have shaped Titan’s surface. These SAR swaths cover about20% of the surface, at a spatial resolution ranging from �350 m to �2 km. The SAR data are distributedover a wide latitudinal and longitudinal range, enabling some conclusions to be drawn about the globaldistribution of processes. They reveal a geologically complex surface that has been modified by all themajor geologic processes seen on Earth – volcanism, tectonism, impact cratering, and erosion and depo-sition by fluvial and aeolian activity. In this paper, we map geomorphological units from SAR data andanalyze their areal distribution and relative ages of modification in order to infer the geologic evolutionof Titan’s surface. We find that dunes and hummocky and mountainous terrains are more widespreadthan lakes, putative cryovolcanic features, mottled plains, and craters and crateriform structures thatmay be due to impact. Undifferentiated plains are the largest areal unit; their origin is uncertain. In termsof latitudinal distribution, dunes and hummocky and mountainous terrains are located mostly at low lat-itudes (less than 30�), with no dunes being present above 60�. Channels formed by fluvial activity arepresent at all latitudes, but lakes are at high latitudes only. Crateriform structures that may have beenformed by impact appear to be uniformly distributed with latitude, but the well-preserved impact cratersare all located at low latitudes, possibly indicating that more resurfacing has occurred at higher latitudes.Cryovolcanic features are not ubiquitous, and are mostly located between 30� and 60� north. We examinetemporal relationships between units wherever possible, and conclude that aeolian and fluvial/pluvial/lacustrine processes are the most recent, while tectonic processes that led to the formation of mountainsand Xanadu are likely the most ancient.

� 2009 Elsevier Inc. All rights reserved.

1. Introduction

Results from the Cassini–Huygens mission obtained so far haverevealed a wide range of geological and climatological processes onTitan and a complex interaction between the surface and atmo-sphere. The Cassini Titan Radar Mapper (henceforth RADAR) is pro-

ll rights reserved.

.C. Lopes).

viding an unprecedented view of Titan’s surface geology. RADAR isable to penetrate haze and clouds and to produce high-resolution(175 m pixels, equivalent to �350 m resolution at best) SyntheticAperture Radar (SAR) images. Analysis of the RADAR data has al-ready significantly advanced our knowledge of Titan’s surface(e.g., Elachi et al., 2005a, 2006; Stofan et al., 2006, 2007; Lorenzet al., 2006, 2007, 2008a; Lopes et al., 2007a; Lunine et al., 2008;Paganelli et al., 2007; Radebaugh et al., 2007, 2008; Soderblomet al., 2007; Wall et al., 2009).

Fig. 1. The Cassini Titan Radar Mapper obtained Synthetic Aperture Radar (SAR) images over Titan. Swaths from Ta flyby through T30, which are used in this paper, are shownhere over a background of near-infrared images obtained by Cassini’s Imaging Subsystem (ISS, Porco et al., 2005). The bright area (in both imaging and SAR) centered at�100W is Xanadu. All figures are in cylindrical projection with north at the top unless noted.

Table 1SAR image swaths characteristics.

Flyby Date (d/m/year) Lat, long range Incidence angle range

Ta 26-10-2004 32N–53N, 130W–10W 2–46T3 15-02-2005 3S–22.5N, 0.5W–133W 1–30T7 7-9-2005 31S–70S, 23W–350W 7–35T8 27-10-2005 7S–11S, 186W–314W 1–31T13 01-05-2006 10S–12S, 65W–170W 10–29T16 07-22-2006 15N–18N, 138W–358W 9–40T17 09-07-2006 5N–14N, 33W–68W 29–45T18 09-23-2006 15N–65N,105W–346W 19–45T19 10-09-2006 2S–42N, 160W–318W 9–39T21 12-12-2006 25S–50N, 195W–290W 9–57T23 01-13-2007 35S–53N, 95W, 335W 9–39T25 02-22-2007 33S–62N, 215W–55W 8–39T28 04-10-2007 18S–62N, 5W–37W 6–36T29 04-26-2007 2S–45N, 35W–210W 8–39T30 05-12-2007 35N–68N, 225W–325W 9–48

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Planetary surfaces are shaped by the interplay of endogenic(volcanism, tectonism) and exogenic (impact cratering, erosionand surficial) processes. Understanding the distribution and inter-play of endogenic and exogenic processes on a planet is importantfor constraining models of the interior, surface–atmosphere inter-actions and climate evolution. Titan’s atmosphere is the seconddensest in the Solar System and present day surface–atmosphereinteractions make aeolian, fluvial, pluvial, and lacustrine processesimportant on a scale previously seen only on Earth.

The SAR coverage obtained to date, although only of a relativelysmall portion of the surface, probably reveals the main geologicprocesses that have shaped Titan’s surface. In this paper, we inter-pret the SAR images from October 2004 to May 2007, covering20.1% of Titan’s surface, in terms of characteristic morphology ofgeological features and their radar backscatter in order to deter-mine possible emplacement sequences and the overall distributionof geologic processes on Titan. As with any geological mapping, theultimate aim is to be able to interpret how geologic processes haveaffected a planetary surface in space and time and to provide con-straints on surficial and interior structures and processes.

2. Cassini Titan Radar Mapper observations and fly-by geometry

Cassini carries a multimode Ku-band (13.78 GHz, k = 2.17 cm)radar instrument (Elachi et al., 2005b) designed to map the surfaceof Titan and to observe other targets in the Saturn system in fouroperating modes – SAR, altimetry, scatterometry, and radiometry.An overview of initial Titan results using all four modes is givenby Elachi et al. (2005a, 2006). At Titan, the SAR mode is used at alti-tudes under �4000 km, resulting in resolution cell size rangingfrom �350 m to > 1 km. Images are acquired either left or rightof nadir using 2–7 looks. At each Titan encounter used by the RA-DAR, a swath 120–450 km wide and ranging from 1000 to 5000 kmin length is created from five antenna beams, with coverage largelydetermined by spacecraft range and orbital geometry. Radar back-scatter variations in SAR images can be interpreted in terms ofvariations of surface slope at the pixel scale, near-surface rough-ness at the wavelength scale, and near-surface dielectric proper-

ties. On Titan, the candidate surface materials (water–ice, water–ammonia ice and other ice mixtures, hydrocarbons, tholins, e.g.Soderblom et al., 2007) are different from the rocky surfaces moreusually imaged with radars; in particular, volume scattering at Ti-tan may be significant (Elachi et al., 2005a, 2006; Stofan et al.,2006; Janssen et al., 2009; Radebaugh et al., submitted forpublication). The SAR swaths have revealed a wide variety of geo-logic features, including cryovolcanic flows and constructs (Lopeset al., 2007a,b), channels (Lorenz et al., 2008a), mountains and tec-tonic features (Radebaugh et al., 2007), vast fields of dunes (Lorenzet al., 2006; Radebaugh et al., 2008), and radar-dark lakes (Stofanet al., 2007; Mitri et al., 2007; Lopes et al., 2007b; Mitchell et al.,submitted for publication; Brown et al., 2008; Turtle et al., 2009).

The SAR swaths of Titan, as of May 2007, comprise a rich datasetthat covers 20.1% of Titan’s surface (excluding overlap), well dis-tributed in both latitude and longitude (Fig. 1, Table 1), althoughwith more coverage on the leading (0–180�W) and northern hemi-spheres. Some additional data taken from higher altitudes (referredto as HiSAR, West et al., 2006) and covering an additional 2% at

542 R.M.C. Lopes et al. / Icarus 205 (2010) 540–558

significantly coarser resolution are available but were not used inthe mapping.

2.1. Description of SAR data swaths

Table 1 gives basic information about the SAR swaths. It is use-ful to briefly describe the overall geologic character of each swathbefore discussing the relationships between individual units. For adetailed description of Ta and T3 see Stofan et al. (2006). For T7 andT8, see Lunine et al. (2008).

Ta crosses the mid-northern latitudes in an approximately E–Wdirection. Ganesa Macula, radar-bright lobate features, and two cir-cular features with associated lobate deposits may be due to cryo-volcanism. All of these features appear to be superposed on thesurrounding plains units. Channels interpreted to be fluvial in ori-gin also are located throughout the swath, along with small, iso-lated hills.

T3 crosses the low northern latitudes in an approximately E–Wdirection. Fields of dunes are located throughout the swath, andthere are two impact craters: Menrva (�450 km diameter, 19.6N,87.1W) and Sinlap (�70 km diameter, 11.3N, 16.1W). The dunefields in T3 are superposed on both intermediate and low backscat-ter plains. Isolated hills are also seen in the swath between 16–21Nand 39–54W and between 1–11N and 10–23W. Small circular fea-tures of unknown origin are located near 15N, 33W. Channels offluvial origin are found throughout the swath. A distinct featureis Elivagar Flumina, interpreted as a large fluvial deposit. A fewsmall features interpreted as cryovolcanic flows are also seen.

T7 is a partial swath (Lunine et al., 2008). Channels are ubiqui-tous throughout the swath. There are several deeply incised chan-nels and also some dendritic channels of somewhat differentappearance (Lorenz et al., 2008a). Channels are particularly con-centrated in the intermediate to bright unit in the swath centerthat terminates abruptly into a relatively radar-dark unit. A circu-lar feature in the eastern portion of the swath appears to be aneroded impact crater.

T8 is mostly equatorial. Closely-spaced dunes cover much of theswath. Linear mountain ranges extend from 13–5S to 198–225W. Afew channels interpreted to be fluvial in origin are seen, as well assmall circular features of unknown origin. This swath also includesthe Huygens landing site (Lunine et al., 2008).

T13 covers the central portion of Xanadu (from 10S, 65W to 12S,170W) and is also essentially equatorial. The Xanadu radar-brightterrain has a ‘crinkled’, hummocky texture and a rugged surface,and is characterized by irregular but not regionally raised topogra-phy. Xanadu has multiple channels cutting its surface; at the mar-gins these channels appear to drain towards the dark plainssurrounding Xanadu, particularly to the south. Some circular fea-tures within Xanadu are interpreted to be eroded impact craters(Wood et al., in press). Overall, the morphology of Xanadu indicatesthat the terrain has been highly eroded, suggesting that at onepoint it was topographically higher but has been worn down byfluvial and other processes (Radebaugh et al., submitted for publi-cation). The plains surrounding Xanadu contain abundant dunesand Guabonito, an 80 km across circular feature with a broad, rug-ged rim, interpreted to be possibly an eroded impact basin thougha cryovolcanic origin cannot be ruled out.

T16 includes the north polar region and contains scattered lakesnorth of about 70N (Stofan et al., 2006; Hayes et al., 2008a; Mitch-ell et al., submitted for publication). Some lakes are only partiallyfilled with dark material interpreted to be liquids (Stofan et al.,2006; Lopes et al., 2007b; Mitchell et al., submitted for publica-tion), while other basin-shaped features with no dark fill are inter-preted as empty lakes. Channels occur throughout the swath anddrain into some of the lakes. Small hills and patches of rough, ra-

dar-bright terrain are found throughout the swath. Dunes are onlyseen at the southern end.

T17 is a partial swath in which the main feature is the impactcrater Ksa, about 29 km in diameter with a well-defined ejectablanket and a central peak. Dunes from the sand sea Fensal areseen throughout this swath.

T18 extends over the north polar lakes region. Mottled terrain isimaged south of the lakes, with bright patches, some with a tex-tured surface, occurring throughout the swath. No dunes are foundin T18, and fewer channels are imaged than in T16 or T19.

T19 also extends over the northern polar lakes region. Bright,featureless patches occur throughout the swath, as well as dunesin the southernmost extent of the swath. Channels are seenthroughout the region, particularly in association with the lakes.Some circular features of unknown origin are seen (Wood et al.,in press).

T21 contains large areas covered by dark, relatively featurelessplains, scattered bright patches, and dunes and streaks. Sets of lin-ear mountains with interspersed dunes cover about half the swath.A number of circular features are defined by a faint bright ring, andare of unknown origin (Wood et al., in press).

T23 overlaps some of the Ta swath. It contains a partial circularfeature with morphology similar to that of Guabonito, as well asabundant dunes, channels, and hills, and the northern part of abright flow interpreted as cryovolcanic. Dunes are superposed onterrains that have low to moderate backscatter.

T25 covers some of the mid-northern latitudes and contains lin-ear hills surrounded by fields of dunes. At 15S, 40W there is a can-didate cryovolcanic feature, surrounded by lobate deposits. Somecircular features may be of impact origin (Wood et al., in press).In the north, the swath extends into the lakes region, covering Li-geia Mare, one of the larger bodies of liquid.

T28 passed over the northern lakes region and some of the lar-ger seas. At its southernmost end are several circular features ofpossible cryovolcanic origin, and textured bright terrain, sur-rounded by dune fields. Moving north, more fields of dunes andstreaks superpose both intermediate and dark backscatter plains,with scattered small hills. Circular to oval bright rim features areinterspersed among the dark plains, and may be either emptylakes, impact craters, or cryovolcanic features. Towards the seas,the terrain becomes more rugged and cut by channels.

T29 is similar in coverage and terrain to T25 and T28 and showsseveral dark lakes, multiple circular features of unknown origin,relatively dark plains, and dune fields at low latitudes.

T30 covers the northern seas which are surrounded by texturedradar-bright terrain. Channels are abundant. To the south of theseas, some radar-bright streaks can be seen that have also beennoted in ISS images (Perry et al., 2007).

3. Identification and description of terrain units

All the major planetary geologic processes – volcanism, tecto-nism, impact cratering and erosion – have played a role in shapingTitan’s complex surface. Their global distribution can constraingeologic and climatological models. The SAR swaths are well dis-tributed in latitude and longitude, sufficient for patterns in the dis-tribution of the various processes to be discerned. Here we presentmaps of their overall distribution and discuss their possible origins.

Geological mapping using radar data presents numerous chal-lenges, as discussed by Ford et al. (1993) and Hansen (2000) and,for the Cassini RADAR, by Stofan et al. (2006), who mapped theTa and T3 SAR swaths. For example, geologic units will only beapparent if their backscatter characteristics are sufficiently differ-ent from those of surrounding terrains. Also, a mapping techniqueusing a combination of SAR and high-resolution radiometry was

Fig. 2. Top: The Ganesa Macula region (50N, 87W), showing the 180-km diameterstructure on the left (west) and numerous overlapping flows to the north and east,including Leilah Fluctus, the radar-bright flows to the east that have beeninterpreted as fluvial in origin (Paganelli et al., 2005). Data from the Ta and T23flybys. Bottom: Cryovolcanic units are colored pink, the outline of Ganesa Macula isshown as a yellow line. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)

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used by Paganelli et al. (2007), in which plotted normalized radarcross-section versus brightness temperature revealed differing sig-natures that characterize various terrains and surface features. Theresults indicated brightness temperature variations consistentwith variable slopes, or with surface material, compositional prop-erties and volume scattering. Other complexities may arise, forexample, structures are more easily recognized when oriented per-pendicular to the RADAR’s look direction (Stofan et al., 2006, Paga-nelli et al., 2006). Given these complexities, and the fact thatdetailed maps using radar backscatter have been done for the Ta,T3, T7 and T8 swaths (Stofan et al., 2006; Lunine et al., 2008), wedid not attempt to map the full remaining swaths in terms of theradar backscatter of different units. Instead, we mapped only theparts of each swath where we could identify the dominant geo-morphological unit and inferred geologic origin, such as dunefields, putative cryovolcanic flows and edifices, mountains andridges of possible tectonic origin, lakes, river channels, and impactcraters.

For this paper, we have used the SAR swaths and the global im-age map by Cassini Imaging Science Subsystem (ISS; Porco et al.,2005) to identify the geomorphological units and to perform a geo-spatial analysis of their distribution on Titan’s surface using GIS(Geographical Information System) software to combine datasetsin a cartographic system. Titan’s geologic units were mapped indi-vidually; then their distribution and relative coverage on the sur-face were examined statistically.

For each SAR swath, ArcGIS shapefiles demarking the bound-aries of prominent landforms and terrains were drawn. In the caseof previously mapped geologic units (e.g. cryovolcanic units in Tawere mapped in detail by Lopes et al., 2007a), the existing mapwas imported and used as a baseline, though in some cases simpli-fied for the purposes of this broad-coverage project. For example,individual cryovolcanic flows mapped by Lopes et al. (2007a) weregiven a single shapefile designation in this project and are simplyreferred to as cryovolcanic units. For areas having markedly darkor bright radar return, the contrast allowed for another methodof shapefile creation in ArcGIS. Areas of markedly dark radar return(in many cases below the instrument noise floor) are well corre-lated with features interpreted as hydrocarbon lakes (Stofanet al., 2007). Areas of markedly bright return often correspond tofeatures interpreted as mountains and tectonic ridges (Radebaughet al., 2007), which are likely rough and fractured and have perhapsbeen stripped of surface hydrocarbon mantling by rain (Soderblomet al., 2007). We took advantage of the radar contrast of these fea-tures with the surrounding terrain to create contours of equal ra-dar return to define shapefiles.

We mapped both geomorphologic and surficial units. Geomor-phologic units are cryovolcanic features (volcanic constructs andflows, with Ganesa Macula and cryovolcanic flows as sub-units),impact unit (craters, basins, and their ejecta), and hummocky andmountainous terrain (radar-bright mountains, ridges, and hum-mocky terrain interpreted as likely tectonic in origin). This latterunit includes Xanadu, a complex, radar-bright region that appearsheavily dissected but may contain cryovolcanic, tectonic, and otherunits within. Because the major geomorphological unit withinXanadu is mountainous, radar-bright, hummocky terrain, for thepurposes of the distribution analysis in this paper Xanadu is in-cluded as part of the hummocky and mountainous terrain unit.

Surficial units are lakes, dunes, and channels. A third type areunits of uncertain or multiple origin. Here we include crateriformstructures (quasi-circular features that may be impact, volcanic,or tectonic in origin), and plains units. Most of the plains units thatmake up much of the surface, with the exception of the mottledplains unit, are not distinguished in this paper as different typesof plains (as done by Stofan et al., 2006 for the Ta and T3 swaths)and are referenced as undifferentiated plains.

3.1. Geomorphologic units

3.1.1. Cryovolcanic unitsThe SAR swaths revealed several features that may have re-

sulted from cryovolcanism, or water–ice-based volcanism, includ-ing radar-bright flows, caldera-like features, and a circular volcanicfeature named Ganesa Macula (Lopes et al., 2007a). The GanesaMacula and cryovolcanic flows sub-units are the main candidatecryovolcanic units identified from the SAR data. The possible originof these units was discussed in detail by Lopes et al. (2007a). Flowscomposition is unknown, it is possibly ammonia–water or ammo-nia–water–methanol. A mechanism for the transport of ammonia–water cryomagmas to the surface was proposed by Mitri et al.(2008).

3.1.1.1. Ganesa Macula. This is the most prominent feature in theSAR Ta swath. It is a large apparently circular feature about180 km in diameter (Fig. 2). The main feature is radar-dark, withbrighter edges and a series of radar-bright, flow-like units to theeast and north that appear related to the radar-dark unit. Initially,Ganesa Macula was interpreted as a volcanic dome or shield (Ela-chi et al., 2005a,b; Lopes et al., 2007a). Niesh et al. (2008) modeledthe feature as a viscously emplaced dome between 2 and 5 km inheight. However, recent topographic results using SAR stereo (Kirket al., 2008) and overlapping SAR beams (Stiles et al., 2009) showed

544 R.M.C. Lopes et al. / Icarus 205 (2010) 540–558

that Ganesa’s current shape is not that of a dome or shield. It ap-pears to be a heavily eroded and dissected region, and is possiblyan eroded shield or dome that has suffered deflation or collapse.For the purposes of this paper, we map Ganesa Macula and its asso-ciated flows as a single unit and interpret it as a cryovolcanic re-gion, although a heavily eroded one. Stofan et al. (2006)separated Ganesa dark materials (gdm) and Ganesa mottled mate-rials (gmm) and Lopes et al. (2007a) mapped the region in detail,separating several flow units. The Ganesa area and its surroundingsare shown in Fig. 2.

3.1.1.2. Cryovolcanic flows. The SAR swaths also revealed severalflow features interpreted as resulting from cryovolcanism. Theunits are generally bright and lobate, but clearly different fromthe radar-bright, hummocky and mountainous unit discussed later.Cryovolcanic units from Ta through T8 were discussed by Lopeset al. (2007a) and those in Ta and T3 were mapped by Stofanet al. (2006) as ‘‘bright lobate unit” (blu). Examples of candidatecryovolcanic flows are shown in Fig. 3. There are two main typesof flows, which are not separated for the purposes of this analysis.Both types are shown in Fig. 3. Winia Fluctus is an example of a

Fig. 3. Bright lobate units interpreted as cryovolcanic flows (highlighted in pink inlower figure). Data are from the Ta and T23 flybys. The largest flow unit so faridentified on Titan, Winia Fluctus (45N, 30W), is >90,000 km2 and is an example offlows with diffuse edges. The smaller Rohe Fluctus (on the right) is at least1230 km2 and is an example of a flow with radar-bright edges associated with acrater. (For interpretation of the references to color in this figure legend, the readeris referred to the web version of this article.)

Fig. 4. Cryovolcanic feature near Sotra Facula (highlighted in pink in lower figure)imaged by SAR in T25 and T28. The feature is at 15S, 40W. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version ofthis article).

flow complex in which the lobes have diffuse edges. The signifi-cantly smaller Rohe Fluctus is an example of a flow with radar-bright edges and associated with a crater (see Lopes et al.(2007a) for more detailed descriptions and other examples). Small,radar-bright flows interpreted as cryovolcanic are seen mostly inthe Ta and T3 swaths. One of these flows (in the T3 swath; seeFig. 10 of Lopes et al., 2007a,b) was imaged by VIMS, showing itto have a different composition from the surrounding terrain (LeCorre et al. 2009).

There are other features on Titan besides those mapped herethat are possibly cryovolcanic. Another potential volcanic edificeis located near Sotra Facula at 15S, 40W (Stofan et al., in prepara-tion). The feature (Fig. 4) was imaged in both the T25 and T28swaths, allowing for some topographic information from stereo.We interpret this feature to be a cryovolcanic edifice consistingof a �30 km diameter partial caldera superposed by a �40 kmacross, relatively steep-sided dome-like feature. A 180 km,bright-edged lobate flow-like feature extends to the south of theedifice, and is partially covered by dunes.

Lopes et al. (2007a) and Lunine et al. (2008) discussed the small,radar-dark spots 1–2 km in diameter that are located in clusters

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and may be similar to pit craters on Earth. Due to their very smallarea and inconclusive interpretation, they were not included in ourglobal map. There are larger depressions in the north polar regionof Titan, many containing lakes, that may be cryovolcanic, karsticor thermokarstic in origin, or a combination (Mitchell et al., sub-mitted for publication). Because the most likely origin is consid-ered to be karstic, these features were not mapped ascryovolcanic units in this paper, but as part of the lakes unit. De-tailed mapping of lake depressions is reported in Hayes et al.(2008a).

VIMS results must be included in the overall picture of cryovol-canism on Titan, though they are not included in our mapping. Tworegions have been interpreted as cryovolcanic from VIMS dataalone. These are Tortola Facula (Sotin et al., 2005) and Tui Regio(Barnes et al., 2006). Tortola Facula (9S, 143W) is a snail-shapedfeature about 30 km in diameter, interpreted by Sotin et al.(2005) as a dome-type feature or thick cryolava flow. However, re-cent SAR data (Hayes et al., 2008b) indicates that Tortola Facula is apatch of the hummocky and mountainous terrain unit. The westernpart of Tui Regio (125W, 24S) is a large (>3 � 104 km2), lobate re-gion that appears bright at 5 lm and was interpreted by Barneset al. (2006) to be a field of cryovolcanic flows. Candidate flow fea-tures in this region are seen in more recent SAR data (Stofan et al.(2009)). Other VIMS results may indicate that volcanism on Titan ismore widespread and possibly still ongoing. Nelson et al. (2009a)noted spectral changes in the area around Hotei Regio, an arc-likefeature that is bright at visible wavelengths and is unusually brightat 5 lm (Barnes et al., 2005). Spectral changes were also observedby Nelson et al. (2009b) over the western part of Xanadu that mayalso indicate ongoing changes. Patches of what may be CO2 icehave been identified in VIMS spectra of Tui and Hotei Regio (Hayneet al., 2008). Recent SAR images from T41 and T43 (Wall et al.,2009) showed possible cryovolcanic flows in the Hotei Regio re-gion. Candidate flows that appear similar to Hotei’s are seen in

Fig. 5. Titan’s bright, hummocky and mountainous terrain shows ridges and mountainshighlighted in yellow in the lower figure. Other radar-bright units that are not highligsufficient for these areas to be unambiguously mapped. (For interpretation of the referearticle.)

SAR data located on the western part of Xanadu, coinciding witha second region where photometric changes have been reported(Wall et al., 2009; Nelson et al., 2009b).

3.1.2. Hummocky and mountainous terrain unitTitan’s surface has numerous patches of radar-bright, textured

terrains that have been interpreted to be of tectonic origin (Radeb-augh et al., 2007). Among these features are ridge-like features(Fig. 5), elevated blocks which stand generally isolated, and moun-tainous and hummocky terrain. Hummocky and mountainous ter-rain consists of isolated patches of radar-bright hummockymaterial, hills, mountain chains, and blocks. This terrain is mostlyfound as isolated patches or long mountain chains. In general, thepatches of bright, hummocky terrain, including mountain ridges,are small in area, except for Xanadu, which is discussed below asa sub-unit.

Individual mountain heights, determined from radarclinometry(similar to shape-from-shading), are mostly between 200 and2000 m (Radebaugh et al., 2007). Diffuse, bright units are oftenseen around mountains, and these may be a product of peak ero-sion and flank deposition. For the purposes of this analysis, theaprons of depositional materials around mountains are mappedas part of the hummocky and mountainous terrain unit.

3.1.2.1. Xanadu. The 4500 km-wide Xanadu (Fig. 6) is bright at RA-DAR’s wavelength as well as at optical and infrared wavelengths. Itwas the first distinct region to be identified on Titan’s surface, bothfrom Earth and from spacecraft (Lemmon et al., 1993; Noll andKnacke 1993; Richardson et al., 2004). This region is often thoughtto be distinct from the rest of Titan’s surface due to its large size,near-infrared brightness contrast with its surroundings, and dis-tinctive microwave backscatter and radiometry (natural emission)properties (Janssen et al., 2009; Wye et al., 2007; Radebaugh et al.,submitted for publication). Radiometric observations done by the

, such as seen in the T8 swath. The ridge-like features that are part of this unit arehted are considered part of the undifferentiated plains, as the resolution was notnces to color in this figure legend, the reader is referred to the web version of this

Fig. 6. Part of Xanadu region, showing SAR swaths (T13 and T3) over imaging data.Xanadu is bright at visible and radar wavelengths. The crateriform structureGuabonito (Fig. 7) can be seen to the west of Xanadu, a similar crateriform structurecan be seen at top left, at the south end of the T16 swath. The large impact basinMenrva is seen on the T3 swath near top right. Image is centered at �10S, 120W.Xanadu and other patches of hummocky and mountainous terrain, are mapped inthe radar swaths in the bottom image.

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Cassini RADAR (Janssen et al., 2009) show that Xanadu and the Tse-gihi regions are associated with significantly lower emissivity andlow (�1) retrieved dielectric constant. Preliminary backscattermodeling of the scatterometry radar data show that the hum-mocky and Xanadu terrains are similar in nature, but suggests thatthe hummocky and mountainous terrain may be of a slightly higherdielectric constant material that may also be less efficient at vol-ume scattering. The data are consistent with the possibility of bothterrains being composed largely of water–ice, but with ice in theinterior of Xanadu being cleaner than the hummocky ice (Wyeet al., 2007). Although the resolution of the polarized radiometryis too low to address the dielectric constant for this terrain, it isassociated with low brightness temperatures that are neverthelesshigher than Xanadu, consistent with cleaner ice in Xanadu at thedepths of penetration by radar.

Geomorphologically (in SAR data) Xanadu is very similar to theterrains seen in mountainous and hummocky patches elsewhereon Titan, and is therefore mapped as part of the hummocky andmountainous terrain unit. However, we note that several othertypes of units are seen within Xanadu’s large area, such as struc-tures that may be flows (Wall et al., 2009), channels, and crateri-

form structures (Wood et al., in press; Radebaugh et al.,submitted for publication). Although the Xanadu province as awhole may have origins different than those of other hummockyand mountainous terrains mapped in this paper, we assume herethat Xanadu is simply a much larger exposure of the same typeof geomorphologic unit. We note that, from VIMS spectra, thereare differences between Xanadu and other hummocky and moun-tainous materials (Barnes et al., 2007), implying differences in ter-rain at least at the top few microns.

Explanations for why such large mountainous regions in Xan-adu are exposed include that methane rain has washed veneersof hydrocarbon particles off of mountain ridges and into the low-lying areas, leaving the topographically higher regions bare (Smithet al., 1996; Barnes et al., 2007; Soderblom et al., 2007). A detailedstudy of the Xanadu region is presented in Radebaugh et al. (sub-mitted for publication).

3.1.3. Impact crater unitOnly three impact structures have been conclusively identified

on the SAR swaths from Ta to T30 (Fig. 7): Menrva, Sinlap, andKsa (Lorenz et al., 2007; Wood et al., in press), but there are a num-ber of quasi-circular structures in these swaths that may be of im-pact origin (see crateriform structures below). A fourth impactcrater, Afekan, with a diameter of about 115 km and located at26N, 200W, was identified in a SAR swath obtained in May 2008(T43, Wood et al., in press). Another crater, Selk (7N, 199W), wasidentified initially from VIMS data (J. Soderblom et al., in prepara-tion) and later on the T36 SAR swath obtained in October 2007.

The craters Menrva, Sinlap, and Ksa (Fig. 7) and their associatedejecta blankets, where those are discernable, are mapped as the im-pact crater unit. The three craters have markedly different morphol-ogies. Menrva is an impact basin 392 km in diameter (440 from outerrim to outer rim), with a radar-bright, well defined outer ring and amore degraded inner ring (Elachi et al., 2006; Stofan et al., 2006).Menrva and Sinlap were imaged in the T3 flyby. Sinlap, 80 km indiameter, has a well-defined ejecta blanket. Ksa, 29 km in diameter,has the only central peak so far seen on Titan, and an ejecta blanketthat was partially imaged by SAR during the T17 flyby.

According to Lorenz et al. (2007), the rarity of medium sizedcraters on Titan (20–100 km in diameter) implies rapid resurfacing,with erosion and burial likely being the dominant means of craterobliteration. Viscous relaxation plays a lesser role, though if thecrust was thin over most of Titan’s history (Tobie et al., 2006), largecraters would not have survived because of crustal breaching andflooding. The paucity of impact craters implies that Titan’s surfaceis geologically young, having a crater retention age between 0.2and 1 Gyr (Lorenz et al., 2007).

3.2. Surficial units

3.2.1. DunesAeolian transportation and deposition is a major and wide-

spread process on Titan. Features resembling linear dunes on Earthhave been identified by SAR, covering regions hundreds, some-times thousands, of kilometers in extent (Lorenz et al., 2006). Theirpresence indicates the production of a significant supply of easilymobilized sand-sized particles, the generation of winds capableof saltation, and conditions dry enough to permit sand transportnear Titan’s equatorial latitudes. Dunes were first seen in patchesin T3, where they were mapped by Stofan et al. (2006) as aeolianfeatures. Large regions of dunes were then seen in T8, an equatorialpass (Lorenz et al., 2006), when a correlation of dunes with low lat-itudes was first noted. Dunes have since been seen in the T13, T16,T17, T21, T23, T25, T28, and T29 passes, almost exclusively within�30� of the equator, as discussed previously by Radebaugh et al.(2008) from a more limited set of flybys.

Fig. 7. Three major impact structures on Titan: Menrva (upper left, on T3 swath), Sinlap (upper right, on T3 swath), and Ksa (center, on T17 swath). The structures areoutlined in yellow in the bottom image. Center of image is at �55W, 20N. (For interpretation of the references to color in this figure legend, the reader is referred to the webversion of this article).

Fig. 8. SAR images of dunes over ISS basemap from the T3 (top, showing Sinlap at12N, 15W,), T17, T25, and T29 flybys. The dune units are highlighted in brown in thelower figure. Dune units seen in SAR images correspond well to dark areas in ISSimages. (For interpretation of the references to color in this figure legend, the readeris referred to the web version of this article.)

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Because the dunes are dark to RADAR and ISS and correlate wellwith dark surface units identified by VIMS, they are likely com-prised of organics (Soderblom et al., 2007). This material probablybegan as photochemical debris rained out from the atmosphere,then was deposited and perhaps eroded by fluvial processes intosand-sized particles suitable for dune formation (Atreya et al.,2006; Soderblom et al., 2007). It is estimated that dunes cover�20% of Titan’s surface, and thus represent a major geological unit.An example of the dune unit is shown in Fig. 8.

3.2.2. Fluvial channels and lakesChannels, lakes, and radar-dark terrains are candidates for liq-

uids, either past or present. There is plentiful morphological evi-dence that liquids have flowed on Titan (Tomasko et al., 2005;Atreya et al., 2006; Stofan et al., 2007; Lorenz et al., 2008a; Brownet al., 2008). Lakes at high northern latitudes provide evidence forcurrent surface liquids (Stofan et al., 2006; Lopes et al., 2007b;Mitchell et al., submitted for publication). Numerous channelsare seen associated with these lakes and, in some cases, connectthem. In this paper, channels associated with lakes are mappedas part of the lakes sub-unit, while other channels and depositsinterpreted as fluvial are mapped as part of the fluvial featuressub-unit. Although both channels and lakes result from methaneprecipitation, we map them together where they are connectedto study the relationship between fluid-formed features, and wemap channels not associated with lakes separately in order to bet-ter understand their distribution at lower latitudes.

3.2.2.1. Channels. Both radar-bright and radar-dark channels havebeen seen in SAR images and are easily mapped. Channels arewidely distributed in latitude. It is likely that many more channelsexist on Titan which are too small for SAR data to resolve. Ourmapping therefore is likely to underestimate the area covered bychannels. We have attempted to differentiate fluvial and cryovol-canic flows, based on previous work and also the geological contextof these features. As more SAR data become available, the interpre-tation of some surface features has been revised. For example, a

Fig. 9. Elivagar Flumina, located at about 19N, 79W, is a radar-bright featureinterpreted as a large fluvial deposit. This image, acquired during the T3 fly-by,shows braided channels (highlighted in dark blue in the bottom image) converginginto the area interpreted as a catchment basin (highlighted in light blue). Note thefaint patch of dune material on the top right, overlying the catchment area. (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of this article.)

Fig. 10. Lakes and seas at high northern latitudes on Titan (in polar projection). Thelarge central feature is Ligeia Mare (highlighted in blue in bottom image), centeredat approximately 250W, 80N. Lakes and seas appear distinctively dark in radar data.Ligeia Mare images from T25, T28, and T29. (For interpretation of the references tocolor in this figure legend, the reader is referred to the web version of this article).

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large catchment area into which several channels converge, Eliva-gar Flumina, seen in the T3 swath (Fig. 9), was previously mappedas ‘‘bright lineated unit” (blnu) by Stofan et al. (2006), who sug-gested it might be either a fluvial deposit or a tectonically de-formed region. Lorenz et al. (2008a) interpreted Elivagar Fluminaas alluvial due to its relationship to channels and its similar radarbackscatter to that of channel materials. We interpret the linea-tions on this feature as faint dune deposits overlying the fluctus,rather than tectonic features. The relationship between the fluvialand dunes units is discussed in Section 4. Channels also often beara relationship with hummocky, mountainous terrain at lower lati-tudes, possibly indicating orographic rainfall.

3.2.2.2. Lakes. The northern polar regions of Titan were first imagedusing SAR in the T16 flyby, revealing a myriad of very radar-darkpatches interpreted as lakes filled with liquid hydrocarbons (Stofanet al., 2007). Later coverage revealed that lakes are mostly found athigh northern latitudes (Fig. 10), with only a few so far at high south-ern latitudes (Lopes et al., 2007b; Brown et al., 2008; Turtle et al.,2009), possibly in part due to lack of coverage. At high southern lat-itudes, a feature resembling a shoreline was partially imaged by SARin T7 (Paganelli et al., 2007; Lunine et al., 2008), but the area pole-ward of the shoreline is not as radar-dark as the northern lakes. Re-cent data over high southern regions from the T36 flyby show threelakes that are radar-dark and appear to be filled (Lopes et al., 2007b).Filled lakes are characterized by extremely low (often below theinstrument noise floor) radar returns, indicative of an unusuallysmooth or absorptive material at 2.2-cm wavelength, and are inter-preted to be standing bodies of liquids. Supporting this hypothesis isthe identification by VIMS of liquid ethane in Ontario Lacus (Brownet al., 2008), a feature at high southern latitudes which ISS imagesidentified as a lake. Some of the filled lakes at high northern latitudesare sufficiently large that they have been designated maria (seas).

Many but not all lakes occupy steep-sided depressions that ap-pear to exist primarily in the same high-latitude regions, implying

a causal relationship. Mitchell et al. (submitted for publication)have proposed that these are limnogenic in origin, on the basis ofcorrelation of location with the liquids and lack of consistency ofdistribution with other proposed origins; hence, the depressionsare caused by the presence of the liquids interacting with the solidsubstrate, with terrestrial karst landscapes being a possible analog.However, alternative origins cannot be completely ruled out in allcases. Some of the depressions are not currently filled with darkmaterials, although often they exhibit different radar propertiesfrom the surrounds (Hayes et al., 2008a). These are considered tohave drained or evaporated. Lakes likely form due to rainfall (Mitriet al., 2007), but the location and extent of lakes may be affectedconsiderably by evaporation, surface runoff and subsurface flow(Hayes et al., 2008a; Mitchell et al., submitted for publication). Sev-eral features at high latitudes that appear to be dry lake beds havealso been imaged. These are not mapped here, but are discussed byHayes et al. (2008a) and Mitchell et al. (submitted for publication).

3.3. Units of uncertain origin

3.3.1. Crateriform structuresWood et al. (in press) identified 44 possible impact craters with

a wide variety of morphologies, in addition to the four acceptedones. Radar-bright complete or incomplete circles are seen all over

Fig. 11. Guabonito (right) and the crater in Shikoku Facula bright region (left),located just to the west of Xanadu, are two well-defined crateriform structures seenin T13. They are outlined in yellow in the bottom SAR image. Their origin could beimpact or volcanic. Image is centered at �10S, 160W. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version ofthis article.)

Fig. 13. Example of mottled plains area on Titan, from T28 SAR swath. Image iscentered at �10W, 65N.

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the surface, but these features are not sufficiently well-defined toallow a confident interpretation of their origin. In some cases thecircular outline is clear (for example, the crater in the region

Fig. 12. This semi-circular feature that may be part of an impact crater was imagedby SAR during the T23 flyby. This crater is roughly 180 km (110 miles) wide. Thebright material is probably part of the crater’s ejecta blanket, while the darkinterior, smooth at the radar wavelength, possibly represents dunes that havecovered the inside of the crater. The image is approximately 240 km wide and iscentered at about 26N, 9W.

named Shikoku Facula and the Guabonito crater, shown inFig. 11), but burial of surrounding material obscures possible ejectablankets or volcanic flows, which would be important clues to theirorigin. In one case, only part of the structure was imaged by RA-DAR. The T18 swath revealed part of a structure (Fig. 12) that ismost likely an impact crater (Wood et al., in press), but interpreta-tion is still uncertain due to the lack of data showing the wholestructure.

Lorenz et al. (2007) analyzed the size-frequency distribution ofthe three impact craters known on Titan and concluded that thedistribution is similar to Earth’s, indicating obliteration of smallerstructures by erosion and burial. Indeed, crateriform structuresare often seen to be partially buried by dunes (discussed later). Adetailed analysis of crateriform structures is presented by Woodet al. (in press). While their origin continues to be uncertain, in thispaper we examine their global distribution and relationship toother units.

3.3.2. Plains unitsSeveral other units on Titan remain of uncertain origin.

There are vast expanses of plains at mid-latitudes, some ofwhich are mottled in radar appearance (mapped by Stofanet al. (2006) for the Ta and T3 swaths) which we map in thispaper as the mottled plains unit (Fig. 13). The plains units thatmake up large areas of the surface are not mapped as separateunits in this paper, but are considered together as the undiffer-entiated plains (Fig. 14). These areas are often relatively homo-geneous and radar-dark, and are classified as plains becausethey are areally extensive, relatively featureless, and generallyappear to be of low relief (e.g., Elachi et al., 2005a; Stofanet al., 2006). Plains units tend to have gradational boundaries,and occur in patches that are generally hundreds of kilometersin extent. The gradational boundaries make it difficult to distin-guish possible different types of plains within this unit. Withinthe undifferentiated plains are some small areas of uncertain ori-gin, such as possible karstic depressions (discussed by Mitchellet al., submitted for publication) and possible glacial features(Robshaw et al., 2008).

The mottled plains are irregularly shaped with dominantlyintermediate backscatter, and patches of radar-darker and bright-er material. They sometimes flank hummocky and mountainousterrain. Patches of mottled plains extend from tens to hundredsof kilometers. Both the mottled plains and the undifferentiatedplains units are interpreted to be composed predominantly ofwater–ice, with some likely component of tholins and hydrocar-bons (McCord et al., 2006). Stofan et al. (2006) interpreted the

Fig. 14. Example of undifferentiated plains unit in the Ta SAR swath. Ganesa (at 50N, 87W) can be seen on the larger context image.

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radar brightness variations in these units to be likely dominatedby roughness variations produced by fracturing and erosion of Ti-tan’s icy surface, along with some contribution from volume scat-tering and compositional variations. The units may besedimentary in origin, resulting from fluvial or lacustrine deposi-tion, or they may be cryovolcanic – but their relatively featurelessnature and gradational boundaries prevent interpretation usingSAR data alone.

Fig. 15. Distribution of geological units on Tita

4. Global distribution of mapped units and correlation withlatitude

We have measured areas of swaths and the various types ofgeomorphological units in order to quantify the global distribu-tions of these units. The relative distribution by area of the featuretypes mapped is shown in Fig. 15 and Table 2. Undifferentiatedplains make up about 61% of the total area of the swaths. As

n using SAR swaths from Ta through T30.

Table 2Global distribution of different types of geomorphological units as mapped in SARswaths (up to T30). The total swath area is 1.7 � 107 km2, or 20.1% of Titan’s area.

Geomorphological unit Area(�103 km2)

Percentage areawithin swaths

Candidate cryovolcanic (Ganesa andcryovolcanic flows)

202.7 1.2

Hummocky and mountainous, includingXanadu

2032.6 12.1

Impact crater 158.3 0.9Crateriform structures 53.9 0.32Fluvial channels and lakes 418.4 2.5Dunes 2813.7 16.8Mottled plains 821.0 4.9Undifferentiated plains 10 264.0 61.2

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previously discussed, there are features within these plains thatcould not be interpreted given the current data. Of the unitsmapped, the largest in area are dunes and hummocky and moun-tainous terrains. The lakes and fluvial unit is dominated by lakes,as the area of fluvial channels is very small compared to the totalareas of the swaths, although channels are common. Lorenz et al.(2008a) found that areas in which channels are prominent amountto only about 1% of Titan’s surface.

Geomorphologic units such as dunes are clearly not distributeduniformly in latitude. We examined how the different types ofunits are distributed with latitude (Table 3), though it must benoted that most SAR coverage through T30 is in Titan’s northernhemisphere. We divided the swath areas into 30� bins, and calcu-lated the area of each of the major geomorphologic terrain typeswithin each 30� band. The percentage of Titan’s surface coveredby SAR swaths varies widely in these bins, which must be takeninto account when interpreting the results. While 45% of the north-ern polar region has been covered by SAR (60N–90N), only 1.74% ofthe south polar region was imaged by SAR up to T30.

The strongest latitude dependence is apparent in the dune andthe lakes units. Lakes are only present above 60� latitude, and cur-rently predominate in the northern hemisphere. Dunes are foundmostly between 30� north and south latitudes, and are not presentabove 60� north and south. The possible reasons for this latitudinaldependency are discussed below, and also by Mitchell et al. (submit-ted for publication), Lorenz et al. (2006), and Radebaugh et al. (2008).

4.1. Cryovolcanic units

The global distribution of cryovolcanic features (Fig. 15) showsthat these units are mostly small in area, except for the units in Ta

Table 3Areas and percentage areas of different mapped units sorted by 30-degree latitude bins. N

Latitude bin Swath area in bin(�103 km2) andpercentage of totalarea in bin

Hummocky(including Xanadu)(�103 km2)

Lakes(�103 km2)

Dunes(�103 km2)

90N-60N 2507.0 93.9 384.6 –(45.3%) (3.8%) (15.3%)

60N–30N 4437.4 146.7 – 26.4(29.2%) (3.3%) (0.6%)

30N-0 4056.4 537.9 – 1089.9(19.5%) (13.2%) (26.9%)

0–30S 4964.2 1204.8 – 1697.4(23.5%) (24.3%) (34.2%)

30S-60S 703.0 50.3 – –(4.6%) (7.2%)

60S-90S 96.7 (1.74%) – 33.9 –(35%)

and T23. The conclusion reached in an earlier study (Lopes et al.,2007a) is that there is an apparent concentration of cryovolcanismin the Ganesa Macula region, a conclusion validated by data fromsubsequent flybys and presented here. In our earlier study, we sug-gested that the Ganesa region may represent a possible hot spot orTharsis-type analogue (Lopes et al., 2007a).

Cryovolcanic units comprise �1.2% of the area mapped by SARthrough the T30 flyby. This may be an underestimate, becauseinterpretation of some features in the SAR swaths is still tentative,such as the lake depressions discussed above. If most of the lakedepressions are volcanic, it would imply that there is a significantconcentration of volcanism at high latitudes, and that heat flow ishigher in the north (and possibly south) polar regions. This wouldhave implications for interior models. However, the analysis byMitchell et al. (submitted for publication) does not favor the volca-nic hypothesis for the origin of lake depressions. The other majorunknown is the origin of the plains, some or much of which maybe composed of cryovolcanic flows.

4.2. Hummocky and mountainous terrain

The global distribution of the radar-bright hummocky andmountainous unit, including Xanadu, is shown in Fig. 15. Patchesof this terrain can be found all over Titan, with mountain rangespredominantly located in the equatorial regions between 30Nand 30S. This unit is absent at high southern latitude (where ter-rain instead shows evidence of fluvial incision into plateaus) butthis is likely an effect of the small coverage of these latitudes upto T30. Radebaugh et al. (2007) examined the distribution ofmountains and ridges up to T13 and also found them to be locatedpredominantly at lower latitudes.

The distribution of this terrain gives clues to the origin of tec-tonic features on Titan. Radebaugh et al. (2007) discuss possibleorigins for the mountains and ridges. These included crustal com-pressional tectonism and upthrusting of blocks, creation of highblocks and low grabens through extension, dissection and erosionof a pre-existing layer, and deposition of blocks as impact ejecta.Radebaugh et al. (2007) favored crustal compression thrusting upblocks (ice) from below for the origin of mountain chains andblocks and, for Xanadu, a pre-existing layer of icy material strippedaway to leave mesas and pinnacles. Based on the Radebaugh et al.(2007) results, we propose that mountain ridges, which are mostlyin the equatorial regions, are compressional in origin, although allprocesses may be at work. A possible explanation of the contrac-tional tectonism on Titan was proposed by Mitri et al. (submittedfor publication).

ote that the majority of observations up to T30 were of the northern hemisphere.

Cryovolcanism(�103 km2)

Crateriform(�103 km2)

Craters(�103 km2)

Mottled plains(�103 km2)

Undifferentiatedplains (�103 km2)

– 1.9 – 383.7 1642.9(0.1%) (15.3%) (65.5%)

177.0 3.9 – 337.8 3745.6(4.0%) (0.1%) (7.6%) (84.4%)6.4 25.4 158.3 47.9 2191.6(0.2%) (0.6%) (3.9%) (1.2%) (54.0%)19.3 22.2 – 14.2 2006.4(0.4%) (0.5%) (0.3%) (40.4%)– 0.45 – 37.5 615.2

(<0.1%) (5.3%) (87.5%)– 0.5 – – 62.3

(0.5%) (64.5%)

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4.3. Impact craters and crateriform structures

It is not possible at present to say anything conclusive about theglobal distribution of impact craters on Titan, given their verysmall number. It is interesting that these three craters are locatedwithin �1000 km from one another (Fig. 7), but this cannot be con-sidered significant given the available data. The impact cratersidentified on Titan so far have been confined to a fairly narrow lon-gitude band, from 15W to 90W. The presence of craters on theleading hemisphere (0–180�) is, according to Lorenz et al. (2007),consistent with a leading:trailing asymmetry as favored by theimpactor model of Korycansky and Zahnle (2005). However, theSAR coverage at present is insufficient to draw definite conclusions.

Crateriform structures are not distributed uniformly. They aremore common in the 0–30S range due to Xanadu’s crater densityof 2–4 times the Titan average (Wood et al., in press). However,no structures clearly identified as impact craters are seen on Xan-adu, likely because they have been degraded. In contrast, the den-sity of crateriform structures in the large dune areas is about 0.5the Titan average (Wood et al., in press). The difference is likelydue to the relatively older age of Xanadu, while dunes are likelyan ongoing process still covering up impact structures (see sectionbelow).

4.4. Dunes

Dunes cover 16.8% of Titan’s surface as seen in the current RA-DAR swaths, or at least 20% of the surface contains dunes whenextrapolated to all regions (assuming that the dark areas in ISS dataare dune deposits). These are dominantly linear in form, similar insize and morphology to longitudinal dunes in Earth’s Saharan,Namibian, and Saudi Arabian deserts (Radebaugh et al., 2008).These dunes are 1–4 km wide, up to several hundred meters high(Kirk et al., 2005) and often over 100 km long (Radebaugh et al.,2008). The dunes form sand ‘‘seas” thousands of kilometers in ex-tent, similar to sand seas on Earth, and are found on or close to theequator (Fig. 15). Poleward of the equator, dunes also collect intodune fields, smaller in extent with more widely spaced dunes thanin the sand seas (Radebaugh et al. 2008). All dunes are concen-trated mostly at lower latitudes, within 30� of the equator, as in-ferred from earlier studies with less data (Lorenz et al., 2006;Radebaugh et al., 2008). Dunes in RADAR correlate well with darkareas in imaging (Fig. 8), which also occur mostly at equatorial lat-itudes, consistent with lower latitudes being relatively dry (Ran-nou et al., 2006). Dunes in SAR are also well correlated with theVIMS dark brown unit, again found mostly at equatorial regions(Soderblom et al., 2007; Barnes et al., 2007).

Longitudinal dunes typically form on Earth from winds in twodominant directions averaging to parallel to the dune long axis.Dunes on Titan are nearly all oriented west to east with detailsindicating eastwards time-averaged winds at Titan’s low latitudes.Dunes also require a sediment supply and deposition region, bothof which require mostly dry conditions so that particles do notstick together and dune fields are not consumed into lake basins.Thus, Titan’s mid-latitudes are probably generally dry. This findingcorrelates well with the observations of lakes and filled channels atthe high latitudes and dunes and dry river beds at low latitudes(Stofan et al., 2006; Lunine et al., 2008; Lorenz et al., 2008a; Radeb-augh et al., 2008).

4.5. Channels and lakes

Fluvial processes are extremely important on Titan, are distrib-uted over all RADAR swaths and also seen at Huygens landing site(Tomasko et al., 2005) at scales smaller than the RADAR resolution.

Channels are therefore major contributors to the modification ofthe morphology of other geologic features.

Lorenz et al. (2008a) found that radar-bright, braided channelsare predominantly located at low and mid-latitudes, and proposedthat these resemble desert washes where fine material has beenremoved by energetic surface liquid flow, presumably from meth-ane rainstorms. Radar-dark channels are found predominantly athigh latitudes and meander or drain into polar lakes, suggestingslower-moving flow depositing fine-grained sediments. A thirdtype, seen predominantly at mid and high latitudes, has radar-brightness patterns indicating topographic incision, with valleywidths up to 3 km across and depths of several hundred meters.

Up to T30, lakes were only found at high northern latitudes, la-ter data showed a few at high southern latitudes (Lopes et al.,2007b). The distribution of lakes is shown in Fig. 15, but is dis-cussed in detail by Hayes et al. (2008a) and Mitchell et al. (submit-ted for publication). In the northern hemisphere, where 55.4% ofthe surface has been mapped, lakes only occur above 55N, withthe darkest lakes at radar wavelengths occurring at latitudes high-er than 70N (Hayes et al., 2008a). Lakes likely form due to rainfall(Mitri et al., 2007) but the origin of lake depressions may be varied(Mitchell et al., submitted for publication). Lakes range in size fromthe limits of detectability by SAR to seas of over 100,000 km2.Hayes et al. (2008a) found that lakes and seas are segregated bylongitude, with those with areas in excess of 104 km2 being gener-ally confined to a region between 0W and 140W. This may be afunction of the pre-existing topography in different regions.

The areal extent of the lakes is consistent with the model of Mi-tri et al. (2007) in which the high relative humidity of Titan’s loweratmosphere is sustained by evaporation of seasonally variablenear-polar lakes of liquid methane–ethane composition. As dis-cussed above, there are fewer lakes in the southern polar regions(considering the distribution of lakes per swath area), consistentwith seasonal variations. Recent results from the VIMS experiment(Brown et al., 2008) indicate that Ontario Lacus, located in thesouthern polar regions and first imaged by ISS (Turtle et al.,2009), is liquid-filled.

4.6. Plains

The distribution of undifferentiated plains is fairly uniform, andthese plains occur in all swaths. Mottled plains are predominantlylocated at high northern latitudes (Fig. 15), in the region between30N and 75N, though this could be an effect of coverage. These ter-rains are distinct from both Xanadu and other patches of hum-mocky and mountainous terrains, as they are not as radar-bright,have more variable brightness, and do not form chains. It is unclearwhy these mottled plains appear to be located predominantly athigh latitudes. It could possibly be an effect of more dune materialobscuring the lower latitude plains, but at this point it is not pos-sible to say whether the mottled plains have the same origin as theplains at lower latitudes that are more uniform in radarbackscatter.

5. Temporal relationships between units

5.1. Cryovolcanic features

The interaction of cryovolcanic features with those produced byother major processes is not easy to ascertain, partly because of thepaucity of cryovolcanic constructs. Ganesa Macula and the variouscryoflows mapped overlie the featureless plains (here the undiffer-entiated plains unit) discussed by Stofan et al. (2006). There are noexamples of cryovolcanic units having contact with hummocky andmountainous terrains or mottled plains in the current data, except

Fig. 16. Winia Fluctus (at �45N, 30W), a cryovolcanic flow field, showing dunes over the northern part of the radar-bright flow field.

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for a possible small cryovolcanic region on the western part of Xan-adu (discussed by Wall et al., 2009 and Nelson et al., 2009b). Theclearest interaction between a cryovolcanic unit and another unitis in the Winia Fluctus area (Fig. 16), where dunes can be seensuperposing the radar-bright, cryovolcanic flow field. Dunes canalso be seen superposing the cryovolcanic feature near Sotra Facula(Fig. 4).

Ganesa Macula is cut by channels (Lopes et al., 2007a; see alsoFig. 2), but it is unclear whether these are cryovolcanic or fluvialchannels. If the channels are fluvial in origin, they postdate the edi-fice. The other cryovolcanic units identified in the current data donot show any evidence of fluvial erosion. There is also no evidenceof impact craters superposing any cryovolcanic feature, and onlyWinia Fluctus and the feature near Sotra Facula show clear evi-dence of being covered by dunes in relatively small areas. This

Fig. 17. Dunes divert around patches of bright, hummocky, and mountainousterrain. As winds blow on average from west to east (left to right in this scene) theyinteract with topographic obstacles, causing changes in the direction and style offlow of materials. Image of Titan’s eastern equatorial region, 6S, 39W, from the T25orbit, February 2007.

may imply that these cryovolcanic units are relatively young. Mitriet al. (2008) argued that the cryovolcanic features could have beenassociated with episodic (potentially late) geological activity. Tobieet al. (2006) suggested that episodic methane outgassing associ-ated with cryovolcanic activity recommenced on Titan relativelyrecently (�500 Ma), which would be consistent with these unitsbeing still exposed and well preserved.

5.2. Hummocky and mountainous terrain

Nowhere do patches of the hummocky and mountainous terrain(including Xanadu) appear to be stratigraphically younger than anyother units. Many patches of hummocky and mountainous terrainappear to be locally embayed by undifferentiated plains units;other mountain patches appear to have erosional debris apronsthat obscure the contact between the unit and the surroundingplains (Radebaugh et al., 2007). Most commonly, dunes – thoughtto be among the youngest units on Titan – are seen to be either

Fig. 18. Channels cutting through bright, hummocky, and mountainous terrain andsmoother lowlands in Xanadu. Channels are narrow in the most mountainousregions, and can often be difficult to trace, but they widen out in flat lowlands, asseen in the channels at lower left. From Titan’s eastern equatorial region, 11S 128W,obtained during the T13 flyby, April 2006.

Fig. 19. Menrva impact basin (19N, 87W), showing channels cutting through the outer western rim and dune material against the outer edge of the rim. Some dune materialappears to have entered the basin, and accumulated near the inner edge of the eastern rim.

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diverted around or climbing onto patches of the hummocky andmountainous terrain unit, indicating that this terrain is topograph-ically higher than surrounding areas. One such case is shown inFig. 17; such a contact between dunes and hummocky and moun-tainous terrain is common on Titan’s surface. Dunes are seen to di-vert around the Xanadu area on the western side, are not seenwithin Xanadu, and begin to re-appear in patches on the easternside (e.g., Radebaugh et al., 2008). Channels, another stratigraphi-cally young unit, are seen to cut through the hummocky andmountainous terrain, particularly in the Xanadu area (Fig. 18).

Mountainous terrains also contain impact craters, as for exam-ple in some of the highland terrains in Xanadu and on the highlandShikoku Facula in the dune field Shangri-La (Fig. 11). A more thor-ough description of the relationship between craters and their sur-roundings is in the following section.

5.3. Impact craters and crateriform structures

The three craters mapped in this paper differ in apparent degreeof erosion and modification they have suffered, though overall therelationship to dunes and channels is the same.

Fig. 20. Sinlap (11N, 16W) and its surrounding ejecta blanket: Dunes abut thewestern edge of the ejecta. Small patches of topographically high bright,hummocky, and mountainous terrain are seen to the west, also partially coveredby dunes. Blocks of presumably the same mountainous material can be seenprotruding from the ejecta blanket south of the crater.

Menrva appears to be the most degraded of the structures. Theterrain surrounding this impact basin indicates that erosional pro-cesses have degraded the SW outer rim in particular (Fig. 19).Channels are seen to cut through this part of the rim, while dunesabut the rim. Clearly, the basin is being modified by the action ofaeolian and fluvial processes. Sinlap’s ejecta blanket is also par-tially covered by dunes (Fig. 20). Radar-bright material is seen toprotrude from the ejecta to the southeast of Sinlap, interpretedto be mountain blocks (Radebaugh et al., 2007), perhaps thrownout by the impact event. Here they are considered part of the hum-mocky and mountainous terrain unit. This is the only instance sofar where an association has been seen between impact cratersand the hummocky and mountainous terrain unit. A more detailedmap of the Sinlap region using both VIMS and RADAR data is de-scribed in Le Mouelic et al. (2008).

Ksa (Fig. 21) also shows an ejecta blanket. Although the partialimaging of the ejecta blanket makes it difficult to see the relation-ship between the crater and surrounding units, dunes abut theejecta blanket to the south, possibly overlying it, as the rim is less

Fig. 21. Ksa crater (14N, 65W), showing dunes overlying the ejecta blanket to thewest. Note the small patches of bright, mountainous, and hummcky material to theeast of the crater and ejecta.

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well-defined in this region. An embayment contact between dunematerial and the ejecta blanket is seen on the western part of theejecta blanket. To the east of the ejecta blanket are patches of hum-mocky and mountainous terrain, which appear to be topographi-cally high, as dunes abut them, though in places (presumablytopographically lower areas) dunes overlie the hummocky mate-rial. A large, dark, sinuous feature runs north–south on the easternedge of the Ksa ejecta blanket. This could be a river channel that isexploiting the low terrain outside of the likely higher blanket ofimpact materials.

A greater density of crateriform structures is seen on Xanaduthan on any other terrains (Wood et al., in press), and fewer onthe dunes. This is consistent with Xanadu being one of the olderunits on Titan’s surface, and the dune deposits being young mate-rials. Crateriform structures are often covered by dune materials,such as in Fig. 11.

We have identified no direct contacts between crateriformstructures and cryovolcanic units. There are several instances ofcrateriform structures being superposed by dune materials. Crater-iform structures appear younger than Xanadu, and younger thanother hummocky and mountainous terrains where they are seen.Some crateriform structures on Xanadu are exploited by or cutby channels but, elsewhere, we have not identified direct contactsbetween crateriform structures and channels.

5.4. Dunes

Dunes are among the youngest units on Titan, and dune forma-tion may be an ongoing process (Barnes et al., 2008). Dune depositsare seen to partially cover some cryovolcanic materials, also super-pose patches of hummocky and mountainous terrain and impactmaterials (including crateriform structures, discussed below). Therelationship between dunes and channels is not clear, as both ap-pear to be young processes. SAR data shows no regions wherechannels clearly cut dunes; although Barnes et al. (2008) haveidentified a channel from the overlap of VIMS and RADAR data thatappears to cut through dune material, indicating activity withinthe channel postdates particle movement on that portion of thedunes. In regions on Earth such as in Namibia, dry channels cutthrough dunes. Given that there are channels and lakes on Titanthat appear to be filled with liquids probably recently rained out,channel formation may also be an ongoing process. Some localtemporal relationships are apparent – in Fig. 9, dune depositsclearly overlay the fluvial unit Elivagar Flumina.

5.5. Channels and lakes

Fluvial and lacustrine processes are among the most recent onTitan. These processes are expected to be ongoing, and changesare expected to be detected by the continuing Cassini mission,although it is possible that some of the lakes and seas are persis-tent. Fluvial channels cut through other geologic terrains on Titan(except dunes, with which they may be penecontemporaneous, seeabove section).

5.6. Mottled plains and other units

Most units we have mapped – including cryovolcanic features,dunes, rivers, craters and crateriform structures – superpose themottled and undifferentiated plains. Some occurrences of hum-mocky and mountainous terrain appear to be either embayed bythe plains (Stofan et al., 2006), or are surrounded by an apron ofbright material that is interpreted to have been eroded from thehummocky terrain (Radebaugh et al., 2007). Therefore, we inter-pret the plains to generally postdate the hummocky and moun-tainous unit. We have seen no occurrence of a tectonic feature

that is convincingly younger than the plains units. However, wecannot definitively constrain the origin of the hummocky andmountainous unit; it may be composed of deformed and/or erodedpre-existing plains units. While we do see lobate units that areeither cryovolcanic or depositional in origin, we cannot determinethe contribution of these processes to the origin of the plains.

6. Interaction of processes and relative ages

A picture of Titan’s geologic evolution is emerging from thestratigraphic relationships and the distribution of geomorphologi-cal units. It is clear that Titan is a very active world, and that ero-sional and depositional processes, both from fluvial and aeolianactivity, play a significant and ongoing role in modifying thesurface.

The majority of Titan’s surface is young, that is, less than a bil-lion years, even assuming that all the crateriform structures dis-cussed are due to impact (Wood et al., in press). Impact cratershave been removed by burial and erosion, as shown by the cuttingand burying of craters by fluvial and aeolian materials. It is likelythat numerous craters have been buried or partially buried by aeo-lian deposits; in some cases, only the rims are left exposed. Fluvialerosion also seems to have played a major role, particularly inareas such as Xanadu. Lorenz et al. (2007) pointed out that thesize-density distribution of craters on Titan is similar to Earth’s,indicating perhaps similar rapid processes of obliteration by ero-sion and burial. It is not yet clear what the role of cryovolcanismhas been on crater obliteration, as so far cryovolcanic processesare not seen to be as widespread as erosional processes.

The oldest units on Titan are the many patches of hummockyand mountainous terrain, including Xanadu. There are crateriformstructures superposed on Xanadu, but it is unclear from the avail-able data if Xanadu and other areas of hummocky and mountain-ous terrain are older than the three main impact craters seen upto T30. It is clear, however, that the patches of hummocky andmountainous terrain are scattered all over the surface and that no-where do they appear uneroded or stratigraphically younger thananother local terrain type. We consider these to be the oldest geo-logic formations on Titan that are still preserved.

The hummocky and mountainous terrains are likely tectonic inorigin, and one possibility is that all the patches of hummocky andmountainous terrain formed at the same time by the same mech-anism, during an early phase of Titan’s history when tectonic activ-ity was taking place. Another possibility is that the patches of thisterrain formed at different times by different mechanisms, but inan earlier era when tectonics was more persistent. It is also possi-ble that some of the patches of hummocky and mountainous ter-rain may not be tectonic, but purely erosional. Erosion hascertainly played a role on the morphology of these terrains. Radeb-augh et al. (2007) point out that mountain peak morphologies andsurrounding, diffuse blankets are suggestive of erosion. Most of thepossible tectonic features appear to be at least partially degradedand embayed by surrounding plains units. Radebaugh et al.(2007) argue that fluvial runoff has played a significant role in ero-sion. Cassini VIMS data have identified that, at least for one moun-tainous area east of Xanadu, there are spectral differences betweenthe mountainous area and the surrounding terrain (Barnes et al.,2007), perhaps implying that material that coats the surroundingplains is not present on the mountains. The same analysis showedthis to be the case for channels as well; perhaps the thin coatinghas been removed by erosion on channels and mountainous areas.

Mottled and undifferentiated plains appear to be younger thanpatches of hummocky and mountainous terrains, although anearlier phase of plains units may have been deformed and erodedinto the hummocky and mountainous units. The processes of

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deposition and cryovolcanism are likely to have contributed toplains formation, but the featureless nature of the undifferentiatedplains provides few clues to their mode(s) of origin. An exception isthe brighter plains flanking mountainous units, which are inter-preted to be erosional aprons (Radebaugh et al., 2007).

Cryovolcanism may be a relatively young process or possiblyongoing, as suggested by Nelson et al. (2009a,b). The large flowfields mapped so far do not show any evidence of fluvial erosion,perhaps implying that they are quite young.

Tobie et al. (2006) and Mitri et al. (2008) have shown that cryo-volcanic activity is likely a marginal geological process on Titanwith episodic activity. Mitri et al. (2008) proposed a model of cryo-volcanism that involves cracking at the base of the ice shell andformation of ammonia–water pockets in the ice. Large-scale tec-tonic stress patterns (tides, non-synchronous rotation, satellite vol-ume changes, solid state convection, or subsurface pressuregradients associated with topography) would enable the ammo-nia–water pockets to erupt effusively onto the surface. Solid stateconvection may also be responsible for cryovolcanic features onthe surface of Titan, either the result of cryomagmatic liquids beingentrained in the convection process, accumulating and erupting(Mitri et al., 2008), or heating and melting/vaporising near-surfacepockets of materials (ammonia hydrates or methane clathrate hy-drates), causing them to erupt (Choukroun et al., 2008). Tobie et al.(2006) proposed that episodic outgassing associated with cryovol-canic activity of methane stored as clathrate hydrates within anice-I shell, possibly in the manner described by Choukroun et al.(2008), can explain the present atmospheric methane abundance.

Fluvial and aeolian deposits appear to be very young features onTitan. At high latitudes, fluvial erosion appears to be the dominantmodification process, at least at the high northern latitudes. Ero-sion and deposition by aeolian activity may dominate the lowerlatitudes and dunes are seen in places to overlay fluvial deposits.

The interplay of dunes and fluvial activity is not clear. Based onthe presence of fluids in some lakes and rivers and the stratigraphicyouth and relationship to underlying materials of the dunes, it islikely that there is current activity in the form of both dunes andchannels, at least on seasonal time scales. The abundance of dunesat lower latitudes suggests that these regions are predominantlydry. Rain, and by implication, the formation of fluvial features,may be infrequent, perhaps analogous to the deserts in Arizonaand California.

7. Conclusions and discussion

Titan shows a variety of geological processes similar to those onEarth. Each of the SAR swaths has provided insights into the geo-logic processes that shape the complex and youthful surface of Ti-tan. These snapshots await integration into the larger set of swathsto be obtained during the Cassini mission before a comprehensivemodel of the geologic evolution of Titan can be developed.

Limiting our ability to interpret the mapping of geological andgeomorphological units in terms of a crustal history is the generallack of accurate topographic information over much of Titan’s sur-face, the lack of detailed compositional information, and the lack ofa large enough impact crater population to determine relative agesof units. The radar system cannot work simultaneously in altimet-ric and SAR mode, so little direct altimetric data exist, while radarstereogrammetry methods are limited in area and accuracy. Know-ing, for example, how the topography changes across the large im-pact craters and in the boundary between Xanadu and adjacentunits would provide major breakthroughs in understanding theorigins of these features. Stiles et al. (2009) have recently devel-oped a method to obtain topography using SAR, future mappingstudies will utilize results from this technique to better constrain

the timeline of geologic processes on Titan. In addition, the lackof detailed compositional information limits our ability to classifymany of the units as geological units, leaving us to group by mor-phology rather than composition. VIMS data has provided limitedinformation on the composition of units such as Xanadu and thelakes (e.g. Barnes et al., 2005, 2006; Brown et al., 2008), but differ-entiating between plains units or obtaining a detailed understand-ing of cryovolcanic units requires higher resolution compositionalinformation, which is currently limited (Le Mouelic et al., 2008). Interms of age dating units, while we can determine that Xanadu hasa higher population of crateriform structures, and the channelshave no superimposed craters, we cannot utilize this informationto make definitive statements about relative age given the paucityof the impact crater population. Superposition and cross cuttingrelationships do provide useful information such as the relativeolder age of the hummocky and mountainous unit, but these rela-tionships are local and cannot be related to an absolute agerelationship.

Titan’s surface is overall very young, given the small number ofimpact craters and the clear evidence of lacustrine, fluvial, and aeo-lian processes on the surface. Although the larger patches of exposedhummocky and mountainous terrain, such as Xanadu, show crater-iform structures interpreted as impact in origin, the number of cra-ters is too small for relative age dating. Ancient impact cratersmust have been subsequently destroyed by tectonic activity, cryo-volcanism, atmospheric deposition and fluvial and pluvial erosion.The ubiquity of hummocky and mountainous terrain on the surfaceseems to indicate a past episode of tectonics, possibly crustal short-ening. No strong evidence of extension is seen by the radar to date.

Plains, both undifferentiated and mottled that are now exposed,are of unknown origin, but could possibly be composed of cryovol-canic flows. The plains are younger than the hummocky and moun-tainous terrains, but older than impact features, dune and fluvialfeatures, even the highly eroded Menrva. Several cryovolcanicunits were mapped and appear to be relatively young, at least lo-cally, indicating some variation in volcanic styles. The distributionand relative importance of cryovolcanism are still unknown. If asignificant portion of the crateriform structures are volcanic in ori-gin, then cryovolcanism has been more widespread on Titan thanindicated by the features mapped in this paper as cryovolcanic.However, the analysis of Wood et al. (in press) argues that mostcrateriform structures are impact in origin.

An approximate timeline of geological processes on Titan can beinferred from the analysis presented in this paper. The oldest unitsstill exposed are hummocky and mountainous terrains, includingXanadu. While it is clear that impact and tectonic processes oc-curred relatively early in the surface history, other processes suchas cryovolcanism may have been happening at the same time.

The youngest processes are those of a surficial nature, namelydunes, fluvial and lacustrine features, although it is possible thatsome cryovolcanism may still be happening on the surface (Nelsonet al., 2009a,b; Wall et al., 2009). Atmospheric deposition in theform of the raining out of organic particles is probably acting to ob-scure and bury surface features, and is likely very significant insome areas, particularly the dune fields. Fluvial erosion and depo-sition also appear to be significant everywhere, and to have playedan important role in eroding terrains that were initially topograph-ically higher, such as mountains and Xanadu. There is evidencethat fluvial erosion has also happened at Menrva and possiblyhas been important in erasing evidence of earlier impacts. Aeolianprocesses dominate in the equatorial region, with dunes superpos-ing craters, volcanic features, and hummocky and mountainousterrain. Dunes on the whole appear younger than fluvial channels,though Barnes et al. (2008) have argued from VIMS data that atleast one channel overlies dunes. It is likely that both aeolian depo-sition and fluvial activity are still ongoing.

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Several studies have shown that the thermal evolution of Titanproduced changes in the thermal state (conductive and convective)and thickness of the ice shell (Tobie et al., 2006; Mitri et al., 2008).Thermal models suggest that a subsurface liquid layer might bepresent during all the evolution of Titan’s interior (Tobie et al.,2005, 2006; Mitri and Showman, 2008; Mitri et al., 2008). Duringaccretion and differentiation of Titan, ammonia and other volatilesmay have been released from the interior and ammonia might bepresent in a subsurface liquid layer (Lunine and Stevenson,1987). The long-term cooling of Titan and freezing of the ammo-nia–water liquid sublayer have likely caused a large variation inthe thickness of the outer ice shell from a few tens kilometers tolarger than 70 km (Tobie et al., 2005; Mitri et al., 2008). The onsetof thermal convection in the ice shell can cause large (20–30 km)and rapid (�107 year) variations in the ice shell thickness (Mitriand Showman, 2008). Tobie et al. (2006) argued that during Titan’sevolution, methane could be stored as clathrate hydrates withinthe ice shell. In this scenario, the crust was quite thin, around10–20 km, over much of Titan’s history, but has thickened to morethan 50 km in the last 0.5–1 Ga (Tobie et al., 2006). However, For-tes et al. (2007) proposed a different model of the interior structureof Titan in which the crust has remained thick throughout Titan’shistory except near the beginning. This model assumes the pres-ence of sulfur leached into a subsurface liquid layer, however, sofar no sulfur has been detected on Titan. Based on this model,Grindrod et al. (2008) proposed that the outer layer has evolvedto form a heterogeneous crust (�170 km thick) of methane clath-rate with a small amount of water–ice and ammonium sulfateoverlying an aqueous ammonium sulfate ocean (�50 km thick).

The estimated age of the surface from crater counts and the ab-sence of a significant number of large craters would seem to favormaintaining a thin crust up until the last ½–1 Gyr, but the exis-tence of Menrva requires an explanation. Menrva is too large tohave been formed when the crust of Titan was significantly lessthan 100 km in thickness, a value maintained in the Fortes et al.(2007) model for most of Titan’s history. The state of degradationof Menrva does, however, imply that it is ancient and significantcrustal modification occurred subsequent to its formation, includ-ing flow of presumably methane liquids across its rim. This mightbe consistent with an early episode in the Tobie et al. (2006) ther-mal history in which a thick crust (exceeding 50 km) of methaneclathrate hydrate exists and is subsequently thinned. It is of inter-est to ask whether the kind of degradation observed in Menrva isconsistent with crustal thinning and possibly circulation of meth-ane as an impact generated hydrothermal system; good topo-graphic information on this crater is needed to evaluate thishypothesis.

It may be that on Titan some features of the landscape we ob-serve today cannot be explained by present conditions. For exam-ple, there is some suggestion (e.g. Soderblom et al., 2007) that Titanhas fluvial features that are considerably larger in scale than can beeasily explained even with the heavy (�1 m rainfall) downpoursthat can happen today.

Two major perturbations can be considered, though theremay be others. First, over time the solar luminosity has changedand an initial inventory of methane may have been photochem-ically processed into ethane and higher hydrocarbons. The neteffect is a decrease in the amount of methane in the surface–atmosphere system, but typically an increase in total atmo-spheric pressure and temperature (e.g. Lorenz et al., 1999). Sincelatitudinal and seasonal effects seem significant even today incontrolling aeolian transport and pluvial erosion, it is difficultto assess how these processes might have been different in thepast. A second perturbation (e.g. Lunine et al., 1998) may bethe episodic injection, by cryovolcanism, of methane from theinterior into the surface–atmosphere system. Although there is

no direct evidence of such events, they seem likely given thatthe observed lake inventory is inconsistent with photolysisthroughout Titan’s history (Lorenz et al., 2008b). Such injectionevents may have warmed or cooled Titan, depending on the roleof cloud feedbacks (not well understood on Earth), but wouldhave certainly made Titan wetter.

Some major unknowns remain, including whether fluvial activ-ity occurred early in Titan’s history, which requires a better under-standing of the atmospheric history. Understanding the nature ofthe plains, which occupy a large area on the surface, is particularlyimportant. If the plains are cryovolcanic in origin, this would implythat cryovolcanism has been widespread at least for part of Titan’ssurface history. It is unclear whether tectonic processes have oc-curred after the formation of the features now seen as part of thehummocky and mountainous terrain. As Cassini progresses anddata sets from various instruments become more complete, someof these issues may become clearer or at least better constrained.However, mapping of Titan’s surface by Cassini instrumentationmay not tell the whole story. Much of Titan’s geological history -not least many impact structures – may lie buried beneath aeolianand other sediments. Data such as the detailed chemical composi-tion of the surface, direct atmospheric and lake measurements tomore accurately constrain the methane cycle, and determinationof the interior structure to constrain models of interior evolution,will require future exploration of Titan. Future missions, perhapsusing landers and the ground-penetrating radar techniques pres-ently being used to explore Mars, may open new windows into Ti-tan’s history.

Acknowledgments

We are grateful for helpful reviews from Jason Barnes and LynnCarter. This work was conducted at the Jet Propulsion Laboratory,California Institute of Technology, under contract with the NationalAerospace and Space Administration (NASA). This research wascarried out at the Jet Propulsion Laboratory, California Instituteof Technology, under a contract with the National Aeronauticsand Space Administration.

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