global geological mapping of ganymede

Icarus 207 (2010) 845–867

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Icarus

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Global geological mapping of Ganymede

G. Wesley Patterson a,c,*, Geoffrey C. Collins b, James W. Head c, Robert T. Pappalardo d,Louise M. Prockter a, Baerbel K. Lucchitta e, Jonathan P. Kay b,f

a Applied Physics Laboratory, Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, MD 20723, USAb Department of Physics and Astronomy, Wheaton College, Norton, MA 02766, USAc Department of Geological Sciences, Brown University, Providence, RI 02912, USAd Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USAe United States Geological Survey, Flagstaff, AZ 86001, USAf Department of Geological Sciences, University of Idaho, Moscow, ID 83844, USA

a r t i c l e i n f o

Article history:Received 29 July 2009Revised 25 November 2009Accepted 28 November 2009Available online 6 December 2009

Keywords:GanymedeSatellites, SurfacesJupiter, Satellites

0019-1035/$ - see front matter � 2009 Elsevier Inc. Adoi:10.1016/j.icarus.2009.11.035

* Corresponding author. Address: Planetary ExplorLaboratory, MP3-E106, 11100 Johns Hopkins Rd., LauStates.

E-mail address: [email protected] (G.W. P

a b s t r a c t

We have compiled a global geological map of Ganymede that represents the most recent understandingof the satellite based on Galileo mission results. This contribution builds on important previous accom-plishments in the study of Ganymede utilizing Voyager data and incorporates the many new discoveriesthat were brought about by examination of Galileo data. We discuss the material properties of geologicalunits defined utilizing a global mosaic of the surface with a nominal resolution of 1 km/pixel assembledby the USGS with the best available Voyager and Galileo regional coverage and high resolution imagery(100–200 m/pixel) of characteristic features and terrain types obtained by the Galileo spacecraft. We alsouse crater density measurements obtained from our mapping efforts to examine age relationshipsamongst the various defined units. These efforts have resulted in a more complete understanding ofthe major geological processes operating on Ganymede, especially the roles of cryovolcanic and tectonicprocesses in the formation of might materials. They have also clarified the characteristics of the geolog-ical units that comprise the satellite’s surface, the stratigraphic relationships of those geological units andstructures, and the geological history inferred from those relationships. For instance, the characteristicsand stratigraphic relationships of dark lineated material and reticulate material suggest they represent anintermediate stage between dark cratered material and light material units.

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1. Introduction

The surface of Ganymede can generally be divided into twomaterial types that exhibit differences in albedo, crater density,and surface morphology. Approximately one-third of the surfaceis covered by ‘‘dark” material, which is heavily cratered, coveredwith relatively low albedo regolith, and commonly transectedby large-scale arcuate fracture systems termed furrows. Craterdensity measurements suggest that dark material represents theoldest preserved surfaces on Ganymede, though it generally haslower crater densities than the surface of neighboring Callisto,suggesting that it is not a primordial surface (Shoemaker et al.,1982). The other two thirds of Ganymede’s surface is coveredby vast globe-encircling swaths of ‘‘light” material, which has asurface with higher relative albedo and significantly lower craterdensity than dark material. The swaths of light material are them-

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ation Group, Applied Physicsrel, MD 20723-6099, United

atterson).

selves divided into elongated and polygonal shaped areas. Viewedat resolutions >500 m/pixel, these areas may appear relativelysmooth or they may be modified by troughs termed grooves. Ithas been suggested that light material formed predominantlythrough the modification of dark material by tectonic and cryo-volcanic resurfacing processes (e.g., Pappalardo et al., 2004 andreferences therein).

The dichotomy between these two basic terrain types leads to anumber of fundamental questions about the formation and evolu-tion of the surface of Ganymede. What is the origin of the albedoheterogeneity of the surface? How have dark and light materialsevolved through time? What internal forces led to the formationof tectonic structures like furrows and grooves? Does the forma-tion of grooves primarily reflect a local or global stress regimeand how has that stress regime changed through time? What arethe properties of craters on Ganymede, and what are the relativeage relationships among geologic features on the surface? Under-standing the geological record of Ganymede is crucial to answeringthese questions. In this paper, we present a global geologic map ofGanymede based on combined Voyager and Galileo data and ex-plore its implications for the geologic history of the satellite.

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846 G.W. Patterson et al. / Icarus 207 (2010) 845–867

1.1. The Voyager perspective

Both Voyager spacecraft observed Ganymede during their Jupi-ter flybys (Smith et al., 1979a,b). However, data of sufficient reso-lution (better than 2 km/pixel) to allow geologic interpretation waslimited primarily to areas surrounding the subjovian and antijo-vian points. A synthesis of the results from these two spacecraftis provided by Shoemaker et al. (1982) and McKinnon and Parmen-tier (1986). From these data, a series of geologic maps covering thesurface were produced at the 1:5 M scale. These maps representthe application of Voyager-based geologic interpretations.

Based on Voyager images, dark material on Ganymede wasinterpreted to represent a heavily cratered, primordial surfacemodified by cryovolcanic activity (Croft and Strom, 1985; Croftand Goudreau, 1987; Murchie et al., 1989; Croft et al., 1990). Cryo-volcanism as a modification process was supported by an apparentabsence of small craters, embayment relationships observed inassociation with large craters, and smooth areas associated withfurrows (Cassachia and Strom, 1984; Murchie et al., 1990;Lucchitta et al., 1992). Schenk and Moore (1995) proposed that vol-canic activity on Ganymede included extrusion of icy materialsinto crater floors to form lobate domes and into furrow floors tocreate smooth dark deposits.

Light material was interpreted to represent regions where darkmaterial had been resurfaced by cryovolcanic flows, which weresubsequently tectonized in some areas to form grooves (Golombekand Allison, 1981; Golombek, 1982; Parmentier et al., 1982; Shoe-maker et al., 1982; Squyres, 1982; Allison and Clifford, 1987). Inthis scenario, polygons of light material originate as broad fault-bounded grabens in dark material. This is followed by the volcanicextrusion of relatively clean (silicate-poor) liquid water, slush, orwarm ice to flood the broad graben and create light smooth mate-rial. A second phase of extension is then required after the extru-sion to produce the grooves observed within many polygons oflight material. The grooves were interpreted either as sets of nar-row subparallel graben (Golombek and Allison, 1981) or as cre-vasse-like tension fractures subsequently modified throughviscous relaxation and mass wasting (Squyres, 1982).

1.2. The post-Galileo perspective

Two fundamental objectives of Galileo imaging at Ganymedewere to fill the gaps in Voyager coverage of the surface and to ac-quire high-resolution data of characteristic features and terraintypes. It was anticipated that these data would enable more de-tailed observation of the cryovolcanic and tectonic processesaffecting light and dark material, and illuminate the interactionsamong these processes. However, using high-resolution imagingof dark material, no unequivocal observation could be made of lo-bate materials with an identifiable source vent or any other iden-tifiable morphology related to cryovolcanism (Prockter et al.,2000). Candidate cryovolcanic units identified from Voyager dataat lower resolution on the basis of embayment and texture insteadappear to be the result of fluidized impact ejecta (Pappalardo et al.,2004); moreover, dark smooth materials in topographic lows ap-pear to have accumulated by downslope movement of loose mate-rial (Prockter et al., 1998). Though evidence for volcanismassociated with dark material could potentially exist below thecurrent limits of resolution, the hypothesis that cryovolcanismplayed a primary role in the evolution of dark material is not sup-ported by existing imaging data (Pappalardo et al., 2004).

High-resolution images of light material have also lacked clearand abundant morphological evidence for lava flow fronts, sourcevents, embayment relationships, or any other evidence suggestiveof cryovolcanic emplacement. This suggests that if cryovolcanismon Ganymede were to result in the formation of such features, they

may be too subtle to be resolved in the data available, or they mayhave been destroyed by fracturing, impact erosion, or mass wast-ing. Instead, the high-resolution data supported a prominent rolefor tectonism in the formation of light materials. All light materialis modified to some extent by tectonism, from sets of faint parallellineaments to high-relief sets of parallel ridges interpreted to resultfrom tilt-block extensional faults (Pappalardo et al., 1998). Exami-nation of the boundaries between adjacent polygons of light mate-rial at high resolution supported the hypothesis that tectonismalone could alter the surface sufficiently to wipe out some preex-isting features (Pappalardo et al., 2004). Though the extent of vol-canism in the formation and evolution of light material remainssomewhat enigmatic, indirect evidence for volcanic resurfacinghas been identified in the form of small isolated caldera-like fea-tures (Lucchitta, 1980; Schenk and Moore, 1995; Head et al.,1998; Kay and Head, 1999; Spaun et al., 2001) and smooth, topo-graphically low bright lanes (Schenk et al., 2001).

In response to these new developments, we have compiled aglobal geological map of Ganymede that represents the most re-cent understanding of the satellite based on a combination of datafrom the Galileo and Voyager missions. This contribution builds onimportant previous accomplishments in the study of Ganymede(e.g., Lucchitta, 1980; Shoemaker et al., 1982; Murchie et al.,1986; Pappalardo et al., 1998; Prockter et al., 1998, and many oth-ers). The map will help to elucidate: (1) the major geological pro-cesses operating on Ganymede, (2) the characteristics of thegeological units making up its surface, (3) the stratigraphic rela-tionships of geological units and structures, and (4) the geologicalhistory inferred from these relationships.

2. Data

The Galileo spacecraft made six close encounters with Gany-mede (orbits G1, G2, G7, G8, G28, and G29), enabling the acquisi-tion of high-resolution imaging (�100 m/pixel and better) by theSolid State Imaging (SSI) camera. Due to the failure of Galileo’shigh-gain antenna, high-resolution imaging was concentrated ona few characteristic terrain and feature types. Since the high-reso-lution coverage of Ganymede is limited in area, Voyager data re-main central to understanding the satellite.

Lower-resolution images were obtained during the more dis-tant encounters of Ganymede during other Jupiter orbits to fill gapsin the Voyager coverage, most notably in the areas from 40�W to115�W and 245�W to 305�W longitude (see Carr et al., 1995).The illuminated leading hemisphere was best observed on orbitC9 (2 km/pixel) and the trailing hemisphere on orbit E6 (3.6 km/pixel). Information regarding all 14 Galileo orbits where Ganymederemote sensing data were successfully obtained are listed inSupplementary materials found in Bagenal et al. (2004).

Utilizing the best available data from the Galileo and Voyagermissions, the USGS assembled a global image mosaic of the surface(Becker et al., 2001) resampled at a resolution of 1 km/pixel(Fig. 2). To reach this uniform resolution target, 12% of the surfacearea imaged at slightly higher resolution was degraded. In general,the subjovian region was imaged by Voyager 1, the antijovian re-gion was imaged by Voyager 2, and the leading and trailing hemi-spheres between these areas were imaged by Galileo, as statedabove. Two additional areas imaged by Galileo at resolutions high-er than 1 km/pixel were also included in the mosaic. In areas whereimages from different spacecraft overlapped, higher-resolutionimages were placed on top of lower-resolution images, and near-terminator (high-incidence angle) images were placed on top ofhigh-Sun (low-incidence angle) images.

The imaging data that make up this mosaic vary widely withrespect to their lighting and viewing geometries. These

G.W. Patterson et al. / Icarus 207 (2010) 845–867 847

inhomogeneities can result in the incomplete recognition of terrainfeatures and subsequently hinder the characterization of geologicunits. We tested the effects of potential image-geometry basedmapping biases by counting the numbers of features and unitsmapped as a function of image resolution, incidence angle (anglebetween the Sun-surface vector and the surface normal vector),emission angle (angle between the surface-spacecraft vector andthe surface normal vector), and phase angle (angle between theSun-surface vector and the surface-spacecraft vector). While thereis likely to be natural variation in the numbers of different types offeatures from location to location, a systematic decrease in mappedfeatures with a change in imaging geometry would indicate incom-plete mapping of such features.

There are no trends in feature and unit recognition with respectto phase angle, so we will not consider this factor further. There is afactor of two decrease in groove features recognized at very low-incidence angles (<20�), with a corresponding increase in recog-nized furrow features. This can be explained because most of thelow-incidence angle images cover dark terrain (where furrows

Fig. 1. (Upper) Plot of the resolutions of image data across the surface of Ganymede. Highdata in light tones (white P5 km/pixel). (Lower) Plot of the emission angles associated wdark tones (black = 0� emission angle) and high emission angles in light tones (white = 9

are present and not grooves), and the only low-incidence angleareas primarily covering light terrain also have poor resolution.Since the same topographic shading factors that would makegrooves difficult to recognize at low-incidence angle should alsoaffect furrows, we believe that this decrease in groove recognitionwith lower incidence angle is primarily an artifact of low resolu-tion and happenstance of geographical location. Thus, from thegeometric factors that can influence the recognition of terrain fea-tures, it appears that low resolution and high emission angle arethe primary concerns for the global mosaic of Ganymede (Fig. 1).

Resolution does not have a strong influence on defining the ba-sic categories of geologic materials on Ganymede (i.e., light, dark,and impact in Section 3), though the ability to confidently outlinethe boundaries of these units drops off at resolutions >3 km/pixel(Fig. 2). The recognition of reticulate material (Section 3) and var-ious subunits (e.g., dark cratered unit, light irregular unit, etc. inSection 3) within broadly defined material units requires imageresolutions better than approximately 2 km/pixel. The ability torecognize linear features such as grooves and furrows drops off

er-resolution data is shown in dark tones (black60.5 km/pixel) and lower resolutionith image data across the surface of Ganymede. Low emission angles are shown in0� emission angle).

Fig. 2. (Upper) Plot of image resolution versus the percent of Ganymede’s surface that has been defined as dark or light undivided terrain. (Lower) Plot of emission angleversus the percent of Ganymede’s surface that has been defined as dark or light undivided terrain.


dramatically at resolutions >2.5 km/pixel, which is the half-widthof many of these topographic features. In the base mosaic, 94% ofthe area is covered by image resolutions better than 3.5 km/pixel,76% of the area is at resolutions better than 2.5 km/pixel, and74% of the area is at resolutions better than 2 km/pixel (Fig. 1).

At high emission angles, positive topography can obscure theview of adjacent negative topography. For example, slopes at theangle of repose oriented away from the camera will become fore-shortened with increasing emission angle, and disappear fromview entirely at an emission angle of about 60�. This effect primar-ily influences our ability to distinguish tectonic features such asgrooves within material units on the surface. The number ofmapped tectonic features begins to monotonically decline as afunction of emission angle at angles >30�, and beyond 45� thecount of tectonic features decreases to more than a factor of twobelow the baseline at low emission angles (Fig. 2). In the base mo-saic, 24% of the surface of Ganymede is viewed at emission angles<30�, and 48% of the surface is viewed at emission angles <45�(Fig. 1).

In general, the global geological map of Ganymede presentedhere (Plate 2) was produced directly from the digital image mosaic

of the satellite’s surface released by the USGS (Becker et al., 2001).However, higher resolution Galileo data not included in the mosaicwas often used, where available, to serve as a guide in locating fea-tures and unit boundaries in the lower resolution base mosaic.Additionally, two broad areas of the subjovian hemisphere wereimaged at better than 1 km/pixel on the final flyby of Ganymede(G29), after the USGS assembled the global base mosaic. This dataserved to elucidate relationships between features and units insome areas covered by low-incidence angle, high emission angleVoyager 1 data.

3. Material units

On the basis of our mapping, dark material on Ganymede hasbeen subdivided into three units: cratered, lineated, and undivided,while light material has been subdivided into four units: grooved,subdued, irregular, and undivided. The percentage of Ganymede’ssurface area covered by each of these material units is presentedin Table 1. We also recognize two other basic geologic units onGanymede, reticulate material and impact material. Impact mate-rial encompasses crater, palimpsest, and basin materials.

Table 1Area covered by mapped material units.

Map unit Number of mappedexposures

Area (106 km2) Proportion ofsurface areaa (%)

Dark materials 528 30.66 35.4Cratered unit (dc) 131 22.35 25.8Lineated unit (dl) 92 1.61 1.9Undivided unit (d) 305 6.70 7.7

Light materials 1355 55.59 64.1Grooved unit (lg) 571 12.27 14.1

Oldest (lg1) 106 2.17 2.5Intermediate (lg2) 226 4.98 5.7Youngest (lg3) 239 5.12 5.9

Subdued unit (ls) 519 15.58 18.0Oldest (ls1) 262 7.21 8.3Intermediate (ls2) 163 4.51 5.2Youngest (ls3) 94 3.86 4.5

Irregular unit (li) 156 3.19 3.7Oldest (li1) 94 2.07 2.4Intermediate (li2) 55 1.00 1.2Youngest (li3) 7 0.12 0.1

Undivided unit (l) 109 24.55 28.3

Reticulate material (r) 24 0.42 0.5

a Includes area of superposed impact materials contained within each unit.

1 For interpretation of color in Plate 2, the reader is referred to the web version ofthis article.


Palimpsest material is further subdivided into four units: three ofthese are distinguished by their stratigraphic relationship withlight material units and the fourth is an interior plains unit. Fiveunits comprise the crater materials: fresh, partially degraded, de-graded, unclassified, and ejecta, and two units comprise the basinmaterial: rugged and smooth.

In principle, geological map units are material units and shouldbe independent of structure (ACSN, 1961), and most terrestrialmappers adopt this approach. In planetary mapping, however,exceptions have commonly been made because of the local and re-gional dominance of structural features, the synoptic and remotesensing aspect of the data, and the scale of the mapping (e.g.,Wilhelms, 1972, 1990). On Earth, a rock-stratigraphic unit is de-fined as ‘‘a subdivision of the rocks in the Earth’s crust distin-guished and delimited on the basis of . . .observable physicalfeatures [commonly lithologic]. . . and independent from time con-cepts. . . and . . .inferred geologic history” (ACSN, 1961). In plane-tary mapping, stratigraphic units must be defined by remotesensing, so it is difficult to approach the process of geologic map-ping of planetary surfaces using a strict ‘lithologic’ definition ofunits. As outlined by Wilhelms (1972, 1990), planetary mappersuse the broader definition of rock-stratigraphic units as those ‘‘dis-tinguished and delimited on the basis of observable physical fea-tures”, which might include surface morphology, albedo, etc. (seealso Head et al., 1978).

Shoemaker et al. (1982) defined Ganymede’s grooved terrainas a major geologic unit, considering it ‘‘both lithologically andstructurally distinct from the ancient cratered terrain”. In addi-tion, they mapped additional structural subunits where ‘‘eachline is the boundary of a structural cell that contains a unifiedstructural pattern”. Later workers in the 1:5 M-scale geologicmapping program, such as Lucchitta et al. (1992), found thatthe ‘‘surface of Ganymede seems to be dominated by units ofsimilar compositions but diverse structural patterns”, and alsonoted that ‘‘many map units are subdivided mainly on the basisof structural differences. . .” and that ‘‘. . .locally the structuraldeformation may have been so intense or pervasive that it cre-ated a new and distinct material, distinguished from the parentmaterial by a different physical state rather than a differentcomposition”.

The use of structure to help characterize geological units, evenin the qualified manner described above, has been controversial(e.g., Hansen, 2000). This then raises the question, should such fea-tures be mapped as structures only, should they be mapped as geo-logical units, or is there some combination of approaches that isappropriate, perhaps depending on scale?

For the global geologic map of Ganymede, we believe a combi-nation approach is appropriate, one that is similar to that outlinedby Greeley et al. (2000) for the Europa global mapping effort, or byTanaka et al. (1994) for geologic mapping on Venus. This approachenables the maximum amount of geological information to beclearly conveyed by the map, with a minimum of confusion andextraneous symbols. At the 1:15 M scale of our map, furrows canbe distinguished from surrounding terrain and may be representedon it such that they do not obscure recognition of the underlyingunits. Therefore, furrows are mapped as separate structural fea-tures from the underlying material units described in the unit def-initions. However, in light material units, the grooves generallyhave small enough dimensions and spacing that representing themas separate structural features on the map would be illegible whenprinted at the 1:15 M scale. This necessitates their inclusion as partof the unit definitions for light material units, and instead the gen-eral trend of grooves is represented on the map. We also adopt theconvention started by Shoemaker et al. (1982) of mapping lightmaterial unit boundaries that enclose unified patterns of groovesthat are distinct from neighboring light materials. High-resolutionGalileo images have revealed that the boundaries between adja-cent polygons of light material generally occur at the edge of agroup of cross-cutting faults, and do not represent the transitionto a compositionally distinct material.

In the remainder of this section, we discuss the geologic unitsthat we have defined based on our mapping efforts. We identifythe characteristics that define each unit and, where applicable,its relative age with respect to other mapped units, its relation tounits previously mapped at the 1:5 M scale from Voyager data(e.g., Guest et al., 1988; Murchie and Head, 1989; Croft et al.,1990; Lucchitta et al., 1992; Wilhelms, 1997), and its type localityon the surface. Boundaries between dark, light, reticulate, and im-pact material units are generally distinct and have been identifiedbased on relative albedo and/or physical characteristics such asmorphology and/or the presence of distinctive structures (e.g.,reticulate material and light material). Units crossing image resolu-tion boundaries (Plate 1) that obscure the characteristics which de-fine them (e.g., when light material units lg, ls, and li transition inlight undivided material) or boundaries within undivided materi-als that are not distinct are shown in red (Plate 2).1 A summaryof unit and structure type localities can be found in Table 2. Craterdensity measurements for all units except light undivided, darkundivided, and crater materials are provided in Table 3 and dis-cussed in more detail in Section 5.

3.1. Dark material units

Dark material comprises 35% of Ganymede’s surface, and is sub-divided into three units for the global geologic map: a cratered unit(dc), a lineated unit (dl), and an undivided unit (d). Crater densities(Table 3) suggest that dark material units are the oldest surfaceunits on the satellite (Shoemaker et al., 1982; Murchie et al.,1989; Neukum, 1997; Neukum et al., 1998; Zahnle et al., 2003;Schenk et al., 2004). Dark material is heterogeneous in albedo atdecameter scales, probably resulting from thermally driven segre-gation of ice and non-ice surface components (Spencer, 1987a,b).

Plate 1. Global image mosaic of Ganymede incorporating Voyagers 1 and 2 and Galileo imagery.

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Plate 2. Geological map of Ganymede. Unit descriptions are given in the text.

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Galileo high-resolution images suggest that the dark material iscomposed of a relatively thin dark lag deposit of non-ice materialoverlying brighter icy material. This lag deposit has been concen-trated on the surface by processes such as sublimation, mass wast-ing, and ejecta emplacement (Prockter et al., 1998, 2000; Mooreet al., 1999; Oberst et al., 1999). While dark material is geologicallyuniform at global mapping scales, complex geological relationshipsincluding numerous distinct units have been recognized on localscales at high resolution (Prockter et al., 1998).

3.1.1. Dark cratered unit (dc)The dark cratered unit represents large areas of low albedo

material with moderate to high crater densities (Table 3), andmakes up the majority of all dark material on Ganymede (Table 1).It commonly occurs as polygons bounded by light material, and theboundaries with light materials appear generally sharp and aresometimes marked by troughs. Craters of all ages are superposed,with degraded craters (c1 – Section 3.4.1.3) in higher abundancethan on light materials. Furrows (Section 4.1) are common in thisunit, but areas lacking furrows are also present.

Dark cratered material is interpreted as the oldest preservedsurface on Ganymede. This material unit has been heavily modifiedvia impact processes (palimpsests, craters, basins, and related fur-rows) as well as sublimation and mass wasting to create the darklag deposit on the surface.

The dark cratered unit encompasses units variously mappedat the 1:5 M scale from Voyager images as cratered material, fur-rowed material, vermicular material, smooth material, and hum-mocky material (e.g., Guest et al., 1988; Murchie and Head,1989; Croft et al., 1990; Lucchitta et al., 1992; Wilhelms,1997). The inclusion of these various units into a single unit isprimarily the result of the scale of the global map and the exclu-sion of furrows from the unit definitions. The type locality forthe dark cratered unit is at 15�S, 337�W in Nicholson Regio(Fig. 3) and was imaged during the G28 encounter at 125 m/pixel.

3.1.2. Dark lineated unit (dl)The dark lineated unit is similar in character to the light

grooved (lg – Section 3.2.1) and light irregular (li – Section 3.2.3)units but with lower relative albedo, and grooves that appear gen-

Table 2Type localities for map units and structures.

Map unita Type location Observation Image Resolution (m/pixel)

Map unitsDark materials

Cratered unit (dc) 15�S, 23�E G28GSNICHOL02 s0552445300, s0552445313 125Lineated unit (dl) 24�S, 42�E G28GSCALDRA02 s0552445613 150

Light materialsGrooved unit (lg) 16�S, 50�E G28GSSOOTH02 s0552445100, s0552445113 116Subdued unit (ls) 16�S, 50�E G28GSSOOTH02 s0552445100, s0552445113 116Irregular unit (li) 32�N, 172�E G2GSTRANST01 s0359942426 188

Reticulate material (r) 28�S, 174�E G8GSCALDRA01 s0394532440 179

Impact materialsCrater materialsFresh crater unit (c3) 62�N, 12�W G7GSACHELS01 s0389923200, s0389923213 178Partially degraded crater unit (c2) 65�N, 12�W G7GSACHELS01 s0389923200, s0389923213 178Degraded crater unit (c1) 16�S, 15�E G28ARBELA02 s0552445426 130Crater ejecta unit (ce) 62�N, 12�W G7GSACHELS01 s0389923200, s0389923213 178

Palimpsest materials (p1, p2, and pu) 14�N, 157�E G8GSBUTOFC01 s0394532139, s0394532152 187Interior plains unit (pi) 14�N, 157�E G8GSBUTOFC01 s0394532139, s0394532152 187

Basin materialsRugged unit (br) 57�S, 130�W E12GSGLGMSH01 s0426117300 160

s0426117313Smooth unit (bs) s0426117326

s0426117339

StructuresFurrows Galileo Regio C2063659 1000Grooves 47�N, 156�E G2GSNIPPUR01 s0359944426, s0359944439 99Depressions 32�S, 170�E G8GSCALDRA01 s0394532478 179Domes 10�S, 174�E G8GSMELKRT01 s0394532265, s0394532278 181Secondary craters 24�N, 166�E G8GSPITCRA01 s0394532965 146

a Undivided units are not included.

Table 3Crater densities on mapped units.

Materialtypea

10 kmb 20 kmb 30 kmb Area of zone used forcounting (106 km2)

LightGrooved 39 ± 2

(44 ± 3)14 ± 1(14 ± 2)

8 ± 1 9.29 (5.71)

Irregular 30 ± 4(20 ± 5)

13 ± 3(5 ± 2)

6 ± 2 1.94 (0.99)

Subdued 42 ± 2(39 ± 3)

18 ± 1(15 ± 2)

9 ± 1 8.24 (4.90)

DarkCratered 85 ± 2

(97 ± 2)32 ± 1(34 ± 1)

15 ± 1 21.9 (16.3)

Lineated 67 ± 8(69 ± 8)

19 ± 4(20 ± 4)

8 ± 3 1.06 (1.01)

ReticulateReticulate 39 ± 12

(39 ± 12)18 ± 8(18 ± 8)

4 ± 4 0.28 (0.28)

ImpactPalimpsest 61 ± 7 23 ± 4 1.37Basin 19 ± 5 11 ± 4 0.80

a Undivided units are not included.b Number of craters P the quoted diameter, normalized to a counting area of

106 km2. Numbers in parentheses indicate values calculated from image data atresolutions <1.5 km/pixel.


erally shallower and more sinuous. The crater density on this unitis only slightly greater than on light materials (Table 3). This unitalways occurs adjacent to light grooved materials (lg), and oftenshares a common structural fabric with relatively older nearbylight material units (e.g., lg1 and li1). This implies that the tectonicforces that modified dark lineated material are related to those thatformed early light material units.

Dark lineated material is interpreted as dark cratered (dc) mate-rial that has undergone significant tectonic deformation. It is likely

to be a precursor unit for light materials, as evidenced by its phys-ical proximity to light material units with similar characteristics.The dark lineated unit mapped at this scale encompasses units pre-viously mapped at 1:5 M as dark lineated and dark grooved mate-rial (e.g., Guest et al., 1988; Murchie and Head, 1989). The typelocality for this unit is at �24�S, 318�W within dark material poly-gons interspersed within Harpagia Sulcus, east of the prominentcrater Enkidu (Fig. 4). This region was imaged during the G28encounter at 150 m/pixel.

3.1.3. Dark undivided unit (d)This unit encompasses all materials viewed at sufficiently low

resolution or high emission angle such that characteristics otherthen relative albedo cannot be confidently assessed at the scaleof this mapping effort, and hence they cannot be assigned to anyof the subdivisions of dark material. This unit also includes irregu-larly shaped patches and small slivers of low relative albedo mate-rial of indistinct morphology interspersed within light material, aswell as areas too small to be identified by criteria other than rela-tive albedo.

3.2. Light material units

Light material covers about 64% of Ganymede’s surface andforms swaths that crosscut older dark material, containing poly-gons tens to hundreds of kilometers wide, which form an intricatepatchwork across the surface. Light material is primarily subdi-vided by the density and orientation of structural grooves (Sec-tion 4.2) that exist within a given polygon. On the globalgeologic map, light material is subdivided into four units: agrooved unit (lg), a subdued unit (ls), an irregular unit (li), andan undivided unit (l). Except for the undivided unit, each of thelight material units has been further subdivided into three differ-ent relative age categories based on cross-cutting relationships

Fig. 3. (Upper) Type locality for dark cratered material. This image data was acquired during the G28 encounter at 120 m/pixel and is located at �15�S, 23�E in NicholsonRegio. (Lower) Portions of Plates 1 and 2 showing context at the resolution of the geologic map (left) and the surrounding geologic units (right). The location of the higher-resolution data is indicated in white.


(see Section 5). Category 3 (lg3, ls3, and li3) includes light materialsthat crosscut all adjacent light materials, category 1 (lg1, ls1, andli1) includes light materials that are crosscut by all adjacent lightmaterials, and category 2 (lg2, ls2, and li2) includes light materialsthat crosscut category 1 materials and are crosscut by category 3materials. Category 3 is dominated by light grooved material, whilecategory 1 is dominated by light subdued and light irregular mate-rial (Table 1). Light material has crater densities about half of thoseon dark material units (Table 3), confirming that it is relativelyyounger.

3.2.1. Light grooved unit (lg)The light grooved unit is defined as a high relative albedo mate-

rial that has a surface dominated by structural grooves. This unit isarranged in lanes or polygons, and within each of these areas, there

are roughly evenly spaced grooves and ridges oriented in a singledominant direction. Boundaries with other units are commonlysharp where the grooves cut across older terrain, or are crosscutby other groove sets. Grooves are roughly linear; locally (on thescale of kilometers) slightly curved, rarely sharply angled. This unitis bounded in places by long, relatively deep grooves.

Light grooved material is interpreted to form from dark, reticu-late, or other light material units via extensional tectonism, with orwithout prior cryovolcanic resurfacing. The extensional tectonismleads to the development of imbricate normal faulted tilt-blocks,which are superimposed on broader topographic ‘‘grooves” thatlikely result from extensional necking instabilities (Collins et al.,1998b; Dombard and McKinnon, 2001; Pappalardo et al., 2004).In a few locations where strain could be measured in this unit, itvaried from �15% to 100% extensional strain (Pappalardo and

Fig. 4. (Upper) The type locality for dark lineated material. This image data wasacquired during the G28 encounter at 150 m/pixel and is located at �24�S, 42�E aspart of dark material polygons interspersed within Harpagia Sulcus, east of theprominent crater Enkidu. (Middle) Portion of Plate 1 showing context at theresolution of the geologic map. (Lower) Portion of Plate 2 showing surroundinggeologic units. The location of the higher-resolution data is indicated in white.


Collins, 2005). As the fault blocks tilt during extension, dark surfacematerial is shed downslope and is currently observed in the bot-toms of the valleys. In this manner, it is possible that light materialcould form from dark material as the veneer that covers dark mate-

rial units is shed into topographic lows, exposing a brighter, moreice-rich substrate on the slopes. If cryovolcanism plays a role increating this unit, evidence for it is thoroughly overprinted by tec-tonism (Pappalardo et al., 2004).

The light grooved unit encompasses materials previouslymapped as grooved material (e.g., Guest et al., 1988; Murchieand Head, 1989; Croft et al., 1990). The type locality for this unitis located at 16�S, 310�W within Harpagia Sulcus, east of the prom-inent crater Enkidu (Fig. 5). This region was imaged during the G28encounter at 116 m/pixel.

3.2.2. Light subdued unit (ls)The light subdued unit consists of material with a moderate to

high albedo and is interspersed with light grooved (lg) and lightirregular (li) materials. It is arranged in polygons, similar to lightgrooved (lg) material, but it is characterized by a smooth surfacewith groove structures that are faint or undetectable at decame-ter to kilometer resolution. Where grooves are present, their mor-phology at high resolution may resemble horst and grabenstructures or they may be dark lineaments without obvious topo-graphic expression (Fig. 5). The light subdued unit typically hassharp boundaries with dark materials, while boundaries withother light material units range from sharp to transitional. Wherethe boundaries with other material units are sharp, they oftentake the form of long, relatively deep, linear to curvilineargrooves.

Light subdued material may form from dark material via exten-sional tectonism, similar to light grooved (lg) material, but theapparently low strain in this unit compared to light grooved mate-rials makes it less likely that tectonism alone could accomplish theresurfacing. Cryovolcanism may have played a more prominentrole in the formation of light subdued material then it has for lightgrooved (lg) material. Indirect evidence for cryovolcanic resurfac-ing in the form of small isolated caldera-like features are foundin association with this unit (Lucchitta, 1980; Schenk and Moore,1995; Head et al., 1998; Kay and Head, 1999; Spaun et al., 2001),and some areas of light subdued material have been found to occu-py topographic lows and have topographically level surfaces(Schenk et al., 2001).

This unit encompasses units previously mapped as smoothmaterial, fine material, and slightly grooved material (e.g., Guestet al., 1988; Wilhelms, 1997). The type locality for this unit is at�16�S, 309�W within Harpagia Sulcus, east of the prominent craterEnkidu (Fig. 5). This region was imaged during the G28 encounterat 116 m/pixel.

3.2.3. Light irregular unit (li)This unit is characterized by moderate to high albedo

material cut by grooves and is found in association with lightgrooved (lg) and light subdued (ls) materials. It is arranged inpolygons imprinted by grooves with irregular spacings and ori-entations. At high resolution, light irregular material could com-monly be divided into smaller subunits of light grooved (lg) orlight subdued (ls) material, but these subunits are too denselypacked to be separated at the scale of the global map. Whilethe boundaries of this unit are readily delineated fromsurrounding materials, the unmapped subunits within a givenexposure of light irregular material may be related to adjacentmapped light grooved (lg) and/or light subdued (ls) unitexposures.

Light irregular material consists of interwoven portions of lightsubdued (ls) and light grooved (lg) material. This unit encompassesmaterials previously mapped as irregular material (Lucchitta et al.,1992) and grooved material (e.g., Guest et al., 1988; Murchie andHead, 1989). The type locality for this unit is located at 32�N,188� along the boundary between Marius Regio and Nippur Sulcus

Fig. 5. (Upper) The type locality for light grooved and subdued material. This image data was acquired during the G28 encounter at 116 m/pixel and is located at �16�S, 50�Ewithin Harpagia Sulcus, east of the prominent crater Enkidu. (Lower) Portions of Plates 1 and 2 showing context at the resolution of the geologic map (left) and thesurrounding geologic units (right). The location of the higher-resolution data is indicated in white.


(Fig. 6). This region was imaged during the G2 encounter at 188 m/pixel.

3.2.4. Light undivided unit (l)This unit encompasses all moderate to high relative albedo

materials imaged at sufficiently low resolution or high emissionangle that characteristics other than albedo cannot be confidentlyassessed. Hence the material in this unit cannot be assigned to anyof the other subdivisions of light material. This unit also encom-passes areas with moderate to high relative albedo that have beenobscured by crater ejecta such that the texture of the underlyingtopography cannot be discerned.

3.3. Reticulate material (r)

This material represents a unique style of structural modifica-tion of both dark and light material units. Reticulate material iscommonly surrounded by light grooved (lg), light subdued (ls),and/or dark lineated (dl) units. Reticulate material is distinguishedfrom other materials by its variable albedo and the presence of twodominant sets of distinct grooves, typically oriented near-orthogo-nal to each other. Boundaries between reticulate material andother material units are typically sharp. Reticulate material is pre-dominantly found in the Sippar Sulcus region of Ganymede, withoutliers near the south pole and near the crater Serapis. This mate-

Fig. 6. (Upper) The type locality for light irregular material. This image data was acquired during the G2 encounter at 188 m/pixel and is located at 32�N, 172�E along theboundary between Marius regio and Nippur Sulcus. (Lower) Portions of Plates 1 and 2 showing context at the resolution of the geologic map (left) and the surroundinggeologic units (right). The location of the higher-resolution data is indicated in white.


rial has a crater density similar to light material units, suggestingcontemporaneous formation.

Reticulate material is comprised of dark and light material thathas been modified by the formation of near-orthogonal sets ofgrooves. At high resolution these grooves resemble horst and gra-ben structures. It has been suggested that the formation of reticu-late material may be the result of block rotation within adistributed shear zone initiated during light material formation(Murchie and Head, 1988). This unit encompasses materials previ-ously mapped as reticulate and dark reticulate material (Guestet al., 1988; Wilhelms, 1997).

The type locality for this unit is located at �32�S, 182� in theSippur Sulcus region, northwest of the prominent fresh crater

(c3) Osiris (Fig. 7). This region was imaged during the G8 encounterat 179 m/pixel.

3.4. Impact material units

Impact features are ubiquitous on Ganymede, and their distri-bution across the surface appears to be controlled by two primaryfactors. One is related to the distribution of light and dark materi-als, with dark materials having a higher density of craters thanlight materials (Table 3). The other results from a slight apex–ant-apex asymmetry in the density of craters associated with lightmaterials (Schenk and Sobieszczky, 1999; Zahnle et al., 2001). Im-pact materials have been subdivided for the global map into crater,

Fig. 7. (Upper) The type locality for reticulate material. This image data was acquired during the G8 encounter at 172 m/pixel and is located at �28�S, 174�E in the SippurSulcus region, northwest of the prominent fresh crater (c3) Osiris. (Lower) Portions of Plates 1 and 2 showing context at the resolution of the geologic map (left) and thesurrounding geologic units (right). The location of the higher-resolution data is indicated in white.


basin, and palimpsest materials based on distinct morphologicaldifferences.

3.4.1. Crater materialsCrater materials have been mapped globally for craters with rim

diameters greater than 30 km. Crater materials are divided into de-graded (c1), partially degraded (c2), and fresh (c3) units based onspecific morphological characteristics. Previous mapping effortsbased on Voyager data and utilizing a 1:5 M mapping scale haveused the degradation state of crater rims as a distinguishing char-acteristic of the relative age of craters (e.g., Guest et al., 1988; Mur-chie and Head, 1989; Croft et al., 1990; Lucchitta et al., 1992;Wilhelms, 1997). As a result of the scale of the global map andthe lighting and viewing geometry issues associated with the datapresently available (Section 2), we are not confident that rim deg-radation state can be consistently determined across Ganymede.

Therefore, we concentrate on two distinguishing characteristicsto categorize craters on Ganymede: relative albedo contrast withrespect to surrounding materials, and the presence or absence ofrays and continuous ejecta deposits. Other subdivisions of cratermaterials include an unclassified (cu) unit to account for craterswhose distinguishing characteristics cannot be determined, andan ejecta material (ce) unit. Overall, 908 craters greater than30 km diameter were mapped: 480 in the c1 unit, 263 in the c2

unit, 95 in the c3 unit, and 70 in the cu unit.Structures found within the craters vary with increasing crater

diameter from central peaks, to central pits, to central domes. Thediameters at which craters transition from one type of interiorstructure to another does not depend on the relative age of the cra-ters. However, it has been suggested that dome morphology has arelationship with the relative ages of craters (Schenk et al., 2004).This is consistent with the global map of Ganymede where 29 of 32

Fig. 8. (Upper) The type locality for degraded (c1) craters (a) taken from the globalmosaic of the satellite (km/pixel scale) and (b) as imaged during the G28 encounterof the Galileo mission at 130 m/pixel. This crater is located at 16�S, 15�E withinNicholson Regio and in close proximity to Arbela Sulcus. (Middle) Portion of Plate 1showing context at the resolution of the geologic map. (Lower) Portion of Plate 2showing surrounding geologic units. The location of the higher-resolution data isindicated in white.


‘anomalous domed’ craters (Ganymede crater database, availableat www.lpi.usra.edu) belong to the degraded crater unit (c1) (Kayet al., 2007).

3.4.1.1. Degraded crater unit (c1). Degraded craters typically haverelative albedos that are similar to surrounding material, and theyare found predominantly in association with dark material units.This unit lacks the presence of rays and continuous ejecta andhas a subdued interior morphology that typically resembles thetarget material. The type locality for this unit is located at 16�S,345� within Nicholson Regio, in close proximity to Arbela Sulcus(Fig. 8). This region was imaged during the G28 encounter at130 m/pixel.

3.4.1.2. Partially degraded crater unit (c2). Partially degraded craterscan have albedos higher or lower than the surrounding materialand are found in association with a variety of other material units.This unit lacks rays but has a readily observable deposit of contin-uous ejecta surrounding the crater rim. The type locality for thisunit is the crater Gula located at �65�N, 12� northeast of PerrineRegio and north of Aquarius Sulcus (Fig. 9). This region was imagedduring the G7 encounter at 178 m/pixel.

3.4.1.3. Fresh crater unit (c3). Fresh craters are typically character-ized by a strong albedo contrast with respect to the surroundingsurface (higher or lower) and are found in association with a vari-ety of other material units. Ray systems (commonly bright, occa-sionally dark) and continuous ejecta are readily observed. Thetype locality for this unit is the crater Achelous located at �62�N,12� northeast of Perrine Regio and north of Aquarius Sulcus(Fig. 9). This region was imaged during the G7 encounter at178 m/pixel.

3.4.1.4. Unclassified crater unit (cu). This unit encompasses any cra-ter materials found in regions imaged at sufficiently low resolutionor high emission angle that the characteristic properties that dis-tinguish degraded (c1), partially degraded (c2), and fresh (c3) cratermaterials cannot be determined.

3.4.1.5. Crater ejecta unit (ce). Crater ejecta represents material thatcontinuously blankets surrounding preexisting terrain concentricto the crater with which it is associated. This unit is observed inassociation with partially degraded (c2) and fresh (c3) crater mate-rials (Fig. 9).

3.4.2. Palimpsest materialPalimpsest material is characterized by moderate to relatively

high albedo material forming flat, generally circular to slightlyelliptical structures. It is interpreted to be the result of impacts intothe crust of Ganymede during a time when there was a higher ther-mal gradient and/or a thinner brittle lithosphere (Shoemaker et al.,1982). Palimpsests superpose dark material units but can super-pose or be superposed by light material units. Palimpsests are typ-ically superposed on furrows, but some furrows can remain visibleon their outer margins. The surface texture of palimpsest materialsis smooth to hummocky and can be locally rugged. Palimpsestslack rims, but the presence of outward facing scarps and internal,concentric ridges are common. The centers of some palimpsestsare characterized by smooth, circular to subcircular patches of highalbedo material.

Similar to crater materials, previous mapping efforts based onVoyager data at the 1:5 M mapping scale used the apparent struc-tural complexity and degradation state of palimpsest interiors asdistinguishing characteristics of the relative age of palimpsests(e.g., Guest et al., 1988; Murchie and Head, 1989; Lucchitta et al.,1992). As a result of the scale of the global map and the lightingand viewing geometry issues associated with the data presentlyavailable (Section 2), we are not confident that these characteris-tics can be consistently determined for all palimpsest materials.Therefore, we have categorized palimpsests based on their cross-

http://www.lpi.usra.edu

Fig. 9. (Left) The type locality for partially degraded (c2) and fresh (c3) crater material are the named craters Gula and Achelous respectively. This image data was acquiredduring the G7 encounter at 178 m/pixel and is located at �62�N, 12�W northeast of Perrine Regio and north of Aquarius Sulcus. (Right) Portions of Plates 1 and 2 showingcontext at the resolution of the geologic map (top) and the surrounding geologic units (bottom). The location of the higher-resolution data is indicated in white.


cutting relationships with light materials. This scheme leads tothree subdivisions of palimpsest materials. Ancient palimpsests(p1) are superposed by light materials, young palimpsests (p2)superpose light materials, and unclassified palimpsests (pu) donot have a discernible relationship with light materials. We havealso defined a palimpsest interior plains (pi) unit to represent theinterior smooth patches evident within some palimpsest interiors.In all, 54 palimpsests were mapped: 22 in the p1 unit, 5 in the p2

unit, and 27 in the pu unit.The type locality for all palimpsest materials is Buto Facula lo-

cated at �14�N, 203� within Marius Regio and northeast of TiamatSulcus (Fig. 10). This region was imaged during the G8 encounter at187 m/pixel.

3.4.3. Basin materialBasin material encompasses the deposits of Gilgamesh basin

(�590 km in diameter) centered at 57�S, 130� (Fig. 11). Schenket al. (2004) suggest that the Gilgamesh basin can be divided intothree concentric zones. The central zone is a low dome that risesasymmetrically �500 m in elevation and is surrounded by a dis-continuous inward-facing quasi-concentric scarp �1 km high. Thisis in turn surrounded by an annulus of material that is character-ized by hummocks punctuated by rugged, somewhat angular mas-sifs and quasi-concentric but discontinuous ridges. This zone isroughly bound by a prominent contiguous concentric inward-fac-ing scarp �1 km high, which has been interpreted as the rim of

the basin (Schenk et al., 2004). An exterior annular zone of materialis also present and can be characterized by a mottled texture. It hasbeen suggested that this mottled material has modified or mantledpreexisting light material and represents the continuous ejecta de-posit of Gilgamesh (Schenk et al., 2004).

These morphologic subdivisions of Gilgamesh basin are repre-sented on the global map by a basin interior plains (bi) unit forthe central zone, a basin rugged (br) unit for the interior annularzone, and a basin smooth (bs) unit for the exterior annular zone.Differing somewhat from previous estimates (Schenk et al.,2004), we measure the diameter of the interior plains unit to be�80 km and the widths of the interior and exterior annuli to beapproximately 200 and 60 km, respectively. A portion of Gilga-mesh basin was imaged at high resolution (160 m/pixel) and highsolar incidence angle during the E12 encounter of the Galileo mis-sion, though most of our knowledge of Gilgamesh is derived fromof over half of the basin by low-incidence angle Voyager 2 imagingat 1 km/pixel resolution.

4. Structural features and landforms

4.1. Furrows

Furrows are the oldest recognizable structures on the surface ofGanymede, occurring only on dark material and predating essen-tially all craters larger than 10 km in diameter (Passey and Shoe-

Fig. 10. (Upper) Buto Facula, the type locality for all palimpsest materials. This image data was acquired during the G8 encounter at 187 m/pixel and is located at �14�N,157�E within Marius Regio and northeast of Tiamat Sulcus. (Lower) Portions of Plates 1 and 2 showing context at the resolution of the geologic map (left) and the surroundinggeologic units (right). The location of the higher-resolution data is indicated in white.


maker, 1982). The majority of the furrows are arranged in regionalsets which are close to being concentric around a point, but a fewfurrows crosscut the regional sets at high angles (Cassachia andStrom, 1984; Schenk and McKinnon, 1987; Murchie and Head,1988). They date to an epoch when Ganymede had a higher ther-mal gradient (Nimmo and Pappalardo, 2004) with a thinner brittlelithosphere, overlying a deeper ductile ice interior, in turn likelyabove liquid water (Schenk, 2002; Schenk et al., 2004). Becausedark material is disrupted and resurfaced by younger swaths oflight material, the record of ancient furrow systems is incomplete.When intact, however, the largest known system (the Lakhmu Fos-sae in Galileo Regio) would have been hemispherical in scale(Schenk and McKinnon, 1987).

Individual furrows (Fig. 12) are linear to curvilinear troughsbounded by raised rims, which are generally bright. They are rep-resented on the global map as lines drawn on dark material units.

They extend from tens to hundreds of kilometers in length and aretypically �6–20 km wide, with generally flat or u-shaped floorsand sharp raised rims (Smith et al., 1979a; Shoemaker et al.,1982; Prockter et al., 1998). Inter-furrow spacing is fairly uniformat �50 km, although spacing is generally closer towards the centerof a concentric system (Passey and Shoemaker, 1982). Topographicmodels derived from high resolution stereo images within GalileoRegio show that one furrow rim rises a full kilometer above thefurrow floor and 900 m above the level of the surrounding terrain(Prockter et al., 1998), consistent with estimates from broad-scaleshadow measurements of furrow depth (Murchie and Head, 1988).

Voyager-era mapping and photogeological analysis of the fur-row systems led to a variety of models for their formation (e.g.,Cassachia and Strom, 1984; Schenk and McKinnon, 1987; Murchieet al., 1990). On the basis of morphology and planform, along withtheir similarity to multi-ringed structures on Europa and Callisto

Fig. 11. The type locality for basin materials: Gilgamesh basin (57�S, 130�W). (Left) Portion of Plate 1 showing the basin at the resolution of the geologic map. (Right) Portionof Plate 2 showing the basin and surrounding geologic units.

Fig. 12. Furrows within Galileo Regio, as imaged by Voyager 2 at �1 km/pixel.Furrows of the Lakhmu Fossae system trend NW–SE in the image, while the ZuFossae system trends N–S cutting them obliquely. North is up in the image.


(Moore et al., 1998, 2001; Kadel et al., 2000; Schenk, 2002), theyare now generally accepted to be fault-induced troughs formedin response to large impacts into a relatively thin lithosphere earlyin Ganymede’s history (McKinnon and Melosh, 1980). Individualfurrows probably formed rapidly during basin collapse, as the re-sult of asthenospheric flow radially inward toward the impactpoint, with accompanying brittle failure of the overlying litho-sphere (McKinnon and Melosh, 1980; Melosh, 1982; Moore et al.,1998, 2001). The type and extent of the resulting fault patternwould have depended on the scale of the impact and the rheolog-ical structure of the satellite at the time of formation. Many fur-rows apparently have been reactivated and modified by latertectonic activity (Murchie et al., 1990; Prockter et al., 2000). Ithas been proposed that raised furrow rims formed when thelong-wavelength components of fault-induced relief relaxed,

resulting in the flexural and/or viscous uplift of the boundingescarpments (McKinnon and Melosh, 1980; Nimmo and Pappa-lardo, 2004).

4.2. Grooves

Light material is typically modified by sets of subparallel ridgesand troughs, commonly referred to as ‘‘grooves”. The formation ofthese grooves has been attributed primarily to extensional tecto-nism (Shoemaker et al., 1982; Pappalardo et al., 1998, 2004). Themorphology of grooves in light grooved (lg) material has beenshown to be generally characteristic of normal fault formationsuperimposed on pinches and swells caused by the formation ofnecking instabilities as the lithosphere was extended (Collinset al., 1998b; Dombard and McKinnon, 2001; Bland and Showman,2007). The presence of horst and graben style normal faults hasalso been inferred (Pappalardo et al., 1998, 2004) and is typicallyassociated with light subdued (ls) material units. Individualgrooves within light material units are not represented on the glo-bal map, as they are too densely packed to be legible at the 1:15 Mscale. Instead, the general trend of grooves within a polygon oflight material is represented on the map with a short line orientedto illustrate the trend (Plate 2).

Grooves associated with light grooved (lg) material exhibit twosuperposed spacing scales of grooves. The larger-scale grooves arespaced between 5 and 10 km apart, as derived from Fourier analy-sis of photometric profiles across groove sets (Grimm and Squyres,1985; Patel et al., 1999). These grooves appear in low solar inci-dence angle images as dark and bright stripes, while in near-termi-nator images the topography of the grooves is revealed to bebroadly sinusoidal in cross-section. Galileo topographic data sug-gests a close correlation between albedo and topography on localscales (Oberst et al., 1999), and the albedo striping observed atlow-incidence angles in light grooved materials closely correlatesto topography (Collins et al., 1998b), with the dark lineamentsmarking the locations of broad topographic lows.

Galileo spacecraft images of light grooved material at betterthan 100 m/pixel also reveal smaller-scale grooves, with a mean

Fig. 13. Image mosaic acquired during the G2 encounter at 99 m/pixel illustratingthe morphology of grooves observed in light grooved and subdued material units.The grooves oriented NW–SE (north is up in the image) are part of Nippur Sulcus.They are indicative of imbricate, normal faulted grooves. This morphology istypically classified as light grooved material. A lane of very smooth material marksthe boundary between Nippur and Philus Sulci. Philus Sulcus displays grooves witha horst-and-graben-like morphology. This morphology is typically classified as lightsubdued material. Philus Sulcus is bound to the south, in this image, by darkcratered material associated with Marius Regio.

Fig. 14. Oblique view of a depression found within Sippar Sulcus and acquiredduring the G8 encounter at 179 m/pixel. This feature has a surface texture that maybe indicative of flow toward its open end, consistent with it being a source regionfor icy volcanic material. Here south is up.

Fig. 15. A dome associated with the crater Melkart, on the boundary between darkmaterial of Marius Regio (to the northeast) and light material. This image data wasacquired during the G8 encounter at 181 m/pixel and is located at 10�S, 174�E.


spacing on the order of �1 km (Belton et al., 1996; Patel et al.,1999) superposed on the larger grooves. For example, Fig. 5 showsa high resolution image of light terrain superimposed on a lowerresolution image. Following a large-scale groove across this bound-ary, one may observe many more ridges and troughs in the higherresolution image. In topographic lows, the small-scale grooves aresmaller and more closely spaced, and on topographic highs thesmall-scale grooves are more widely spaced (Pappalardo et al.,1998). These small-scale grooves are likely widespread across Gan-ymede’s grooved terrain and should be considered as commonlycharacteristic of the mapped larger-scale grooves of light grooved(lg) material. The most plausible mechanism to form these small-scale grooves, based on their morphology, is deformation of thesurface by tilt-block extensional faulting (Pappalardo et al.,1998). An example of this material may be seen in the top half ofFig. 13. Analysis of fault scarp geometry (Collins et al., 1998b)

and impact craters cut by these fault sets (Pappalardo and Collins,2005) indicates that extensional strains of 50% and over are typicalof well-developed light grooved material.

Grooves within polygons of light subdued (ls) material com-monly have a morphology at high resolution resembling grabenand intervening flat-topped horst ridges (e.g., in the bottom halfof Fig. 13). Rather than downdropping along prominent and dis-tinct bounding faults, offset appears to have often occurred bycumulative displacement along several sub-kilometer scale frac-tures or faults which are pervasive across these grooved terrains(Pappalardo et al., 2004). A few measurements of strained craters(Pappalardo and Collins, 2005) indicates that extensional strainsof 15% and below are typical in light subdued material, and inmany areas the strain is too small to measure.

4.3. Depressions

At least 30 depressions with scalloped walls (‘‘paterae”) havebeen identified on Ganymede. These depressions have beeninterpreted to represent caldera-like source vents for icy volca-nism (Lucchitta, 1980; Schenk and Moore, 1995; Head et al.,

Fig. 16. Secondary craters found within the crater Lugalmeslam, situated on thesouthwestern margin of Nippur Sulcus (146 m/pixel; G8GSPITCRA01). Clusters ofsecondaries are observed to the northeast and west of the crater center.


1998; Kay and Head, 1999; Schenk et al., 2001; Spaun et al.,2001).

High-resolution Galileo images show that the largest patera ofseveral within Sippar Sulcus (Fig. 14) is associated with a ridgeddeposit in its interior, possibly an icy flow (Head et al., 1998).Stereo data show that the rim elevation of this patera reachesup to 800 m above surrounding light material, with floor depos-its of similar elevation to surrounding light material (Schenket al., 2001). Preexisting grooves continue unmodified up topatera rims, suggesting that the elevated rims probably formedthrough isostatic adjustment and are not the result of construc-tional volcanism. Evidence for embayment relationships andtruly smooth regions that might indicate icy volcanism has beenelusive in high-resolution images of light material. However,some limited embayment is suggested in the Sippar Sulcus re-gion surrounding the paterae (Head et al., 1998; Schenk et al.,2001).

Fig. 17. Correlation of map units. Younger units are shown above older units; diagonal luncertain boundary ages.

4.4. Domes

Domes on Ganymede are features associated exclusively withthe interiors of craters having diameters between �60 km and�175 km (Fig. 15). They are typically circular in map view and oc-cur within central pits (Fig. 15). In cross-section they are steep-sided with flat-topped profiles that can reach heights of up to�1.5 km above surrounding materials (Schenk et al., 2004). Domesare commonly surrounded by a trough, which in turn is sur-rounded by an annular ridge or ring of rugged massifs that havebeen interpreted as structurally equivalent to pit rims in smallercraters (Schenk et al., 2004). At high resolution, web-like networksof narrow fractures are commonly visible on the top surfaces of do-mes. Domes are interpreted to result from ductile icy materialbeing pushed upward from the subsurface during the modificationstage of large impact crater formation (Moore and Malin, 1988)suggested that these domes might be diapiric whose rise was ini-tiated by post impact subsurface adjustments.

4.5. Secondary craters

Secondary craters are represented by fields of uniform smallpits surrounding large fresh (c3) and partly degraded craters (c2),palimpsests (p1, p2, and pu), and basin materials (bs and br). Theycommonly occur as clusters or chains oriented radially or subra-dially to the centers of craters, palimpsests, and the Gilgamesh ba-sin. The morphology of secondary craters can include shallow,circular to elongate craters, or irregular bowl-shaped craters thatoverlap in chains or clusters (Fig. 16). Secondary craters are inter-preted to form from blocks ejected from a primary crater duringthe crater formation process. The locations of individual secondarycraters observable at the scale of the map have indicated (Plate 2).

5. Relative age relationships

To determine relative age relationships among the various unitsof the global map of Ganymede (Plate 2), we utilized observedcross-cutting relationships and crater density measurements (Ta-ble 3). The conclusions drawn from these relationships and craterdensity measurements are broadly supported by previous crater

ines represent transitional or overlapping boundaries; ‘saw-tooth’ pattern indicates


counting efforts based on Voyager data (e.g., Shoemaker et al.,1982; Murchie et al., 1989).

Dark cratered materials have the highest crater densities of themapped units for Ganymede (Table 3) and are consistently super-posed by all other units (Fig. 17). Based on these characteristics,dark cratered material is recognized as the oldest geologic unitpresent on the surface. Dark lineated material has a crater densitysimilar to light material units. This supports the interpretation thatit is a transitional material that has undergone part of the resurfac-ing process that created light material units by the tectonization ofdark cratered material (Pappalardo et al., 2004).

Light material crater densities are approximately half those ofdark cratered material (Table 3) and consistently superpose darkmaterials and some of the oldest impact materials (c1 and p1). Cra-ter density differences among the different light material units arenot significant, and so cannot be used to determine the relativeages of these units. Previous relative age mapping in the Uruk Sul-cus region showed that light subdued (ls) material is often olderthan light grooved (lg) material (Collins et al., 1998a). However,another study of topography and cross-cutting relationships in asmall area imaged at high resolution in the Sippar Sulcus regionfound that light subdued (ls) material is the youngest of the lightmaterial units (Schenk et al., 2001). On the global map, we havedetermined the relative age relationships among all light materialunits based on cross-cutting relationships and divided the lightmaterial into three relative age categories (see also Collins, in prep-aration). The oldest category contains light material that is crosscutby all adjacent light materials, the youngest category contains lightmaterial that crosscuts all adjacent light materials, and the inter-mediate category crosscuts the oldest and is crosscut by the youn-gest adjacent light materials. Since these age categories are basedon local cross-cutting relationships, we cannot say that all thematerial within an age category formed at the same time, norcan we definitively assess the relative age of the oldest light mate-rial on one side of Ganymede versus the youngest light material onthe other side of Ganymede. However, there are a few groove setsthat are continuous for thousands of kilometers, which help to tietogether regional age relationships and give us some confidencethat the age categories are broadly consistent across Ganymede.The surface area covered by light units divided in different age cat-egories is roughly even: the oldest light units cover 13.2% of thesurface, intermediate age light units cover 12.1% of the surface,and the youngest light units cover 10.5% of the surface (see Ta-ble 1). Within the oldest light units, 19% is light grooved material,63% is light subdued material, and 18% is light irregular mate-rial. Within the youngest light units, 56% is light groovedmaterial, 43% is light subdued material, and 1% is light irregularmaterial. Thus there appears to be a shift through time towardsthe formation of light material with more tectonic overprinting.

Voyager era geological mapping of reticulate material suggestedthat, based on cross-cutting relationships, reticulate materialsformed shortly before or during the initiation of light material for-mation (Guest et al., 1988; Wilhelms, 1997). Schenk et al. (2001)indicate that reticulate material is older than light material unitswithin the Sippar Sulcus region based on cross-cutting relation-ships and topographic data. Based on our mapping, the variable al-bedo of reticulate material suggests that the tectonic event thatresurfaced it has affected both light and dark materials. Crater den-sity measurements indicate reticulate material formation is near-contemporaneous with light material formation (Table 3). Reticu-late material is always crosscut by adjacent light material, whichindicates a relative age similar to or slightly older than the oldestage category of light material units.

Impact materials span the surface history of Ganymede. Almostall palimpsest materials are found within dark cratered materialunits, and crater density measurements (Table 3) indicate that they

are generally older than light materials. The identification of fivepalimpsests that superpose light material units suggests thatpalimpsest formation continued during the early stages of lightmaterial formation. Basin materials superpose light material andhave crater densities consistent with being younger than lightmaterial units (Table 3).

The determination of absolute ages for material units on Gany-mede is complicated by uncertainty regarding the impactor popu-lation and flux in the jovian system. Neukum (1997) and Neukumet al. (1998) have suggested that the production function for Gan-ymede is lunar-like (asteroidal) and assign ages of 3.6–4.2 Gyr forlight and dark materials respectively. Others (Shoemaker andWolfe, 1982; Zahnle et al., 1998, 2003) suggest the flux of impactsto Ganymede is wholly different (cometary) and assign a muchyounger age for light material (�2 Gyr). In general, it is agreed thatdark cratered material on Ganymede represents an ancient surfacethat formed shortly after the formation of the planets (4.5 Gyr ago).The current best guess from crater statistics is that light materialsformed at some time during the middle half of Solar System his-tory, but obtaining an exact age is likely to remain elusive for along time.

6. Summary

Data obtained from the Galileo mission has changed our under-standing of the surface of Ganymede, especially the roles of cryo-volcanic and tectonic processes in creating light materials. TheGalileo mission also filled in (at modest resolution) some gaps inthe Voyager imaging coverage of Ganymede, clarifying the rela-tionships between material units on the subjovian and antijovianhemispheres. In response to these developments, we have com-piled a global geological map of the satellite utilizing a global im-age mosaic assembled from the best available data of the Galileoand Voyager missions, with a nominal resolution of 1 km/pixel.This map represents our most recent understanding of Ganymedebased on Galileo mission data.

On the basis of our mapping, we recognize four fundamentalmaterial types: dark material, light material, reticulate material,and impact material. Dark material on Ganymede has been subdi-vided into cratered (dc), lineated (dl), and undivided (d) units.Light material has been subdivided into grooved (lg), subdued(ls), irregular (li), and undivided (l) units. Reticulate material repre-sents a single unit and impact material encompasses crater,palimpsest, and basin materials. Crater materials are subdividedinto degraded (c1), partially degraded (c2), fresh (c3), unclassified(cu) and ejecta (ce) units. Palimpsest material is subdivided intoold palimpsests (p1), young palimpsests (p2), unclassified palimps-ests (pu), and interior plains (pi) units and basin materials are sub-divided into rugged (br) and smooth (bs) units.

Crater density measurements (Table 3) suggest that the highestcrater densities are found on dark cratered materials. Light materi-als on Ganymede have a much lower density of craters, supportingthe view that they formed substantially later. Dark lineated mate-rial and reticulate material have crater densities similar to lightmaterial units, suggesting they mark a transition into the forma-tion of light materials. Palimpsests are older than light materials,dark lineated material, and reticulate material, but younger thandark cratered material. Finally, the Gilgamesh basin appears to beyounger than light materials.

The post-Galileo global geological map of Ganymede presentedhere will serve as an observational benchmark to provide con-straints on models for the formation and evolution of Ganymede.The geological history of Ganymede can be looked upon as a touch-stone for comparing and contrasting the characteristics and evolu-tion of other large to mid-sized icy satellites. This geological map


provides a post-Galileo synthesis of the history of this importanticy satellite that will be extremely useful as a frame of referencefor the future exploration of the jovian system.

Acknowledgments

We thank Ken Tanaka and Jeff Moore for their careful review ofthis manuscript and the useful comments they provided. This workwas supported under a grant awarded through NASA’s PlanetaryGeology and Geophysics program (NNG05GJ787G).

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