Global geological mapping of Ganymede

Icarus 207 (2010) 845–867

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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, USA Department of Physics and Astronomy, Wheaton College, Norton, MA 02766, USA c Department of Geological Sciences, Brown University, Providence, RI 02912, USA d Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA e United States Geological Survey, Flagstaff, AZ 86001, USA f Department of Geological Sciences, University of Idaho, Moscow, ID 83844, USA b

a r t i c l e

i n f o

Article history: Received 29 July 2009 Revised 25 November 2009 Accepted 28 November 2009 Available online 6 December 2009 Keywords: Ganymede Satellites, Surfaces Jupiter, Satellites

a b s t r a c t We have compiled a global geological map of Ganymede that represents the most recent understanding of the satellite based on Galileo mission results. This contribution builds on important previous accomplishments in the study of Ganymede utilizing Voyager data and incorporates the many new discoveries that were brought about by examination of Galileo data. We discuss the material properties of geological units defined utilizing a global mosaic of the surface with a nominal resolution of 1 km/pixel assembled by 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 also use crater density measurements obtained from our mapping efforts to examine age relationships amongst the various defined units. These efforts have resulted in a more complete understanding of the major geological processes operating on Ganymede, especially the roles of cryovolcanic and tectonic processes in the formation of might materials. They have also clarified the characteristics of the geological units that comprise the satellite’s surface, the stratigraphic relationships of those geological units and structures, and the geological history inferred from those relationships. For instance, the characteristics and stratigraphic relationships of dark lineated material and reticulate material suggest they represent an intermediate stage between dark cratered material and light material units. Ó 2009 Elsevier Inc. All rights reserved.

1. Introduction The surface of Ganymede can generally be divided into two material types that exhibit differences in albedo, crater density, and surface morphology. Approximately one-third of the surface is covered by ‘‘dark” material, which is heavily cratered, covered with relatively low albedo regolith, and commonly transected by large-scale arcuate fracture systems termed furrows. Crater density measurements suggest that dark material represents the oldest preserved surfaces on Ganymede, though it generally has lower 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 covered by vast globe-encircling swaths of ‘‘light” material, which has a surface with higher relative albedo and significantly lower crater density than dark material. The swaths of light material are them-

* Corresponding author. Address: Planetary Exploration Group, Applied Physics Laboratory, MP3-E106, 11100 Johns Hopkins Rd., Laurel, MD 20723-6099, United States. E-mail address: [email protected] (G.W. Patterson). 0019-1035/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2009.11.035

selves divided into elongated and polygonal shaped areas. Viewed at resolutions >500 m/pixel, these areas may appear relatively smooth or they may be modified by troughs termed grooves. It has been suggested that light material formed predominantly through the modification of dark material by tectonic and cryovolcanic resurfacing processes (e.g., Pappalardo et al., 2004 and references therein). The dichotomy between these two basic terrain types leads to a number of fundamental questions about the formation and evolution of the surface of Ganymede. What is the origin of the albedo heterogeneity of the surface? How have dark and light materials evolved through time? What internal forces led to the formation of tectonic structures like furrows and grooves? Does the formation of grooves primarily reflect a local or global stress regime and how has that stress regime changed through time? What are the properties of craters on Ganymede, and what are the relative age relationships among geologic features on the surface? Understanding the geological record of Ganymede is crucial to answering these questions. In this paper, we present a global geologic map of Ganymede based on combined Voyager and Galileo data and explore its implications for the geologic history of the satellite.

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1.1. The Voyager perspective Both Voyager spacecraft observed Ganymede during their Jupiter flybys (Smith et al., 1979a,b). However, data of sufficient resolution (better than 2 km/pixel) to allow geologic interpretation was limited primarily to areas surrounding the subjovian and antijovian points. A synthesis of the results from these two spacecraft is provided by Shoemaker et al. (1982) and McKinnon and Parmentier (1986). From these data, a series of geologic maps covering the surface were produced at the 1:5 M scale. These maps represent the application of Voyager-based geologic interpretations. Based on Voyager images, dark material on Ganymede was interpreted to represent a heavily cratered, primordial surface modified by cryovolcanic activity (Croft and Strom, 1985; Croft and Goudreau, 1987; Murchie et al., 1989; Croft et al., 1990). Cryovolcanism as a modification process was supported by an apparent absence of small craters, embayment relationships observed in association with large craters, and smooth areas associated with furrows (Cassachia and Strom, 1984; Murchie et al., 1990; Lucchitta et al., 1992). Schenk and Moore (1995) proposed that volcanic activity on Ganymede included extrusion of icy materials into crater floors to form lobate domes and into furrow floors to create smooth dark deposits. Light material was interpreted to represent regions where dark material had been resurfaced by cryovolcanic flows, which were subsequently tectonized in some areas to form grooves (Golombek and Allison, 1981; Golombek, 1982; Parmentier et al., 1982; Shoemaker et al., 1982; Squyres, 1982; Allison and Clifford, 1987). In this scenario, polygons of light material originate as broad faultbounded grabens in dark material. This is followed by the volcanic extrusion of relatively clean (silicate-poor) liquid water, slush, or warm ice to flood the broad graben and create light smooth material. A second phase of extension is then required after the extrusion to produce the grooves observed within many polygons of light material. The grooves were interpreted either as sets of narrow subparallel graben (Golombek and Allison, 1981) or as crevasse-like tension fractures subsequently modified through viscous relaxation and mass wasting (Squyres, 1982). 1.2. The post-Galileo perspective Two fundamental objectives of Galileo imaging at Ganymede were to fill the gaps in Voyager coverage of the surface and to acquire high-resolution data of characteristic features and terrain types. It was anticipated that these data would enable more detailed observation of the cryovolcanic and tectonic processes affecting light and dark material, and illuminate the interactions among these processes. However, using high-resolution imaging of dark material, no unequivocal observation could be made of lobate materials with an identifiable source vent or any other identifiable morphology related to cryovolcanism (Prockter et al., 2000). Candidate cryovolcanic units identified from Voyager data at lower resolution on the basis of embayment and texture instead appear to be the result of fluidized impact ejecta (Pappalardo et al., 2004); moreover, dark smooth materials in topographic lows appear to have accumulated by downslope movement of loose material (Prockter et al., 1998). Though evidence for volcanism associated with dark material could potentially exist below the current limits of resolution, the hypothesis that cryovolcanism played a primary role in the evolution of dark material is not supported by existing imaging data (Pappalardo et al., 2004). High-resolution images of light material have also lacked clear and abundant morphological evidence for lava flow fronts, source vents, embayment relationships, or any other evidence suggestive of cryovolcanic emplacement. This suggests that if cryovolcanism on Ganymede were to result in the formation of such features, they

may be too subtle to be resolved in the data available, or they may have been destroyed by fracturing, impact erosion, or mass wasting. Instead, the high-resolution data supported a prominent role for tectonism in the formation of light materials. All light material is modified to some extent by tectonism, from sets of faint parallel lineaments to high-relief sets of parallel ridges interpreted to result from tilt-block extensional faults (Pappalardo et al., 1998). Examination of the boundaries between adjacent polygons of light material at high resolution supported the hypothesis that tectonism alone could alter the surface sufficiently to wipe out some preexisting features (Pappalardo et al., 2004). Though the extent of volcanism in the formation and evolution of light material remains somewhat enigmatic, indirect evidence for volcanic resurfacing has been identified in the form of small isolated caldera-like features (Lucchitta, 1980; Schenk and Moore, 1995; Head et al., 1998; Kay and Head, 1999; Spaun et al., 2001) and smooth, topographically low bright lanes (Schenk et al., 2001). In response to these new developments, we have compiled a global geological map of Ganymede that represents the most recent understanding of the satellite based on a combination of data from the Galileo and Voyager missions. This contribution builds on important 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 others). The map will help to elucidate: (1) the major geological processes operating on Ganymede, (2) the characteristics of the geological units making up its surface, (3) the stratigraphic relationships of geological units and structures, and (4) the geological history inferred from these relationships.

2. Data The Galileo spacecraft made six close encounters with Ganymede (orbits G1, G2, G7, G8, G28, and G29), enabling the acquisition of high-resolution imaging (100 m/pixel and better) by the Solid State Imaging (SSI) camera. Due to the failure of Galileo’s high-gain antenna, high-resolution imaging was concentrated on a few characteristic terrain and feature types. Since the high-resolution coverage of Ganymede is limited in area, Voyager data remain central to understanding the satellite. Lower-resolution images were obtained during the more distant encounters of Ganymede during other Jupiter orbits to fill gaps in the Voyager coverage, most notably in the areas from 40°W to 115°W and 245°W to 305°W longitude (see Carr et al., 1995). The illuminated leading hemisphere was best observed on orbit C9 (2 km/pixel) and the trailing hemisphere on orbit E6 (3.6 km/ pixel). Information regarding all 14 Galileo orbits where Ganymede remote sensing data were successfully obtained are listed in Supplementary materials found in Bagenal et al. (2004). Utilizing the best available data from the Galileo and Voyager missions, 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 surface area imaged at slightly higher resolution was degraded. In general, the subjovian region was imaged by Voyager 1, the antijovian region was imaged by Voyager 2, and the leading and trailing hemispheres between these areas were imaged by Galileo, as stated above. Two additional areas imaged by Galileo at resolutions higher than 1 km/pixel were also included in the mosaic. In areas where images from different spacecraft overlapped, higher-resolution images were placed on top of lower-resolution images, and nearterminator (high-incidence angle) images were placed on top of high-Sun (low-incidence angle) images. The imaging data that make up this mosaic vary widely with respect to their lighting and viewing geometries. These

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inhomogeneities can result in the incomplete recognition of terrain features and subsequently hinder the characterization of geologic units. We tested the effects of potential image-geometry based mapping biases by counting the numbers of features and units mapped as a function of image resolution, incidence angle (angle between the Sun-surface vector and the surface normal vector), emission angle (angle between the surface-spacecraft vector and the surface normal vector), and phase angle (angle between the Sun-surface vector and the surface-spacecraft vector). While there is likely to be natural variation in the numbers of different types of features from location to location, a systematic decrease in mapped features with a change in imaging geometry would indicate incomplete mapping of such features. There are no trends in feature and unit recognition with respect to phase angle, so we will not consider this factor further. There is a factor of two decrease in groove features recognized at very lowincidence angles (<20°), with a corresponding increase in recognized furrow features. This can be explained because most of the low-incidence angle images cover dark terrain (where furrows

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are present and not grooves), and the only low-incidence angle areas primarily covering light terrain also have poor resolution. Since the same topographic shading factors that would make grooves difficult to recognize at low-incidence angle should also affect furrows, we believe that this decrease in groove recognition with lower incidence angle is primarily an artifact of low resolution and happenstance of geographical location. Thus, from the geometric factors that can influence the recognition of terrain features, it appears that low resolution and high emission angle are the primary concerns for the global mosaic of Ganymede (Fig. 1). Resolution does not have a strong influence on defining the basic categories of geologic materials on Ganymede (i.e., light, dark, and impact in Section 3), though the ability to confidently outline the boundaries of these units drops off at resolutions >3 km/pixel (Fig. 2). The recognition of reticulate material (Section 3) and various subunits (e.g., dark cratered unit, light irregular unit, etc. in Section 3) within broadly defined material units requires image resolutions better than approximately 2 km/pixel. The ability to recognize linear features such as grooves and furrows drops off

Fig. 1. (Upper) Plot of the resolutions of image data across the surface of Ganymede. Higher-resolution data is shown in dark tones (black 60.5 km/pixel) and lower resolution data in light tones (white P5 km/pixel). (Lower) Plot of the emission angles associated with image data across the surface of Ganymede. Low emission angles are shown in dark tones (black = 0° emission angle) and high emission angles in light tones (white = 90° emission angle).

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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 angle versus 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-width of many of these topographic features. In the base mosaic, 94% of the 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, and 74% of the area is at resolutions better than 2 km/pixel (Fig. 1). At high emission angles, positive topography can obscure the view of adjacent negative topography. For example, slopes at the angle of repose oriented away from the camera will become foreshortened with increasing emission angle, and disappear from view entirely at an emission angle of about 60°. This effect primarily influences our ability to distinguish tectonic features such as grooves within material units on the surface. The number of mapped tectonic features begins to monotonically decline as a function of emission angle at angles >30°, and beyond 45° the count of tectonic features decreases to more than a factor of two below the baseline at low emission angles (Fig. 2). In the base mosaic, 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 presented here (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 mosaic was often used, where available, to serve as a guide in locating features and unit boundaries in the lower resolution base mosaic. Additionally, two broad areas of the subjovian hemisphere were imaged at better than 1 km/pixel on the final flyby of Ganymede (G29), after the USGS assembled the global base mosaic. This data served to elucidate relationships between features and units in some areas covered by low-incidence angle, high emission angle Voyager 1 data. 3. Material units On the basis of our mapping, dark material on Ganymede has been 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’s surface area covered by each of these material units is presented in Table 1. We also recognize two other basic geologic units on Ganymede, reticulate material and impact material. Impact material encompasses crater, palimpsest, and basin materials.

G.W. Patterson et al. / Icarus 207 (2010) 845–867 Table 1 Area covered by mapped material units. Map unit Dark materials Cratered unit (dc) Lineated unit (dl) Undivided unit (d)

528 131 92 305

30.66 22.35 1.61 6.70

35.4 25.8 1.9 7.7

Light materials Grooved unit (lg) Oldest (lg1) Intermediate (lg2) Youngest (lg3)

1355 571 106 226 239

55.59 12.27 2.17 4.98 5.12

64.1 14.1 2.5 5.7 5.9

Subdued unit (ls) Oldest (ls1) Intermediate (ls2) Youngest (ls3)

519 262 163 94

15.58 7.21 4.51 3.86

18.0 8.3 5.2 4.5

Irregular unit (li) Oldest (li1) Intermediate (li2) Youngest (li3)

156 94 55 7

3.19 2.07 1.00 0.12

3.7 2.4 1.2 0.1

Undivided unit (l)

109

24.55

28.3

24

0.42

0.5

Reticulate material (r) a

Number of mapped Area (106 km2) Proportion of exposures surface areaa (%)

Includes area of superposed impact materials contained within each unit.

Palimpsest material is further subdivided into four units: three of these are distinguished by their stratigraphic relationship with light material units and the fourth is an interior plains unit. Five units comprise the crater materials: fresh, partially degraded, degraded, unclassified, and ejecta, and two units comprise the basin material: rugged and smooth. In principle, geological map units are material units and should be independent of structure (ACSN, 1961), and most terrestrial mappers adopt this approach. In planetary mapping, however, exceptions have commonly been made because of the local and regional dominance of structural features, the synoptic and remote sensing aspect of the data, and the scale of the mapping (e.g., Wilhelms, 1972, 1990). On Earth, a rock-stratigraphic unit is defined as ‘‘a subdivision of the rocks in the Earth’s crust distinguished and delimited on the basis of . . .observable physical features [commonly lithologic]. . . and independent from time concepts. . . and . . .inferred geologic history” (ACSN, 1961). In planetary mapping, stratigraphic units must be defined by remote sensing, so it is difficult to approach the process of geologic mapping of planetary surfaces using a strict ‘lithologic’ definition of units. As outlined by Wilhelms (1972, 1990), planetary mappers use the broader definition of rock-stratigraphic units as those ‘‘distinguished and delimited on the basis of observable physical features”, which might include surface morphology, albedo, etc. (see also Head et al., 1978). Shoemaker et al. (1982) defined Ganymede’s grooved terrain as a major geologic unit, considering it ‘‘both lithologically and structurally distinct from the ancient cratered terrain”. In addition, they mapped additional structural subunits where ‘‘each line is the boundary of a structural cell that contains a unified structural pattern”. Later workers in the 1:5 M-scale geologic mapping program, such as Lucchitta et al. (1992), found that the ‘‘surface of Ganymede seems to be dominated by units of similar compositions but diverse structural patterns”, and also noted that ‘‘many map units are subdivided mainly on the basis of structural differences. . .” and that ‘‘. . .locally the structural deformation may have been so intense or pervasive that it created a new and distinct material, distinguished from the parent material by a different physical state rather than a different composition”.

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The use of structure to help characterize geological units, even in the qualified manner described above, has been controversial (e.g., Hansen, 2000). This then raises the question, should such features be mapped as structures only, should they be mapped as geological units, or is there some combination of approaches that is appropriate, perhaps depending on scale? For the global geologic map of Ganymede, we believe a combination approach is appropriate, one that is similar to that outlined by Greeley et al. (2000) for the Europa global mapping effort, or by Tanaka et al. (1994) for geologic mapping on Venus. This approach enables the maximum amount of geological information to be clearly conveyed by the map, with a minimum of confusion and extraneous symbols. At the 1:15 M scale of our map, furrows can be distinguished from surrounding terrain and may be represented on it such that they do not obscure recognition of the underlying units. Therefore, furrows are mapped as separate structural features from the underlying material units described in the unit definitions. However, in light material units, the grooves generally have small enough dimensions and spacing that representing them as separate structural features on the map would be illegible when printed at the 1:15 M scale. This necessitates their inclusion as part of the unit definitions for light material units, and instead the general trend of grooves is represented on the map. We also adopt the convention started by Shoemaker et al. (1982) of mapping light material unit boundaries that enclose unified patterns of grooves that are distinct from neighboring light materials. High-resolution Galileo images have revealed that the boundaries between adjacent polygons of light material generally occur at the edge of a group of cross-cutting faults, and do not represent the transition to a compositionally distinct material. In the remainder of this section, we discuss the geologic units that we have defined based on our mapping efforts. We identify the characteristics that define each unit and, where applicable, its relative age with respect to other mapped units, its relation to units 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 locality on the surface. Boundaries between dark, light, reticulate, and impact material units are generally distinct and have been identified based on relative albedo and/or physical characteristics such as morphology and/or the presence of distinctive structures (e.g., reticulate material and light material). Units crossing image resolution boundaries (Plate 1) that obscure the characteristics which define them (e.g., when light material units lg, ls, and li transition in light undivided material) or boundaries within undivided materials that are not distinct are shown in red (Plate 2).1 A summary of unit and structure type localities can be found in Table 2. Crater density measurements for all units except light undivided, dark undivided, and crater materials are provided in Table 3 and discussed in more detail in Section 5.

3.1. Dark material units Dark material comprises 35% of Ganymede’s surface, and is subdivided 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 surface units 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 at decameter scales, probably resulting from thermally driven segregation of ice and non-ice surface components (Spencer, 1987a,b). 1 For interpretation of color in Plate 2, the reader is referred to the web version of this article.

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

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

Plate 2. Geological map of Ganymede. Unit descriptions are given in the text. 851

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Galileo high-resolution images suggest that the dark material is composed of a relatively thin dark lag deposit of non-ice material overlying brighter icy material. This lag deposit has been concentrated on the surface by processes such as sublimation, mass wasting, and ejecta emplacement (Prockter et al., 1998, 2000; Moore et al., 1999; Oberst et al., 1999). While dark material is geologically uniform at global mapping scales, complex geological relationships including numerous distinct units have been recognized on local scales 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), and makes up the majority of all dark material on Ganymede (Table 1). It commonly occurs as polygons bounded by light material, and the boundaries with light materials appear generally sharp and are sometimes marked by troughs. Craters of all ages are superposed, with degraded craters (c1 – Section 3.4.1.3) in higher abundance than on light materials. Furrows (Section 4.1) are common in this unit, but areas lacking furrows are also present.

Dark cratered material is interpreted as the oldest preserved surface on Ganymede. This material unit has been heavily modified via impact processes (palimpsests, craters, basins, and related furrows) as well as sublimation and mass wasting to create the dark lag deposit on the surface. The dark cratered unit encompasses units variously mapped at the 1:5 M scale from Voyager images as cratered material, furrowed material, vermicular material, smooth material, and hummocky 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 is primarily the result of the scale of the global map and the exclusion of furrows from the unit definitions. The type locality for the 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-

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G.W. Patterson et al. / Icarus 207 (2010) 845–867 Table 2 Type localities for map units and structures. Map unita

Type location

Observation

Image

Map units Dark materials Cratered unit (dc) Lineated unit (dl)

15°S, 23°E 24°S, 42°E

G28GSNICHOL02 G28GSCALDRA02

s0552445300, s0552445313 s0552445613

125 150

Light materials Grooved unit (lg) Subdued unit (ls) Irregular unit (li)

16°S, 50°E 16°S, 50°E 32°N, 172°E

G28GSSOOTH02 G28GSSOOTH02 G2GSTRANST01

s0552445100, s0552445113 s0552445100, s0552445113 s0359942426

116 116 188

Reticulate material (r)

28°S, 174°E

G8GSCALDRA01

s0394532440

179

62°N, 12°W 65°N, 12°W 16°S, 15°E 62°N, 12°W 14°N, 157°E 14°N, 157°E

G7GSACHELS01 G7GSACHELS01 G28ARBELA02 G7GSACHELS01 G8GSBUTOFC01 G8GSBUTOFC01

s0389923200, s0389923200, s0552445426 s0389923200, s0394532139, s0394532139,

57°S, 130°W

E12GSGLGMSH01

s0426117300 s0426117313 s0426117326 s0426117339

Galileo Regio 47°N, 156°E 32°S, 170°E 10°S, 174°E 24°N, 166°E

G2GSNIPPUR01 G8GSCALDRA01 G8GSMELKRT01 G8GSPITCRA01

Impact materials Crater materials Fresh crater unit (c3) Partially degraded crater unit (c2) Degraded crater unit (c1) Crater ejecta unit (ce) Palimpsest materials (p1, p2, and pu) Interior plains unit (pi) Basin materials Rugged unit (br) Smooth unit (bs)

Structures Furrows Grooves Depressions Domes Secondary craters a

Resolution (m/pixel)

s0389923213 s0389923213 s0389923213 s0394532152 s0394532152

C2063659 s0359944426, s0359944439 s0394532478 s0394532265, s0394532278 s0394532965

178 178 130 178 187 187 160

1000 99 179 181 146

Undivided units are not included.

Table 3 Crater densities on mapped units. Material typea Light Grooved Irregular Subdued Dark Cratered Lineated Reticulate Reticulate

10 kmb

20 kmb

30 kmb Area of zone used for counting (106 km2)

39 ± 2 (44 ± 3) 30 ± 4 (20 ± 5) 42 ± 2 (39 ± 3)

14 ± 1 (14 ± 2) 13 ± 3 (5 ± 2) 18 ± 1 (15 ± 2)

8±1

9.29 (5.71)

6±2

1.94 (0.99)

9±1

8.24 (4.90)

85 ± 2 (97 ± 2) 67 ± 8 (69 ± 8)

32 ± 1 (34 ± 1) 19 ± 4 (20 ± 4)

15 ± 1

21.9 (16.3)

8±3

1.06 (1.01)

39 ± 12 (39 ± 12)

18 ± 8 (18 ± 8)

4±4

0.28 (0.28)

Impact Palimpsest 61 ± 7 Basin 19 ± 5

23 ± 4 11 ± 4

1.37 0.80

to be a precursor unit for light materials, as evidenced by its physical proximity to light material units with similar characteristics. The dark lineated unit mapped at this scale encompasses units previously mapped at 1:5 M as dark lineated and dark grooved material (e.g., Guest et al., 1988; Murchie and Head, 1989). The type locality for this unit is at 24°S, 318°W within dark material polygons interspersed within Harpagia Sulcus, east of the prominent crater Enkidu (Fig. 4). This region was imaged during the G28 encounter 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 other then relative albedo cannot be confidently assessed at the scale of this mapping effort, and hence they cannot be assigned to any of the subdivisions of dark material. This unit also includes irregularly shaped patches and small slivers of low relative albedo material of indistinct morphology interspersed within light material, as well as areas too small to be identified by criteria other than relative albedo.

a

Undivided units are not included. Number of craters P the quoted diameter, normalized to a counting area of 106 km2. Numbers in parentheses indicate values calculated from image data at resolutions <1.5 km/pixel. b

erally shallower and more sinuous. The crater density on this unit is only slightly greater than on light materials (Table 3). This unit always occurs adjacent to light grooved materials (lg), and often shares a common structural fabric with relatively older nearby light material units (e.g., lg1 and li1). This implies that the tectonic forces that modified dark lineated material are related to those that formed early light material units. Dark lineated material is interpreted as dark cratered (dc) material that has undergone significant tectonic deformation. It is likely

3.2. Light material units Light material covers about 64% of Ganymede’s surface and forms swaths that crosscut older dark material, containing polygons tens to hundreds of kilometers wide, which form an intricate patchwork across the surface. Light material is primarily subdivided by the density and orientation of structural grooves (Section 4.2) that exist within a given polygon. On the global geologic map, light material is subdivided into four units: a grooved unit (lg), a subdued unit (ls), an irregular unit (li), and an undivided unit (l). Except for the undivided unit, each of the light material units has been further subdivided into three different relative age categories based on cross-cutting relationships

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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 Nicholson Regio. (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 higherresolution data is indicated in white.

(see Section 5). Category 3 (lg3, ls3, and li3) includes light materials that crosscut all adjacent light materials, category 1 (lg1, ls1, and li1) includes light materials that are crosscut by all adjacent light materials, and category 2 (lg2, ls2, and li2) includes light materials that crosscut category 1 materials and are crosscut by category 3 materials. Category 3 is dominated by light grooved material, while category 1 is dominated by light subdued and light irregular material (Table 1). Light material has crater densities about half of those on dark material units (Table 3), confirming that it is relatively younger. 3.2.1. Light grooved unit (lg) The light grooved unit is defined as a high relative albedo material that has a surface dominated by structural grooves. This unit is arranged in lanes or polygons, and within each of these areas, there

are roughly evenly spaced grooves and ridges oriented in a single dominant direction. Boundaries with other units are commonly sharp where the grooves cut across older terrain, or are crosscut by other groove sets. Grooves are roughly linear; locally (on the scale of kilometers) slightly curved, rarely sharply angled. This unit is bounded in places by long, relatively deep grooves. Light grooved material is interpreted to form from dark, reticulate, or other light material units via extensional tectonism, with or without prior cryovolcanic resurfacing. The extensional tectonism leads to the development of imbricate normal faulted tilt-blocks, which are superimposed on broader topographic ‘‘grooves” that likely 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, it varied from 15% to 100% extensional strain (Pappalardo and

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rial units is shed into topographic lows, exposing a brighter, more ice-rich substrate on the slopes. If cryovolcanism plays a role in creating this unit, evidence for it is thoroughly overprinted by tectonism (Pappalardo et al., 2004). The light grooved unit encompasses materials previously mapped as grooved material (e.g., Guest et al., 1988; Murchie and Head, 1989; Croft et al., 1990). The type locality for this unit is located at 16°S, 310°W within Harpagia Sulcus, east of the prominent crater Enkidu (Fig. 5). This region was imaged during the G28 encounter 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 light irregular (li) materials. It is arranged in polygons, similar to light grooved (lg) material, but it is characterized by a smooth surface with groove structures that are faint or undetectable at decameter to kilometer resolution. Where grooves are present, their morphology at high resolution may resemble horst and graben structures or they may be dark lineaments without obvious topographic expression (Fig. 5). The light subdued unit typically has sharp boundaries with dark materials, while boundaries with other light material units range from sharp to transitional. Where the boundaries with other material units are sharp, they often take the form of long, relatively deep, linear to curvilinear grooves. Light subdued material may form from dark material via extensional tectonism, similar to light grooved (lg) material, but the apparently low strain in this unit compared to light grooved materials makes it less likely that tectonism alone could accomplish the resurfacing. Cryovolcanism may have played a more prominent role in the formation of light subdued material then it has for light grooved (lg) material. Indirect evidence for cryovolcanic resurfacing in the form of small isolated caldera-like features are found in 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 occupy topographic lows and have topographically level surfaces (Schenk et al., 2001). This unit encompasses units previously mapped as smooth material, fine material, and slightly grooved material (e.g., Guest et al., 1988; Wilhelms, 1997). The type locality for this unit is at 16°S, 309°W within Harpagia Sulcus, east of the prominent crater Enkidu (Fig. 5). This region was imaged during the G28 encounter at 116 m/pixel.

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

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

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 light grooved (lg) and light subdued (ls) materials. It is arranged in polygons imprinted by grooves with irregular spacings and orientations. At high resolution, light irregular material could commonly be divided into smaller subunits of light grooved (lg) or light subdued (ls) material, but these subunits are too densely packed to be separated at the scale of the global map. While the boundaries of this unit are readily delineated from surrounding materials, the unmapped subunits within a given exposure of light irregular material may be related to adjacent mapped light grooved (lg) and/or light subdued (ls) unit exposures. Light irregular material consists of interwoven portions of light subdued (ls) and light grooved (lg) material. This unit encompasses materials previously mapped as irregular material (Lucchitta et al., 1992) and grooved material (e.g., Guest et al., 1988; Murchie and Head, 1989). The type locality for this unit is located at 32°N, 188° along the boundary between Marius Regio and Nippur Sulcus

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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°E within 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 the surrounding 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 emission angle that characteristics other than albedo cannot be confidently assessed. Hence the material in this unit cannot be assigned to any of the other subdivisions of light material. This unit also encompasses areas with moderate to high relative albedo that have been obscured by crater ejecta such that the texture of the underlying topography cannot be discerned.

3.3. Reticulate material (r) This material represents a unique style of structural modification of both dark and light material units. Reticulate material is commonly surrounded by light grooved (lg), light subdued (ls), and/or dark lineated (dl) units. Reticulate material is distinguished from other materials by its variable albedo and the presence of two dominant sets of distinct grooves, typically oriented near-orthogonal to each other. Boundaries between reticulate material and other material units are typically sharp. Reticulate material is predominantly found in the Sippar Sulcus region of Ganymede, with outliers near the south pole and near the crater Serapis. This mate-

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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 the boundary 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 surrounding geologic units (right). The location of the higher-resolution data is indicated in white.

rial has a crater density similar to light material units, suggesting contemporaneous formation. Reticulate material is comprised of dark and light material that has been modified by the formation of near-orthogonal sets of grooves. At high resolution these grooves resemble horst and graben structures. It has been suggested that the formation of reticulate material may be the result of block rotation within a distributed shear zone initiated during light material formation (Murchie and Head, 1988). This unit encompasses materials previously mapped as reticulate and dark reticulate material (Guest et al., 1988; Wilhelms, 1997). The type locality for this unit is located at 32°S, 182° in the Sippur Sulcus region, northwest of the prominent fresh crater

(c3) Osiris (Fig. 7). This region was imaged during the G8 encounter at 179 m/pixel. 3.4. Impact material units Impact features are ubiquitous on Ganymede, and their distribution across the surface appears to be controlled by two primary factors. One is related to the distribution of light and dark materials, with dark materials having a higher density of craters than light materials (Table 3). The other results from a slight apex–antapex asymmetry in the density of craters associated with light materials (Schenk and Sobieszczky, 1999; Zahnle et al., 2001). Impact materials have been subdivided for the global map into crater,

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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 Sippur Sulcus 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 the surrounding geologic units (right). The location of the higher-resolution data is indicated in white.

basin, and palimpsest materials based on distinct morphological differences. 3.4.1. Crater materials Crater materials have been mapped globally for craters with rim diameters greater than 30 km. Crater materials are divided into degraded (c1), partially degraded (c2), and fresh (c3) units based on specific morphological characteristics. Previous mapping efforts based on Voyager data and utilizing a 1:5 M mapping scale have used the degradation state of crater rims as a distinguishing characteristic of the relative age of craters (e.g., Guest et al., 1988; Murchie and Head, 1989; Croft et al., 1990; Lucchitta et al., 1992; Wilhelms, 1997). As a result of the scale of the global map and the lighting and viewing geometry issues associated with the data presently available (Section 2), we are not confident that rim degradation state can be consistently determined across Ganymede.

Therefore, we concentrate on two distinguishing characteristics to categorize craters on Ganymede: relative albedo contrast with respect to surrounding materials, and the presence or absence of rays and continuous ejecta deposits. Other subdivisions of crater materials include an unclassified (cu) unit to account for craters whose distinguishing characteristics cannot be determined, and an ejecta material (ce) unit. Overall, 908 craters greater than 30 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. The diameters at which craters transition from one type of interior structure to another does not depend on the relative age of the craters. However, it has been suggested that dome morphology has a relationship with the relative ages of craters (Schenk et al., 2004). This is consistent with the global map of Ganymede where 29 of 32

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3.4.1.1. Degraded crater unit (c1). Degraded craters typically have relative albedos that are similar to surrounding material, and they are found predominantly in association with dark material units. This unit lacks the presence of rays and continuous ejecta and has a subdued interior morphology that typically resembles the target 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 at 130 m/pixel. 3.4.1.2. Partially degraded crater unit (c2). Partially degraded craters can have albedos higher or lower than the surrounding material and are found in association with a variety of other material units. This unit lacks rays but has a readily observable deposit of continuous ejecta surrounding the crater rim. The type locality for this unit is the crater Gula located at 65°N, 12° northeast of Perrine Regio and north of Aquarius Sulcus (Fig. 9). This region was imaged during the G7 encounter at 178 m/pixel. 3.4.1.3. Fresh crater unit (c3). Fresh craters are typically characterized by a strong albedo contrast with respect to the surrounding surface (higher or lower) and are found in association with a variety of other material units. Ray systems (commonly bright, occasionally dark) and continuous ejecta are readily observed. The type 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 at 178 m/pixel. 3.4.1.4. Unclassified crater unit (cu). This unit encompasses any crater materials found in regions imaged at sufficiently low resolution or high emission angle that the characteristic properties that distinguish degraded (c1), partially degraded (c2), and fresh (c3) crater materials cannot be determined. 3.4.1.5. Crater ejecta unit (ce). Crater ejecta represents material that continuously blankets surrounding preexisting terrain concentric to the crater with which it is associated. This unit is observed in association with partially degraded (c2) and fresh (c3) crater materials (Fig. 9).

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

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

3.4.2. Palimpsest material Palimpsest material is characterized by moderate to relatively high albedo material forming flat, generally circular to slightly elliptical structures. It is interpreted to be the result of impacts into the crust of Ganymede during a time when there was a higher thermal gradient and/or a thinner brittle lithosphere (Shoemaker et al., 1982). Palimpsests superpose dark material units but can superpose or be superposed by light material units. Palimpsests are typically superposed on furrows, but some furrows can remain visible on their outer margins. The surface texture of palimpsest materials is smooth to hummocky and can be locally rugged. Palimpsests lack rims, but the presence of outward facing scarps and internal, concentric ridges are common. The centers of some palimpsests are characterized by smooth, circular to subcircular patches of high albedo material. Similar to crater materials, previous mapping efforts based on Voyager data at the 1:5 M mapping scale used the apparent structural complexity and degradation state of palimpsest interiors as distinguishing 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 lighting and viewing geometry issues associated with the data presently available (Section 2), we are not confident that these characteristics can be consistently determined for all palimpsest materials. Therefore, we have categorized palimpsests based on their cross-

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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 acquired during 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 showing context 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 to three subdivisions of palimpsest materials. Ancient palimpsests (p1) are superposed by light materials, young palimpsests (p2) superpose light materials, and unclassified palimpsests (pu) do not have a discernible relationship with light materials. We have also defined a palimpsest interior plains (pi) unit to represent the interior 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 located at 14°N, 203° within Marius Regio and northeast of Tiamat Sulcus (Fig. 10). This region was imaged during the G8 encounter at 187 m/pixel.

3.4.3. Basin material Basin material encompasses the deposits of Gilgamesh basin (590 km in diameter) centered at 57°S, 130° (Fig. 11). Schenk et al. (2004) suggest that the Gilgamesh basin can be divided into three concentric zones. The central zone is a low dome that rises asymmetrically 500 m in elevation and is surrounded by a discontinuous inward-facing quasi-concentric scarp 1 km high. This is in turn surrounded by an annulus of material that is characterized by hummocks punctuated by rugged, somewhat angular massifs and quasi-concentric but discontinuous ridges. This zone is roughly bound by a prominent contiguous concentric inward-facing scarp 1 km high, which has been interpreted as the rim of

the basin (Schenk et al., 2004). An exterior annular zone of material is also present and can be characterized by a mottled texture. It has been suggested that this mottled material has modified or mantled preexisting light material and represents the continuous ejecta deposit of Gilgamesh (Schenk et al., 2004). These morphologic subdivisions of Gilgamesh basin are represented on the global map by a basin interior plains (bi) unit for the central zone, a basin rugged (br) unit for the interior annular zone, 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 be approximately 200 and 60 km, respectively. A portion of Gilgamesh basin was imaged at high resolution (160 m/pixel) and high solar incidence angle during the E12 encounter of the Galileo mission, though most of our knowledge of Gilgamesh is derived from of over half of the basin by low-incidence angle Voyager 2 imaging at 1 km/pixel resolution.

4. Structural features and landforms 4.1. Furrows Furrows are the oldest recognizable structures on the surface of Ganymede, occurring only on dark material and predating essentially all craters larger than 10 km in diameter (Passey and Shoe-

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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 surrounding geologic units (right). The location of the higher-resolution data is indicated in white.

maker, 1982). The majority of the furrows are arranged in regional sets which are close to being concentric around a point, but a few furrows crosscut the regional sets at high angles (Cassachia and Strom, 1984; Schenk and McKinnon, 1987; Murchie and Head, 1988). They date to an epoch when Ganymede had a higher thermal gradient (Nimmo and Pappalardo, 2004) with a thinner brittle lithosphere, overlying a deeper ductile ice interior, in turn likely above liquid water (Schenk, 2002; Schenk et al., 2004). Because dark material is disrupted and resurfaced by younger swaths of light material, the record of ancient furrow systems is incomplete. When intact, however, the largest known system (the Lakhmu Fossae in Galileo Regio) would have been hemispherical in scale (Schenk and McKinnon, 1987). Individual furrows (Fig. 12) are linear to curvilinear troughs bounded by raised rims, which are generally bright. They are represented on the global map as lines drawn on dark material units.

They extend from tens to hundreds of kilometers in length and are typically 6–20 km wide, with generally flat or u-shaped floors and sharp raised rims (Smith et al., 1979a; Shoemaker et al., 1982; Prockter et al., 1998). Inter-furrow spacing is fairly uniform at 50 km, although spacing is generally closer towards the center of a concentric system (Passey and Shoemaker, 1982). Topographic models derived from high resolution stereo images within Galileo Regio show that one furrow rim rises a full kilometer above the furrow floor and 900 m above the level of the surrounding terrain (Prockter et al., 1998), consistent with estimates from broad-scale shadow measurements of furrow depth (Murchie and Head, 1988). Voyager-era mapping and photogeological analysis of the furrow systems led to a variety of models for their formation (e.g., Cassachia and Strom, 1984; Schenk and McKinnon, 1987; Murchie et al., 1990). On the basis of morphology and planform, along with their similarity to multi-ringed structures on Europa and Callisto

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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) Portion of Plate 2 showing the basin and surrounding geologic units.

resulting in the flexural and/or viscous uplift of the bounding escarpments (McKinnon and Melosh, 1980; Nimmo and Pappalardo, 2004). 4.2. Grooves

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 Zu Fossae system trends N–S cutting them obliquely. North is up in the image.

(Moore et al., 1998, 2001; Kadel et al., 2000; Schenk, 2002), they are now generally accepted to be fault-induced troughs formed in response to large impacts into a relatively thin lithosphere early in Ganymede’s history (McKinnon and Melosh, 1980). Individual furrows probably formed rapidly during basin collapse, as the result of asthenospheric flow radially inward toward the impact point, with accompanying brittle failure of the overlying lithosphere (McKinnon and Melosh, 1980; Melosh, 1982; Moore et al., 1998, 2001). The type and extent of the resulting fault pattern would have depended on the scale of the impact and the rheological structure of the satellite at the time of formation. Many furrows apparently have been reactivated and modified by later tectonic activity (Murchie et al., 1990; Prockter et al., 2000). It has been proposed that raised furrow rims formed when the long-wavelength components of fault-induced relief relaxed,

Light material is typically modified by sets of subparallel ridges and troughs, commonly referred to as ‘‘grooves”. The formation of these grooves has been attributed primarily to extensional tectonism (Shoemaker et al., 1982; Pappalardo et al., 1998, 2004). The morphology of grooves in light grooved (lg) material has been shown to be generally characteristic of normal fault formation superimposed on pinches and swells caused by the formation of necking instabilities as the lithosphere was extended (Collins et al., 1998b; Dombard and McKinnon, 2001; Bland and Showman, 2007). The presence of horst and graben style normal faults has also been inferred (Pappalardo et al., 1998, 2004) and is typically associated with light subdued (ls) material units. Individual grooves within light material units are not represented on the global map, as they are too densely packed to be legible at the 1:15 M scale. Instead, the general trend of grooves within a polygon of light material is represented on the map with a short line oriented to illustrate the trend (Plate 2). Grooves associated with light grooved (lg) material exhibit two superposed spacing scales of grooves. The larger-scale grooves are spaced between 5 and 10 km apart, as derived from Fourier analysis of photometric profiles across groove sets (Grimm and Squyres, 1985; Patel et al., 1999). These grooves appear in low solar incidence angle images as dark and bright stripes, while in near-terminator images the topography of the grooves is revealed to be broadly sinusoidal in cross-section. Galileo topographic data suggests a close correlation between albedo and topography on local scales (Oberst et al., 1999), and the albedo striping observed at low-incidence angles in light grooved materials closely correlates to topography (Collins et al., 1998b), with the dark lineaments marking the locations of broad topographic lows. Galileo spacecraft images of light grooved material at better than 100 m/pixel also reveal smaller-scale grooves, with a mean

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

Fig. 13. Image mosaic acquired during the G2 encounter at 99 m/pixel illustrating the 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 is typically classified as light grooved material. A lane of very smooth material marks the boundary between Nippur and Philus Sulci. Philus Sulcus displays grooves with a horst-and-graben-like morphology. This morphology is typically classified as light subdued material. Philus Sulcus is bound to the south, in this image, by dark cratered material associated with Marius Regio.

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

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

and impact craters cut by these fault sets (Pappalardo and Collins, 2005) indicates that extensional strains of 50% and over are typical of well-developed light grooved material. Grooves within polygons of light subdued (ls) material commonly have a morphology at high resolution resembling graben and intervening flat-topped horst ridges (e.g., in the bottom half of Fig. 13). Rather than downdropping along prominent and distinct bounding faults, offset appears to have often occurred by cumulative displacement along several sub-kilometer scale fractures 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 strains of 15% and below are typical in light subdued material, and in many areas the strain is too small to measure. 4.3. Depressions At least 30 depressions with scalloped walls (‘‘paterae”) have been identified on Ganymede. These depressions have been interpreted to represent caldera-like source vents for icy volcanism (Lucchitta, 1980; Schenk and Moore, 1995; Head et al.,

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4.4. Domes Domes on Ganymede are features associated exclusively with the interiors of craters having diameters between 60 km and 175 km (Fig. 15). They are typically circular in map view and occur within central pits (Fig. 15). In cross-section they are steepsided with flat-topped profiles that can reach heights of up to 1.5 km above surrounding materials (Schenk et al., 2004). Domes are commonly surrounded by a trough, which in turn is surrounded by an annular ridge or ring of rugged massifs that have been interpreted as structurally equivalent to pit rims in smaller craters (Schenk et al., 2004). At high resolution, web-like networks of narrow fractures are commonly visible on the top surfaces of domes. Domes are interpreted to result from ductile icy material being pushed upward from the subsurface during the modification stage of large impact crater formation (Moore and Malin, 1988) suggested that these domes might be diapiric whose rise was initiated by post impact subsurface adjustments. Fig. 16. Secondary craters found within the crater Lugalmeslam, situated on the southwestern margin of Nippur Sulcus (146 m/pixel; G8GSPITCRA01). Clusters of secondaries 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 of several within Sippar Sulcus (Fig. 14) is associated with a ridged deposit in its interior, possibly an icy flow (Head et al., 1998). Stereo data show that the rim elevation of this patera reaches up to 800 m above surrounding light material, with floor deposits of similar elevation to surrounding light material (Schenk et al., 2001). Preexisting grooves continue unmodified up to patera rims, suggesting that the elevated rims probably formed through isostatic adjustment and are not the result of constructional volcanism. Evidence for embayment relationships and truly smooth regions that might indicate icy volcanism has been elusive in high-resolution images of light material. However, some limited embayment is suggested in the Sippar Sulcus region surrounding the paterae (Head et al., 1998; Schenk et al., 2001).

4.5. Secondary craters Secondary craters are represented by fields of uniform small pits surrounding large fresh (c3) and partly degraded craters (c2), palimpsests (p1, p2, and pu), and basin materials (bs and br). They commonly occur as clusters or chains oriented radially or subradially to the centers of craters, palimpsests, and the Gilgamesh basin. The morphology of secondary craters can include shallow, circular to elongate craters, or irregular bowl-shaped craters that overlap in chains or clusters (Fig. 16). Secondary craters are interpreted to form from blocks ejected from a primary crater during the crater formation process. The locations of individual secondary craters observable at the scale of the map have indicated (Plate 2). 5. Relative age relationships To determine relative age relationships among the various units of the global map of Ganymede (Plate 2), we utilized observed cross-cutting relationships and crater density measurements (Table 3). The conclusions drawn from these relationships and crater density measurements are broadly supported by previous crater

Fig. 17. Correlation of map units. Younger units are shown above older units; diagonal lines represent transitional or overlapping boundaries; ‘saw-tooth’ pattern indicates uncertain boundary ages.

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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 the mapped units for Ganymede (Table 3) and are consistently superposed by all other units (Fig. 17). Based on these characteristics, dark cratered material is recognized as the oldest geologic unit present on the surface. Dark lineated material has a crater density similar to light material units. This supports the interpretation that it is a transitional material that has undergone part of the resurfacing process that created light material units by the tectonization of dark cratered material (Pappalardo et al., 2004). Light material crater densities are approximately half those of dark cratered material (Table 3) and consistently superpose dark materials and some of the oldest impact materials (c1 and p1). Crater density differences among the different light material units are not significant, and so cannot be used to determine the relative ages of these units. Previous relative age mapping in the Uruk Sulcus region showed that light subdued (ls) material is often older than light grooved (lg) material (Collins et al., 1998a). However, another study of topography and cross-cutting relationships in a small area imaged at high resolution in the Sippar Sulcus region found that light subdued (ls) material is the youngest of the light material units (Schenk et al., 2001). On the global map, we have determined the relative age relationships among all light material units based on cross-cutting relationships and divided the light material into three relative age categories (see also Collins, in preparation). The oldest category contains light material that is crosscut by all adjacent light materials, the youngest category contains light material that crosscuts all adjacent light materials, and the intermediate category crosscuts the oldest and is crosscut by the youngest adjacent light materials. Since these age categories are based on local cross-cutting relationships, we cannot say that all the material within an age category formed at the same time, nor can we definitively assess the relative age of the oldest light material on one side of Ganymede versus the youngest light material on the other side of Ganymede. However, there are a few groove sets that are continuous for thousands of kilometers, which help to tie together regional age relationships and give us some confidence that the age categories are broadly consistent across Ganymede. The surface area covered by light units divided in different age categories is roughly even: the oldest light units cover 13.2% of the surface, intermediate age light units cover 12.1% of the surface, and the youngest light units cover 10.5% of the surface (see Table 1). Within the oldest light units, 19% is light grooved material, 63% is light subdued material, and 18% is light irregular material. Within the youngest light units, 56% is light grooved material, 43% is light subdued material, and 1% is light irregular material. Thus there appears to be a shift through time towards the formation of light material with more tectonic overprinting. Voyager era geological mapping of reticulate material suggested that, based on cross-cutting relationships, reticulate materials formed shortly before or during the initiation of light material formation (Guest et al., 1988; Wilhelms, 1997). Schenk et al. (2001) indicate that reticulate material is older than light material units within the Sippar Sulcus region based on cross-cutting relationships and topographic data. Based on our mapping, the variable albedo of reticulate material suggests that the tectonic event that resurfaced it has affected both light and dark materials. Crater density measurements indicate reticulate material formation is nearcontemporaneous with light material formation (Table 3). Reticulate material is always crosscut by adjacent light material, which indicates a relative age similar to or slightly older than the oldest age category of light material units. Impact materials span the surface history of Ganymede. Almost all palimpsest materials are found within dark cratered material units, and crater density measurements (Table 3) indicate that they

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are generally older than light materials. The identification of five palimpsests that superpose light material units suggests that palimpsest formation continued during the early stages of light material formation. Basin materials superpose light material and have crater densities consistent with being younger than light material units (Table 3). The determination of absolute ages for material units on Ganymede is complicated by uncertainty regarding the impactor population and flux in the jovian system. Neukum (1997) and Neukum et al. (1998) have suggested that the production function for Ganymede is lunar-like (asteroidal) and assign ages of 3.6–4.2 Gyr for light and dark materials respectively. Others (Shoemaker and Wolfe, 1982; Zahnle et al., 1998, 2003) suggest the flux of impacts to Ganymede is wholly different (cometary) and assign a much younger age for light material (2 Gyr). In general, it is agreed that dark cratered material on Ganymede represents an ancient surface that formed shortly after the formation of the planets (4.5 Gyr ago). The current best guess from crater statistics is that light materials formed at some time during the middle half of Solar System history, but obtaining an exact age is likely to remain elusive for a long time.

6. Summary Data obtained from the Galileo mission has changed our understanding of the surface of Ganymede, especially the roles of cryovolcanic and tectonic processes in creating light materials. The Galileo mission also filled in (at modest resolution) some gaps in the Voyager imaging coverage of Ganymede, clarifying the relationships between material units on the subjovian and antijovian hemispheres. In response to these developments, we have compiled a global geological map of the satellite utilizing a global image mosaic assembled from the best available data of the Galileo and Voyager missions, with a nominal resolution of 1 km/pixel. This map represents our most recent understanding of Ganymede based on Galileo mission data. On the basis of our mapping, we recognize four fundamental material types: dark material, light material, reticulate material, and impact material. Dark material on Ganymede has been subdivided 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 represents a single unit and impact material encompasses crater, palimpsest, and basin materials. Crater materials are subdivided into degraded (c1), partially degraded (c2), fresh (c3), unclassified (cu) and ejecta (ce) units. Palimpsest material is subdivided into old palimpsests (p1), young palimpsests (p2), unclassified palimpsests (pu), and interior plains (pi) units and basin materials are subdivided into rugged (br) and smooth (bs) units. Crater density measurements (Table 3) suggest that the highest crater densities are found on dark cratered materials. Light materials on Ganymede have a much lower density of craters, supporting the view that they formed substantially later. Dark lineated material and reticulate material have crater densities similar to light material units, suggesting they mark a transition into the formation of light materials. Palimpsests are older than light materials, dark lineated material, and reticulate material, but younger than dark cratered material. Finally, the Gilgamesh basin appears to be younger than light materials. The post-Galileo global geological map of Ganymede presented here will serve as an observational benchmark to provide constraints on models for the formation and evolution of Ganymede. The geological history of Ganymede can be looked upon as a touchstone for comparing and contrasting the characteristics and evolution of other large to mid-sized icy satellites. This geological map

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