**GEOLOGIC MAP OF THE**

**OSIRIS (Jg-12)**

**AND**

**APSU SULCI (Jg-13)**

**QUADRANGLES**

**OF GANYMEDE**

By Don E. Wilhelms

1997

Prepared for the National Aeronautics and Space Administration

by

U.S. Department of the Interior, U.S. Geological Survey

> (Published in hardcopy as USGS Miscellaneous Investigations Series Map
> I-2442, as part of the Atlas of Jovian Satellites, 1:5,000,000
> Geologic Series. Hardcopy is available for sale from U.S. Geological
> Survey, Information Services, Box 25286, Federal Center, Denver, CO
> 80225)

Please direct questions or comments about the digital version to:

> Richard Kozak
>
> U.S. Geological Survey
>
> 2255 N. Gemini Drive
>
> Flagstaff, AZ 86001
>
> e-mail: rkozak@flagmail.wr.usgs.gov

**DESCRIPTION OF MAP UNITS**

LIGHT MATERIALS

s **Smooth material**---Forms predominantly smooth or slightly hummocky
surfaces. Delimited by solid contact lines or structure symbols where
sharply bounded; delimited by dashed contact lines where apparently
gradational with grooved material. Queried where obscured by rays and
could be grooved, palimpsest, or dark material. *Interpretation:*
Undeformed or slightly deformed ice derived from extruded water or
slush. Patches distal to Gilgamesh Basin may be Gilgamesh ejecta

g **Grooved material**---Forms domains predominantly consisting of two
or more parallel grooves, each 3--10 km wide and as long as 500 km; most
grooves gently curved but some straight or sharply curved; raised rims
and U-shaped profiles revealed by low-angle lighting. Queried where
obscured by rays and could be dark or reticulate material.
*Interpretation:* Ice derived from extruded water or slush like that of
smooth material, but deformed after extrusion

gf **Grooved material, fine**---Forms generally small domains. Grooves
and ridges short and narrow, commonly sinuous, produce finer overall
texture than in grooved unit but coarser than in smooth unit.
*Interpretation:* Materials like those of grooved unit but less severely
deformed, possibly because thinner

l **Light material, undivided**---Albedo typical of light units
(0.3--0.5) but topographic features too poorly imaged for further
identification; queried where obscured by rays and could be dark
material. *Interpretation:* Ice derived from extruded water or slush

RETICULATE MATERIALS

r **Reticulate material**---Cut by short grooves meeting at high angles.
Intermediate in albedo between typical dark and light materials,
commonly darkest in the grooves, brightest on the highs.
*Interpretation:* Dark material disrupted by gridlike faults and coated
by thin ice. Predates nearby grooved material

dr **Dark reticulate material**---Similar to reticulate material (unit
r) but darker. *Interpretation:* Formed early in breakup of dark crust;
has little coating of fresh ice

DARK MATERIALS

ds **Dark smooth material**---Forms relatively smooth surfaces, but more
highly textured than brighter smooth material (unit s) except in large,
smooth, lobate patch centered at lat 32˚ S., long 186˚.
*Interpretation:* Extruded ice containing some silicate material.
Patches distal to Gilgamesh Basin may be Gilgamesh ejecta

dl **Dark lineated material**---Contains linear grooves more widely
spaced and with rougher edges than grooves of light grooved material.
Queried where obscured by rays and could be reticulate or light grooved
material. *Interpretation:* Structurally deformed dark material,
possibly precursor of light grooved material

dc **Dark cratered material**---Contains many degraded craters and
crater segments. Darkest extensive unit in map area. *Interpretation:*
Less modified than other units but probably younger than original crust

df **Dark furrowed material**---Extended surfaces with widely spaced
furrows a few kilometers wide having raised, scalloped, ragged rims.
Greatest furrow concentration in map area forms subcircular system
concentric around point in

palimpsest at lat 28˚ S., long 153˚. *Interpretation:* Ancient crust
fractured by shock waves from large impacts followed by structural
modification that opened the fractures (Schenk and McKinnon, 1987)

dh **Dark hummocky material**---Forms small elevated patches having
rough surfaces. *Interpretation:* Ancient ejecta or intensely deformed
part of ancient crust

d **Dark material, undivided**---Albedo typical of dark units (0.2--0.4)
but topographic features too poorly imaged or too limited in extent for
further identification. Queried where obscured by rays and could be a
light or reticulate unit

CRATER AND PALIMPSEST MATERIALS

\[Only craters 20 km in diameter and larger are mapped\]

c3 **Crater material, unit 3**---Interior and exterior materials of
sharply textured, bright-rayed craters. Some albedos as high as 0.73
(McKinnon and Parmentier, 1986). *Interpretation:* Youngest impact
craters in the map area and probably younger than all other units

cs3 **Secondary crater material, unit 3**---Interior and exterior
materials of small, clustered craters within or near crater rays. (Rays
not separately mapped because most show on base map.) *Interpretation:*
Secondary-impact craters responsible for excavating dark and light
substrates

c2 **Crater material, unit 2**---Interior and exterior materials of
well-formed, rayless craters. *Interpretation:* Impact craters of
intermediate age formed throughout long time period

cs2 **Secondary crater material, unit 2**---Interior and exterior
materials of small, clustered craters adjacent to rayless craters.
*Interpretation:* Secondary-impact craters and their deposits

c1 **Crater material, unit 1**---Interior and rim materials of highly
degraded craters. *Interpretation:* Either old or anomalously degraded

cd **Crater material, dark**---Forms floors and patches on flanks of
some craters, including those too small to map at lat 22.5˚ S., long
141˚; lat 27.5˚, long 167.5˚; and lat 28.5˚ S., long 194˚. Albedo as low
as 0.11 (Schenk and McKinnon, 1991). *Interpretation:* Ice contaminated
by projectile, or extrusions of material contaminated by silicates

p **Palimpsest material**---Forms circular or elliptical patches
intermediate in albedo and smoothness between surrounding dark and light
materials. Queried where no rounded form seen. *Interpretation:*
Material of old craters or basins flattened by relaxation of crust;
queried occurrences could be extruded thin light material

GILGAMESH BASIN MATERIALS

bf **Basin floor material**---Smooth to slightly textured material
composing central depressed floor of Gilgamesh. *Interpretation:* Either
melted target material or later interior deposit

br **Basin rugged material**---Coarsely hilly interior and rim materials
of Gilgamesh. May form outlying patches subconcentric with basin and
containing subconcentric small ridges and hummocks. *Interpretation:*
Primary ejecta of Gilgamesh and deformed target material. Outlying
patches possibly decelerated ground-flow ejecta

bs **Basin smooth material**---Smooth or gently hummocky material
transitional with

rugged material. *Interpretation:* Gilgamesh primary ejecta, possibly
mixed with some locally derived substrate material

csb **Secondary crater material of basin**---Interior and surrounding
deposits of clustered and elongate craters as much as 20 km across,
arrayed radially to Gilgamesh. *Interpretation:* Secondary-impact
craters of Gilgamesh and their ejecta

**Contact**---Dashed where gradational; dotted where buried; queried
where obscured by rays (especially west and south of Osiris), by shadows
(especially near terminator), or by poor photographic resolution.
Includes groove-domain boundaries

**Long, linear trough**---Dotted where subdued; queried where continuity
uncertain. Similar to one or pair of grooves in light grooved material,
but generally longer and deeper; may extend into dark terrain; includes
flat-bottomed, narrow troughs. Interpreted as master fracture along
which dark terrain broke up most deeply. Apparently buried where subdued

**Deep, short linear trough in dark and light materials**---Interpreted
as relatively young fracture

**Sharp groove trend**---In grooved material; schematic

**Subdued groove trend**---In grooved or smooth material; schematic

**Reticulate groove trend**---Schematic

**Furrow in dark and palimpsest materials**---Narrow, linear depression
with raised, scalloped rim; shorter and more irregular than long linear
troughs and more widely spaced than light-material grooves. Interpreted
as graben formed by modification of impact-related arcuate fractures
(Schenk and McKinnon, 1987)

**Lineament in dark material**---Schematic

**Ridge in light material**---Dotted where buried. May be caused by
near-surface compression

**Sinuous rille**---Narrow, curvilinear depression. Possibly formed by
flowing water

**Irregular depression**---Widening at end of sinuous rille. Possibly
site of water eruption

**Crater rim crest**

**Buried crater rim crest**

**Inward-facing scarp on crater floor**---Line marks top

**Peak on crater floor**

**Pit on crater floor**---Dot where too small to map

**Dome on crater floor**---Line marks base

**Palimpsest ring**

**Basin ring arc**

**GENERAL FEATURES**

A mosaic of dark-toned and light-toned geologic units about equal in
total area characterizes Ganymede, whose diameter of about 5,262 km
makes it Jupiter's and the Solar System's largest satellite (Burns,
1986). Ganymede's low bulk density (1.94 g/cm3; Burns, 1986), spectral
properties, and high albedos indicate that water ice is the main
component of both the dark and the light units of the crust (Smith and
others, 1979; Johnson and others, 1983; Clark and others, 1986; McKinnon
and Parmentier, 1986). Impact craters pepper but do not saturate the
surface.

Light units predominate in the map area, which includes the parts of two
quadrangles that were imaged at geologically useful scales and
illuminations by Voyager 2 during its flyby of the Jovian system in July
1979 (Smith and others, 1979). Ganymede is locked in synchronous
rotation so that longitudes 90˚--270˚, including the map area, always
face away from Jupiter. Territory east of long 180˚, including the
Osiris quadrangle (Jg-12), is on the hemisphere that leads in Ganymede's
revolution around Jupiter; territory west of 180˚, including the Apsu
Sulci quadrangle (Jg-13), is on the trailing hemisphere. The
southeastern corner of the map area includes Ganymede's and the Jovian
system's largest impact structure with well-preserved morphology, the
Gilgamesh Basin.

**DARK AND LIGHT MATERIALS**

A somewhat higher crater density of the dark materials (Shoemaker and
others, 1982; Chapman and McKinnon, 1986; Murchie and others, 1989) and
interruption of the dark units and their structures by lanes, wedges,
and polygons of light materials demonstrate that the dark are mostly,
though not wholly, older than the light materials (Lucchitta, 1980;
Golombek and Allison, 1981; McKinnon and Parmentier, 1986; Murchie and
others, 1986; Squyres and Croft, 1986). Contamination of pure ice by
less than 10 and possibly only 3 percent of dark grains, possibly of
cometary and meteoroid origin, can explain the albedo difference between
the dark (0.2--0.4) and light (0.3--0.5) materials (Clark and others,
1986; McKinnon and Parmentier, 1986). The generally older dark units
have had more time to accumulate this debris. They are subdivided on the
basis of superposed craters or structures such as furrows and
lineations. Except for the dark smooth material (unit ds), all dark
units have been modified since formation, and none is believed to
represent the original crust (Murchie and others, 1989, 1990). Embayment
of older units and structures suggests that at least one patch of dark
smooth material (centered at lat 32˚ S., 186˚) is as young as some light
material and was emplaced as a fluid, presumably water or slush. The
dark tone suggests that the water or slush contained some silicate
material.

Estimates that Ganymede's light material is only about 1-2 km thick
(Schenk and McKinnon, 1985, 1991) are supported qualitatively by its
commonly splotchy albedo pattern, shallow burial of craters (lat 23˚ S.,
long 165˚), and remnant slivers and islands of dark material. A few
irregular dark-light contacts suggest pinching out of a thin light
veneer (for example, at lat 21˚ S., long 156˚; lat 21.5˚ S., long 133˚).
The dark reticulate material (unit dr) and parts of the lighter
reticulate material (unit r) apparently are coated by so little light
material that the dark substrate shows through.

Most dark-light contacts, however, are sharp and linear or curvilinear
at the map scale, and many of them coincide with evident faults.
Tectonism must therefore have played a major role in deposition of the
light material (Squyres and Croft, 1986). Some types of tectonism seem
excluded. That the dark material did not sink deeply into the crust to
make way for the light follows from the light material's thinness. Major
active rifting like that at Earth's midocean ridges is ruled out because
only the general outlines of the dark patches, but no widely separated
parts of truncated individual craters or other small features, can be
matched across intervening swaths of light terrain (Squyres and Croft,
1986). Signs of compression are few, although compression may have
created some ridges (for example, at lat 28˚ S., long 120˚; lat 47˚,
long 129˚; and in the diamond-shaped block centered in dark

cratered material at lat 62˚ S., long 156˚). Only minor lateral shear is
indicated in the map area, an interpretation also made for the whole of
Ganymede by Schenk and McKinnon (1987); however, Murchie and Head (1988)
favored major shear and rotation in many regions.

Ganymede's principal tectonic regime seems, instead, to have been
crustal extension on the order of 5 to 7 percent caused by global
expansion on the order of only 1 percent (Squyres, 1980; McKinnon, 1981;
Golombek, 1982; Squyres and Croft, 1986). Emplacement of light materials
therefore required less profound modification of the crust than might be
implied by their great extent. The crust stretched enough to open long,
linear troughs (bowtie symbol) and short, ragged lineations in patches
of dark lineated material (unit dl) and reticulate materials. Some of
the linear troughs cross only dark materials, but most of them or their
subdued, probably thinly buried extensions (for example, those near lat
32.5˚ S., long 129˚) bound patches of reticulate or light materials. The
fracturing allowed small amounts of water or slush to flow or spray out
upon the surface. Most light units originated when larger amounts of
this new material filled shallow depressions created where blocks of
dark material dropped passively along the visible and buried faults
(McKinnon and Parmentier, 1986; Squyres and Croft, 1986). Elevation
differences consistent with the downdrop of dark crustal materials are
observed along many dark-light contacts in the favorably illuminated
northeastern part of the map area.

The most typical light material, the grooved unit (unit g), is
characterized by narrow, regularly spaced, parallel grooves bundled in
sets or domains of two or more grooves and locally measuring as much as
100 km in width. Some groove sets parallel long linear troughs, forming
what Murchie and others (1986) called groove lanes. Other groove sets
terminate against the troughs or domains of light material in a
T-relation, in which the crosscutting structure is commonly the older
(Golombek and Allison, 1981; Murchie and Head, 1988). The most common
orientation of the troughs and largest grooves in the map area is
west-northwest. In the northeastern part of the map area, other,
seemingly random trends lie between the west-northwest structures. Some
major troughs and grooves veer off to the west-southwest at about long
180˚ and, in the Apsu Sulci quadrangle, conspicuously truncate the
west-northwest structures.

Light material that is smooth on most available images (unit s) is also
abundant. It constitutes some entire domains, but more commonly it forms
parts of domains in which grooved material is the other component. A few
craters superposed on the smooth material are cut by grooves (lat 24˚
S., long 139˚--141˚). Therefore, the processses that formed grooves and
light materials, although related, are not identical (Squyres and Croft,
1986). Some patches of smooth material embay grooved and other older
units in the manner of an extruded fluid (for example, at lat 25.5˚ S.,
long 126.5˚; lat 43.5˚ S., long 128˚). Smooth units and groove sets
intergrade, however, in most domains of light material (for example, at
lat 22˚ S., long 151˚; lat 31˚ S., long 130˚), as shown by dashed
contacts or suggested by different densities of schematic groove
symbols. In these mixed domains, the smooth material apparently was the
first to form, followed by partial grooving. A reasonable inference is
that domains consisting entirely of grooved material also began as
smooth material.

The global expansion that fractured the dark crust and allowed the light
materials to rise and reach the surface may have directly caused some
grooving as well, at least in the groove lanes (Murchie and others,
1986). A more important determinant of local groove structure may be the
familiar property of the ordinary low-pressure phase of water ice
(ice I) to expand when it freezes. Parmentier and Head (1984)
hypothesized that after water or water-ice slush flooded the crustal
rifts, it formed a low arch as it froze inward from the edges and
expanded. Grooving, which commonly is concentrated at the edges and the
axes of a light domain, is the consequence of the increase of surface
area within each arched domain. Squyres and Croft (1986) pointed out
that each new deposit would freeze from the top and bottom, and the top
and bottom layers might be fractured when the

middle froze and expanded. The finely grooved material (unit gf), which
forms relatively small domains and lacks the many parallel, deep,
raised-rim grooves of the grooved material, may consist of thinner ice
that simply froze, expanded, and deformed itself within its confines
without arching. On the map, each domain that is believed to have been
mechanically isolated during groove formation by any mechanism is
bounded by fault symbols or solid contact lines.

**CRATERS AND PALIMPSESTS**

The impact origin of most craters on Ganymede is not disputed. Many
clusters and fields of satellitic craters having subequal sizes (map
units cs2 and cs3, and unmapped where scattered) attest to the
pervasiveness on Ganymede of the common Solar System process of
secondary impact. Only a few small pits at the heads of apparent sinuous
rilles show the signs of internal origin; flowing water emanating from
these pits may have cut the rilles.

The age of a crater relative to adjacent geologic units is usually
clear, but crater ages cannot easily be correlated in detail from region
to region. Most nonrayed craters are therefore assigned to a single map
unit (c2), and only a few highly degraded craters or crater fragments
are assigned an older age (unit c1). As on other moons and planets,
craters whose ejecta created rays during secondary impact are generally
the youngest (unit c3). Rays obscure other geologic units west and
southwest of Osiris, which has a diameter of 105 km and therefore, is
the largest and most conspicuous young crater in the map area. Crater
rays consist mainly of substrate material exposed and ejected by the
secondary impact of the primary crater's ejecta. Ray brightness,
therefore, depends partly on the brightness of the substrate unit:
brightest on light materials and darkest on dark. The density of mapped
craters (20 km and larger) is greater east of long 180˚ than west of it,
an observation that fits the general finding that more craters of the
mapped sizes were formed on the leading hemisphere than on the trailing
hemisphere during a given time interval (Passey and Shoemaker, 1982).
This distribution suggests that the impacting bodies revolved around the
Sun, not Jupiter (Murchie and others, 1989).

The weakness of Ganymede's icy crust has greatly affected the appearance
and retention of craters. The transition from craters having simple
bowl-shaped interiors to those with central peaks occurs at crater
diameters of only 3--6 km, well below the 19--27 km for the Moon (Passey
and Shoemaker, 1982; Schenk, 1991). Therefore, all the craters mapped
here are complex. Because surface gravity is similar on Ganymede and the
Moon (144 and 162 cm/s2, respectively), substrate properties must be a
more important factor controlling the onset of the rebounds that create
peaks than is gravity (Schenk, 1991). Schenk (1991) also cited crustal
weakness at the time of impact as the cause of a relative shallowness of
Ganymede's craters. Some craters larger than 25 km across possess
central pits in or instead of the peaks, and craters larger than 50 km
possess only pits (Schenk, 1991). The pits may originate by collapse of
the peaks (Passey and Shoemaker, 1982) or by some as yet undetermined
mechanism.

Apparently, the crust was especially weak early in its history because
of a relatively high heat flow (McKinnon and Parmentier, 1986). One
result is the flattening, probably by viscous relaxation (Passey and
Shoemaker, 1982; Shoemaker and others, 1982), of old, large craters and
basins into approximately circular, light-toned patches called
palimpsests (Smith and others, 1979). Viscous relaxation or ice
volcanism (Murchie and others, 1989, 1990) may also have destroyed many
of the smaller old craters that must have formed along with the large
ones. Later, cooling led to a more nearly elastic behavior of the crust
and to the retention of younger craters (units c2 and c3) (Hillgren and
Melosh, 1989).

**THE GILGAMESH BASIN**

The Gilgamesh impact structure is considered a basin rather than a
crater because of its

large size and multiple concentric, mountainous ring arcs. The
relatively low density of superposed craters and the superposition of
its deposits and secondary craters (unit csb) on almost all other nearby
units show that it is relatively young, about 3.5 billion years,
according to an interpretation of its crater density based on
correlation with lunar crater densities (Shoemaker and others, 1982).
The Gilgamesh Basin has not flattened out into a palimpsest and is not
surrounded by concentric furrow systems like other basin-scale craters
on Ganymede; therefore, the lithosphere had presumably cooled and
thickened by the time it formed (Chapman and McKinnon, 1986).
Concentration of the secondary craters north and south of Gilgamesh
indicate that the impacting body approached approximately in the
equatorial or ecliptic plane.

Gilgamesh is typical of lunar basins in some ways but atypical in
others. Its many secondary craters, which appear in abundance about 500
km from its center, closely resemble those of the lunar Orientale Basin
in size (as much as 20 km across), in shape (subcircular to elongate),
and in their radial clustering (Wilhelms, 1987, chapter 4). Outward from
its floor material (unit bf), the deposits of Gilgamesh grade from a
rugged-appearing material (unit br) to a smoother facies (unit bs) that
shows some evidence of outward flow in the form of indistinct lobes.
Outlying patches of the rugged unit (queried) also suggest ground flow,
for they resemble basin-concentric ridges (\"dunes\") that were created
around lunar basins when ejecta flowing along the surface piled up
against obstacles.

However, the ring arcs of Gilgamesh are less well defined than those of
most lunar basins (possibly another effect of the thick lithosphere;
Chapman and McKinnon, 1986). Not even the longest and most conspicuous
arc, whose radius averages about 275 km, defines an unambiguous
topographic basin rim (the ring that encloses most of a basin's
depression). The appearance of abundant secondary craters about 500 km
from the center would indicate a basin diameter of about 500 km on the
Moon (Wilhelms, 1987, chapter 4. However, this distribution of secondary
craters suggests a diameter closer to 450 km on Ganymede because ejecta
particles can fly farther under Ganymede's slightly lower surface
gravity. Even well-defined topographic rims, however, may or may not
bound the excavation cavities of lunar and planetary basins (Wilhelms,
1987, p. 77-81). At least it seems clear that Gilgamesh's excavation was
not limited to its best-defined circular feature---the inner,
smooth-floored depression 125 km in diameter---for the vastly simpler
crater Osiris has a diameter only 20 km less in diameter (Lucchitta and
Ferguson, 1988). Possibly Gilgamesh has no single well-defined
excavation cavity, but rather a poorly defined zone in which excavation
is greatest at the center and increasingly irregular and discontinuous
farther out.

**GEOLOGIC HISTORY**

After accretion, Ganymede differentiated into upper layers rich in ice
and lower layers and a core containing more silicate material (McKinnon
and Parmentier, 1986). Repeated impacts created shallow craters and
basins that relaxed even further into pancake-like palimpsests or that
disappeared altogether. Dark cosmic material progressively darkened the
surface. Dark ice volcanism may have resurfaced part or all of the
ancient crust. Part of the crust was fractured by slight global
expansion, possibly caused by the conversion of dense phases of water
ice in the interior into less dense phases (Squyres and Croft, 1986).
The dark terrain subsided slightly where intensely fractured, and water
or slush from the interior filled the resulting lows. Global expansion
and the freezing and expansion of the water or slush as ordinary ice
created rimmed grooves over much, though not all, of its newly occupied
area. Impacts continued throughout this history, and the resulting
craters have remained moderately well preserved since the major
disruption of dark materials and emplacement of light materials.
Ganymede's largest impact into the thick lithosphere of this relatively
late period created the Gilgamesh Basin about 3.5 billion years ago.

**REFERENCES CITED**

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and Matthews, M.S., eds., Satellites: Tucson, University of Arizona
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satellites, *in* Burns, J.A., and Matthews, M.S., eds., Satellites:
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of natural satellites, *in* Burns, J.A., and Matthews, M.S., eds.,
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to the Nineteenth Lunar and Planetary Science Conference, Houston, March
14--18, 1988: Lunar and Planetary Institute, p. 701--702.

McKinnon, W.B., 1981, Tectonic deformation of Galileo Regio and the
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Murchie, S.L., Head, J.W., and Plescia, J.B., 1989, Crater densities and
crater ages of different terrain types on Ganymede: Icarus, v. 81, p.
271--297.

\_\_\_\_\_\_1990, Tectonic and volcanic evolution of dark terrain and
its implications for the internal structure and evolution of Ganymede:
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Parmentier, E.M., and Head, J.W., 1984, A model for the emplacement and
solidification of bright terrain on Ganymede, *in* Abstracts of papers
submitted to the Fifteenth Lunar and Planetary Science Conference,
Houston, March 12--16, 1984: Lunar and Planetary Institute, p. 633--634.

Passey, Q.R., and Shoemaker, E.M., 1982, Craters and basins on Ganymede
and Callisto:

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Satellites of Jupiter: Tucson, University of Arizona Press, p. 379--434.

Schenk, P.M., 1991, Ganymede and Callisto: Complex crater formation and
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U.S. Geological Survey, 1987a, Shaded relief and surface markings of the
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Base west of long 180° from U.S. Geological Survey, 1987a; base east of
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