2013

U.S. Department of the Interior

U.S. Geological Survey

Global Geologic Map of Ganymede

By Geoffrey C. Collins, G. Wesley Patterson, James W. Head, Robert T. Pappalardo, Louise M. Prockter, 

Baerbel K. Lucchitta, and Jonathan P. Kay

Pamphlet to accompany

Scientific Investigations Map 3237

View looking west across the leading hemisphere of Ganymede, captured by the Galileo spacecraft during orbit C9. The north 

pole is on the right, and the south pole is on the left. On the right, the dark material of Galileo Regio is seen curving off toward the 

limb, separated from the dark material of Perrine Regio on the terminator by a swath of light material called Xibalba Sulcus. On 

the left, the westernmost tip of the dark material in Nicholson Regio is seen at the terminator, and the edge of the large impact 

basin Gilgamesh is just over the limb. (Images from NASA/JPL.)
i

Contents

Introduction.....................................................................................................................................................1

Image data ......................................................................................................................................................1

Relation to previous mapping ......................................................................................................................1

Age determination .........................................................................................................................................2

Geologic history .............................................................................................................................................2

Nicholsonian Period.............................................................................................................................3

Harpagian Period..................................................................................................................................3

Gilgameshan Period .............................................................................................................................3

References Cited............................................................................................................................................3
1

Introduction

Ganymede is the largest satellite of Jupiter, and its icy 

surface has been formed through a variety of impact cratering, 

tectonic, and possibly cryovolcanic processes. The history of 

Ganymede can be divided into three distinct phases: an early 

phase dominated by impact cratering and mixing of non-ice 

materials in the icy crust, a phase in the middle of its history 

marked by great tectonic upheaval, and a late quiescent phase 

characterized by a gradual drop in heat flow and further impact 

cratering. Images of Ganymede suitable for geologic mapping 

were collected during the flybys of Voyager 1 and Voyager 2 

(1979), as well as during the Galileo Mission in orbit around 

Jupiter (1995–2003). This map represents a synthesis of our 

understanding of Ganymede geology after the conclusion of the 

Galileo Mission.

The two fundamental classes of material units on 

Ganymede are dark materials and light materials. The dark/

light distinction is based on sharp relative albedo contrasts at 

terrain boundaries, rather than on absolute albedo, because 

several other types of surface modification (for example, crater 

rays, polar caps) change the absolute albedo within these terrain classes. Dark materials cover 35 percent of Ganymede’s 

surface; almost the entire remainder of the surface is covered by 

light materials.

Dark materials are heavily cratered, though not as heavily cratered as the surface of the neighboring satellite Callisto, 

suggesting that dark materials cannot be a primordial surface 

(Shoemaker and others, 1982). At high resolution, dark materials are dominated by the downslope movement of loose dark 

regolith within impact craters and on the sides of bright ridges 

and hummocks (Prockter and others, 1998). These observations 

suggest that dark materials are covered by a thin lag deposit of 

dark regolith derived by sublimation of a more ice-rich crust 

below (Moore and others, 1999). Dark materials commonly 

exhibit sets of concentric arcuate structures known as furrows. 

Furrows may be the remnants of ancient multi-ring impact 

basins, similar to intact impact basins on Callisto, such as 

Valhalla and Asgard.

Light materials crosscut dark materials and exhibit a lower 

impact crater density, demonstrating that they were formed later. 

Light materials are subdivided into an intricate patchwork of 

crosscutting lineaments called grooves, mixed with areas of relatively smooth terrain. At high resolution, most light materials 

are dominated by extensional faulting (for example, Pappalardo 

and others, 1998; Pappalardo and Collins, 2005). Even light 

materials that appear to be smooth at low resolution are marked 

at high resolution by sets of parallel lineaments of apparent 

tectonic origin. There is an open question on the extent to which 

light terrain is formed by cryovolcanic flooding of dark material with brighter ice versus tectonic destruction of preexisting 

surface features and exposure of brighter subsurface ice in fault 

scarps; it is certainly possible that both of these processes play 

important roles in the formation of light materials (Pappalardo 

and others, 2004). Not all tectonic activity on Ganymede has led 

to the formation of light material—some dark material is cut by 

extensional faults without exhibiting a major change in albedo, 

while reticulate material is cut by two sets of tectonic lineaments and is transitional in albedo between adjacent light and 

dark materials.

Other material units found on Ganymede were created 

by several types of impact features, ranging from impact 

craters to viscously relaxed impact features called palimpsests (Shoemaker and others, 1982) to the large impact basin, 

Gilgamesh, in the southern hemisphere.

Below, we summarize the properties of the imaging dataset used to construct the map, previously published maps of 

Ganymede, our own mapping rationale, and the geologic history 

of Ganymede. Additional details on these topics, along with 

detailed descriptions of the type localities for the material units, 

may be found in the companion paper to this map (Patterson 

and others, 2010).

Image data

This map is based on a global image mosaic of Voyager 

and Galileo data assembled by the U.S. Geological Survey 

(USGS) at a nominal resolution of 1 km/pixel (Becker and 

others, 2001). Voyager 1 partially imaged the subjovian hemisphere of Ganymede, while Voyager 2 partially imaged the 

antijovian hemisphere. Galileo imaging filled in most of the 

gaps in moderate-resolution coverage of the leading and trailing 

hemispheres.

Less than one-eighth of Ganymede’s surface has been 

imaged at higher resolution than 1 km/pixel, but, where available, these higher resolution images were used to elucidate unit 

boundaries and aid geological interpretation. Most of the highresolution Voyager imagery that formed the core of the mosaic is 

available in the southern antijovian hemisphere and the northern 

subjovian hemisphere (fig. 1). Lighting and viewing angles are 

highly variable across the mosaic. Variations in emission angle 

(fig. 2) had the greatest effect on our ability to distinguish material units, with many subunits of light material becoming indistinguishable at high emission angles (near the limb). The uneven 

nature of imaging coverage at Ganymede has led to a global 

geologic map that is more detailed in some areas than others. 

In areas with high-resolution image coverage, the 1:15,000,000 

scale of the map limited us to only delineating units larger than 

30 km (2 mm at map scale). The ability to recognize characteristic morphological features used for defining map units drops 

off precipitously at image resolutions lower than 2 km/pixel and 

emission angles greater than 60° (Patterson and others, 2010). 

Most of the undivided material units on the map occur in areas 

of low-resolution and (or) high-emission angle, most notably in 

the trailing hemisphere (near long 90° E.).

Relation to previous mapping

After the Voyager encounters with Ganymede, several 

regional geologic maps were published at 1:5,000,000 scale, 

covering areas near the subjovian and antijovian points (Guest 

and others, 1988; Murchie and Head, 1989; Lucchitta and 
2

others, 1992; Croft and others, 1994; De Hon and others, 

1994; Wagner and others, 1995; Wilhelms, 1997; Underwood 

and others, 1997). Galileo gap-fill imaging links the separate 

Voyager image sets together, permitting the production of a 

global map to link the understanding from the regional maps 

into a unified picture of Ganymede geology. The selection of 

units for this map was strongly influenced by the need to apply 

unit definitions consistently across images of widely variable 

resolution, incidence angle, and emission angle. Therefore, the 

range of consistently separable units is narrower than that found 

on some previously published regional maps. See Patterson and 

others (2010) for a unit-by-unit comparison to previous maps.

A common tension in planetary mapping is whether to 

explicitly map all structural features or whether to include them 

within the definitions of material units. The treatment of furrow 

structures in relation to dark material units in previous maps 

was inconsistent in this respect; some mappers treated furrows 

as structural features (for example, Guest and others, 1988), 

while others treated furrows as material units (for example, 

Lucchitta and others, 1992) or as both structural features and 

material units (for example, Underwood and others, 1997). 

This map separates furrows from the definition of dark material 

units and displays them as separate structural features. Previous 

regional maps of Ganymede consistently used grooves as part of 

the definition of light material units (for example, Murchie and 

Head, 1989; De Hon and others, 1994). Grooves are generally 

spaced a few km apart, so a map at the 1:15,000,000 scale that 

displayed all grooves as separate structural features would be 

indecipherable. Thus, grooves are included within the definition 

of material units on this map, and only representative groove 

orientations are displayed.

Several previous regional maps of Ganymede used rim 

degradation states to classify crater units (for example, Guest 

and others, 1988; Wagner and others, 1995). Because of highly 

variable lighting and viewing geometry of the source images 

used for this map, classification of craters based on the morphology of rim degradation could not be consistently and 

reliably applied across the globe. Thus, crater classification is 

based on more consistently observable criteria: the presence 

or absence of visible continuous ejecta and the albedo contrast 

of the crater or its rays with the surrounding material. The 

map only displays the continuous ejecta of craters; displaying 

discontinuous ejecta and crater rays obscures too much information about the underlying geology, so they were omitted from 

the map. Only craters larger than 30 km diameter are shown on 

the map; below this diameter the apparent density of mapped 

craters on the surface becomes a strong function of image 

resolution.

Age determination

Relative age relations of mapped units were determined 

based on crosscutting relations and differences in crater density. 

Dark cratered material (dc) is crosscut by grooves to form dark 

lineated material (dl). Dark materials and reticulate material are crosscut by light materials (fig. 3). Light materials are 

divided into three broad age categories based on crosscutting 

relations. The youngest light material units (lg3, ls3, li3) are not 

crosscut by any other light units, while the oldest (lg1, ls1, li1) 

are crosscut by all adjacent light units. Intermediate-age light 

material units (lg2, ls2, li2) are crosscut by the youngest units, 

and intermediate units in turn crosscut the oldest units. Dark lineated (dl) and reticulate (r) materials sometimes share common 

groove spacing, morphology, and orientation with adjacent 

old light materials (lg1, ls1, li1), indicating that they may have 

formed contemporaneously. Palimpsests are divided into ancient 

palimpsests (p1), which are crosscut by light material, young 

palimpsests (p2), which overlie light material, and undivided 

palimpsests (pu), which do not come in contact with light material and, thus, crosscutting relations cannot be used for relative 

age determination. The p2 palimpsests, Epigeous and Zakar, 

overlie all ages of light materials, Teshub overlies undivided 

light materials (l) and is cut by young light grooved material 

(lg3), and Hathor overlies undivided light material (l), while its 

secondary craters overlie old light subdued material (ls1) and 

intermediate light grooved material (lg2). All basin materials 

(br, bs, bi) overlie all ages of light materials. Some degraded 

crater materials (c1) are crosscut by dark lineated (dl) and light 

materials, while other degraded crater materials overlie light 

materials. Partially degraded and fresh crater materials (c2, c3) 

overlie all other material units.

Crater density statistics on the global map have large 

uncertainties due to inconsistent lighting and viewing geometries from one area to another. The shaded area of figure 4 

shows crater density for the material units, using only the craters shown on this map (>30 km diameter) to compute density. 

Craters larger than 30 km diameter can be consistently mapped 

across the globe, but the number of craters is relatively small, 

providing poor statistics for map units with small areal coverage, and thus results are not shown for units br, bs, bi, r, p1, p2,

pu, and pi. The only firm conclusion that can be drawn from the 

>30 km craters is that dark cratered material (dc) is old relative 

to lg3, ls3, li3, lg2, ls2, li2, lg1, ls1, li1, and dl. Better statistics 

using smaller diameter craters cannot be consistently performed 

on a global scale due to wide variations in available image data. 

Results from examining craters larger than 20 km diameter are 

shown in the white area of figure 4, giving better statistics for 

small units, with the caveat that craters may be undercounted in 

some areas with poor image coverage. This data also shows that 

dark cratered material has the highest crater density and, within 

error bars, the rest of the crater densities on material units 

are consistent with their crosscutting relations. See Patterson 

and others (2010) for further discussion of crater densities on 

mapped units.

Geologic history

The age relations on Ganymede, as illustrated in the 

Correlation of Map Units, show that Ganymede’s geologic 

history can be divided into three distinct phases: before, 

during, and after the formation of light materials. The distinction between the first and second phases is typified by the 
3

high-resolution Galileo observation in figure 3 of intermediate 

light subdued material (ls2) in Harpagia Sulcus crosscutting the 

dark cratered material (dc) of Nicholson Regio. The third phase 

begins with the formation of the Gilgamesh basin (br, bs, bi) 

on top of all light material units. Thus, we name the time before 

light material formation the Nicholsonian, we name the time 

during light material formation the Harpagian, and we name the 

time after light material formation the Gilgameshan.

Nicholsonian Period 

Dark cratered material (dc, d) is the oldest material unit on 

Ganymede. The formation of dark material through mixture of 

ice with refractory non-ice materials is a distinguishing process 

during the Nicholsonian. The surface of dark cratered material was bombarded by impactors, forming furrow structures, 

palimpsest materials (p1, pu, pi), and degraded crater materials 

(c1, cu).

Harpagian Period

During the Harpagian, almost two thirds of Ganymede 

was resurfaced by the formation of new material units. Tectonic 

deformation of dark cratered material formed dark lineated material (dl, d) and some reticulate material (r). A combination of 

tectonism and (or) cryovolcanism produced a complex patchwork of light material units (lg3, ls3, li3, lg2, ls2, li2, lg1, ls1, li1,

l) covering the majority of Ganymede’s surface. Grooves and 

depressions formed as structural features during the Harpagian. 

Various areas of light subdued material have been interpreted to 

have a cryovolcanic origin (Schenk and others, 2001) or a crustal 

spreading origin similar to bands on Europa (Head and others, 

2002). The two light units that cover the greatest surface area are 

old subdued light material (ls1) and young grooved light material 

(lg3), indicating a possible shift from cryovolcanic to tectonic 

processes through the Harpagian. Also during this period, large 

impacts continued to produce palimpsest materials (p2, pu, pi), 

and more degraded crater materials (c1, cu) formed.

Gilgameshan Period

Only impact-related materials have formed during the 

Gilgameshan. A very large impact formed basin materials (br, bs, bi) at Gilgamesh. A small amount of palimpsest 

materials (p2, pu, pi) formed early in this period, but later large 

impacts formed features that exhibit more topographic relief and 

more typical crater morphology. An interpretation of this change 

in crater morphology is that heat flow decreased dramatically 

during this time period, thickening the cold elastic layer of 

ice near the surface. The last of the degraded crater materials 

(c1, cu) formed early in this period, while later impacts formed 

partially degraded crater materials (c2, cu), followed by fresh 

crater materials (c3, cu) in the most recent times.

Crater rims and domes appear to have formed throughout 

the history of Ganymede. The secondary craters shown on the 

map are associated with craters formed in the Gilgameshan 

Period.

References Cited

Becker, T., Archinal, B., Colvin, T., and 4 others, 2001, Final 

digital global maps of Ganymede, Europa, and Callisto, in 

Lunar and Planetary Science Conference, XXXII: Houston, 

Tex., Lunar and Planetary Institute, abstract no. 2009.

Croft, S., Cassachia, R., and Strom, R., 1994, Geologic map of 

the Tiamat Sulcus (Jg–9) quadrangle of Ganymede: U.S. 

Geological Survey Miscellaneous Investigations Map 

I–2328, scale 1:5,000,000.

De Hon, R.A., Leith, A.C., and McKinnon, W.B., 1994, 

Geologic map of the Hathor region (Jg–15) of Ganymede: 

U.S. Geological Survey Miscellaneous Investigations Map 

I–2388, scale 1:5,000,000.

Guest, J.E., Bianchi, R., and Greeley, R., 1988, Geologic map 

of the Uruk Sulcus (Jg–8) quadrangle of Ganymede: U.S. 

Geological Survey Miscellaneous Investigations Map 

I–1934, scale 1:5,000,000.

Head, J.W., Pappalardo, R.T., Collins, G.C., and 8 others, 2002, 

Evidence for Europa-like tectonic resurfacing styles on 

Ganymede: Geophysical Research Letters, v. 29, p. 2151.

Lucchitta, B.K., Barnes, C.W., and Glotfelty, M.F., 1992, 

Geologic map of the Memphis Facula (Jg–7) quadrangle 

of Ganymede: U.S. Geological Survey Miscellaneous 

Investigations Map I–2289, scale 1:5,000,000.

Moore, J.M., Asphaug, E., Morrison, D., and 11 others, 1999, 

Mass movement and landform degradation on the icy 

Galilean satellites: Icarus, v. 140, p. 294–312.

Murchie, S.L., and Head, J.W., 1989, Geologic map of the 

Philus Sulcus (Jg–4) quadrangle of Ganymede: U.S. 

Geological Survey Miscellaneous Investigations Map 

I–1966, scale 1:5,000,000.

Pappalardo, R.T., and Collins, G.C., 2005, Strained craters 

on Ganymede: Journal of Structural Geology, v. 27, p. 

827–838.

Pappalardo, R.T., Collins, G.C., Head, J.W., and 6 others, 

2004, Geology of Ganymede, in Bagenal, F., Dowling, T., 

McKinnon, W., eds., Jupiter: Cambridge, UK, Cambridge 

University Press, p. 363–396.

Pappalardo, R.T., Head, J.W., Collins, G.C., and 13 others, 

1998, Grooved terrain on Ganymede: Icarus, v. 135, p. 

276–302.

Patterson, G.W., Collins, G.C., Head, J.W., and 4 others, 2010, 

Global geological mapping of Ganymede: Icarus, v. 207, p. 

845–867.

Prockter, L.M., Head, J.W., Pappalardo, R.T., and 12 others, 

1998, Dark terrain on Ganymede: Icarus, v. 135, p. 

317–344.

Schenk, P.M., McKinnon, W.B., Gwynn, D., and Moore, J.M., 

2001, Flooding of Ganymede’s bright terrains by lowviscosity water-ice lavas: Nature, v. 410, p. 57–60.

Shoemaker, E.M., Lucchitta, B.K., Wilhelms, D.E., Plescia, 

J.B., and Squyres, S.W., 1982, The geology of Ganymede, 

in Morrison, D., ed., Satellites of Jupiter: Tucson, 

University of Arizona Press, p. 435–520.

Underwood, J.R., Casacchia, R., Woronow, A., and Teeling, 

M.J., 1997, Geologic map of the Galileo Regio 
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quadrangle (Jg–3) of Ganymede: U.S. Geological 

Survey Miscellaneous Investigations Map I–2534, scale 

1:5,000,000.

Wagner, R.J., Jaumann, R., and Neukum, G.P., 1995, Geologic 

map of the Etana region (Jg–1) of Ganymede: U.S. 

Geological Survey Miscellaneous Investigations Map 

I–2497, scale 1:5,000,000.

Wilhelms, D.E., 1997, Geologic map of the Osiris (Jg–12) and 

Apsu (Jg–13) quadrangles of Ganymede: U.S. Geological 

Survey Miscellaneous Investigations Map I–2422, scale 

1:5,000,000.