**GEOLOGIC MAP OF THE**

**URUK SULCUS**

**QUADRANGLE (Jg-8)**

**OF GANYMEDE**

By

J.E. Guest, Remo Bianchi,

and Ronald Greeley

1988

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--1934, 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 relatively level surface associated with
light grooved material (unit g). Few isolated grooves; craters \>20 km
in diameter rare. *Interpretation:* Undeformed ice (low silicate-rock
content) that flooded surface or filled spaces created by breakup of
surface

g **Grooved material**---Forms surface cut by sets of long (as much as
400 km), parallel, straight or curvilinear grooves 5 to 12 km wide
arranged in domains showing sharp and intricate crosscutting relations.
Grooves locally orthogonal, forming reticulate pattern.
*Interpretation:* Ice (low rock content) that filled spaces created by
breakup of dark cratered terrain, dark cratered materials converted into
clean ice by melting and sinking of rock particles, or both; patterns of
grooves and ridges may be related to global tectonism or stress fields
developed within sulci as result of physical changes as water froze

DARK MATERIALS

ds **Smooth material**---Forms level plain in topographically low area
in dark cratered terrain near northeast corner of map area
*Interpretation:* Material consisting mainly of silicate rock (and
possibly some ice) emplaced by unknown process, possibly by mantling

dg **Grooved material**---Material of low but variable albedo cut by
closely spaced grooves (about 12 km apart) 25 to 100 km long and about 5
km wide; similar in texture to light grooved material but forms less
regular patterns. Generally occurs adjacent to sulci. *Interpretation:*
Mixture of silicate rock and ice having high rock content. Grooves
formed in same way as those on light grooved material. Forms areas of
incomplete breakup or replacement of dark cratered materials

dl **Lineated material**---Material of low but variable albedo cut by
short, narrow, linear depressions that are closely spaced (about 12 km
apart); unlike grooves of grooved material, depressions tend to be
sinuous and shallow. *Interpretation:* Same as for dark grooved material

dr **Reticulate material**---Material cut by two or more sets of grooves
20 to 70 km long and 4 to 8 km wide intersecting at angles ranging from
30° to 90° (Ferguson and others, 1982). Groove spacing ranges from 8 to
20 km; east-west sets more closely spaced than north-south sets. Grooves
show dark interiors; intervening areas have intermediate to high
albedos. Unit occurs along south edge of map area. *Interpretation:*
Same as for dark grooved material

**Cratered materials**

df **Furrowed**---Low-albedo material extensively cratered; cut by
parallel furrows. Furrows have scalloped edges, and in some places
margins of furrows have higher albedo than surrounding terrain. Area
between furrows generally rough, and small patches of dark, level
terrain occur in floors of topographic hollows. *Interpretation:*
Material consists of rock-ice mixture or ice coated by thin layer of
rock particles. Furrows may be tectonic or possibly part of ancient
multiringed basin and are some of the oldest structures recognized on
Ganymede

dc **Scabrous---**Forms rough, cratered surface that has closely spaced,
small, irregularly shaped hummocks and some randomly oriented linear
troughs a few tens of kilometers long and as wide as 10 km. Grades into
furrowed and vermicular units. *Interpretation:* Similar in lithology
and age to other dark cratered materials; origin of surface features
unknown; some may be remnants of crater rims

dv **Vermicular**---Albedo tends to be lower than that of other dark
cratered materials. Surface generally rough and cut by sinuous troughs a
few tens of kilometers long and as wide as 5 km. *Interpretation:*
Similar in origin and age to that of furrowed unit but may have higher
rock content. Origin of sinuous furrows unknown

CRATER MATERIALS

c3 **Unit 3**---Material forming craters that generally have bright rays
or very bright ejecta; few have dark ejecta. Internal structures include
poorly developed terraces, central peaks, central pits, and dark-floored
central pits having light central domes. Superposed on all other map
units. *Interpretation:* Ejecta and other materials associated with
youngest impact craters

c2 **Unit 2**---Material forming rim and floor of craters that have
relatively narrow, slightly subdued to subdued rim crests. Internal
structures similar to those in c3 craters. Albedo lower than that of
material of c3 craters. *Interpretation:* Degraded impact craters

c1 **Unit 1---**Material forming rim and floor of craters having highly
degraded rim crests. Albedo generally similar to that of dark cratered
materials. Characteristically cut by adjacent light grooved material.
*Interpretation:* Highly degraded impact craters

PALIMPSEST MATERIALS

p3 **Unit 3**---Relatively high albedo material forming circular feature
at north edge of map area; circular ridge segment near middle of
feature. Few superposed craters. Surrounded by small satellitic craters
as large as about 10 km in diameter. Cuts light grooved material.
*Interpretation:* Remnant of large impact crater deformed to low-relief
surface; satellitic craters formed by secondary impact. Postdates light
grooved material

p2 **Unit** **2**---Relatively high albedo material forming subcircular
features; few superposed craters. Circular ridge structures near middle
of features enclose palimpsest smooth material (unit ps). No satellitic
craters. Some features cut by light grooved material. *Interpretation:*
Same as for unit 3 but older and degraded. Most individual palimpsests
predate emplacement of light grooved material although some may be
younger

p1 **Unit** **1**---Forms subcircular to irregular rough areas of
generally higher albedo than surrounding material; no central circular
ridge structure. Cut by light grooved material. *Interpretation:* Origin
same as for unit 3 but degraded by impact craters and deformed by
endogenic activity; formed penecontemporaneously with dark cratered
materials

ps **Palimpsest smooth material---**High-albedo material forming smooth
plains in central parts of unit 2. *Interpretation:* Uncertain; may be
remains of viscously deformed central peak or dome

**Contact**---Dashed where approximately located; dotted where buried.
Includes boundaries between domains in light smooth and light grooved
materials

**Fault---**Ball and bar on downthrown side

**Deep linear trough in dark materials**

**Deep linear trough in light materials**

**Sharp groove trend---**Schematic

**Subdued groove trend---**Schematic

**Reticulate groove trends---**Schematic

**Furrow---**Only representative number mapped

**Lineament**

**Ridge**

**Shallow depression**

**Crater rim crest**

**Highly subdued or buried crater rim crest**

**Circular scarp on continuous ejecta**

**Inward-facing scarp on crater floor**

**Peak on crater floor**

**Pit on crater floor---**Circle outlines rim; dot where rim too small
to map

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

**Palimpsest ring**

**Light ejecta**

**Dark ejecta**

**Secondary craters**

**INTRODUCTION**

The Galilean satellite Ganymede was first imaged at high resolution by
Voyagers 1 and 2 in March and July 1979 (Smith and others, 1979a, b).
The best images available of the Uruk Sulcus quadrangle were obtained by
Voyager 2; their resolution is about 1.5 km per line pair. Their sun
angles range from about 75° to 30° above the horizon, giving good albedo
discrimination but short shadows so that features having shallow slopes
are difficult to discern. These conditions prevail particularly on the
west side of the map area where sun angles were highest.

Albedo, surface morphology and texture, and crater densities were the
principal characteristics used to distinguish one unit from another,
following planetary photogeologic mapping conventions (Wilhelms, 1972).
Crosscutting relations were used to determine the three-dimensional
relations between units. However, unlike the terrestrial planets, for
which the techniques for planetary geologic mapping were developed,
Ganymede has landforms that, apart from impact craters, are largely
structural features. Although every effort was made to identify
individual material units, the map has as much in common with tectonic
maps as with geologic maps on which units are distinguished in terms of
lithology and age. Thus, on this map some units of the same apparent
lithology and age are distinguished from one another by different
surface features.

On the basis of its density (1.93 g cm-3) and spectroscopic studies of
its surface, Ganymede is considered to be composed dominantly of water
ice (Morrison, 1982). Differences in albedo between units may be related
to the amount and composition of rocky material mixed with ice. Because
the physical characteristics of ice are different from those of silicate
rocks, geologic interpretation of Ganymede relies much less on
terrestrial analogs than do interpretations of the terrestrial planets.

PHYSIOGRAPHIC SETTING

The surface of Ganymede may be divided into two distinct regions: older,
dark cratered terrain and younger, light grooved terrain (Shoemaker and
others, 1982). Slightly less than half of the surface of the satellite
is composed of ancient cratered terrain (Shoemaker and others, 1982).

The Uruk Sulcus quadrangle lies along the south margin of Galileo Regio,
a large area of ancient cratered terrain, part of which appears in the
northeast corner of the map area. Galileo Regio is separated from Marius
Regio, another region of ancient terrain to the southwest, by a swath of
grooved terrain known as Uruk Sulcus. Marius Regio is in turn broken by
bands of grooved terrain, including Tiamat Sulcus, which cut across the
southwestern part of the map area. Each of these major terrains is
probably of roughly uniform lithology, but they may be divided into
smaller units of stratigraphic or tectonic significance or both.

Superposed on all these units are impact craters and their associated
deposits. These range in degree of preservation from crisp craters
exhibiting bright or dark rays to shallow craters that have degraded rim
crests and no discernible ejecta deposits. Large subcircular areas of
relatively high albedo, some of which contain low positive-relief
features, may be remnants of large craters. These features have been
termed palimpsests by Smith and others (1979a).

**STRATIGRAPHY**

DARK MATERIALS

The dark cratered units form irregularly shaped areas ranging in size
from several thousand kilometers across (for example, Galileo Regio) to
smaller areas tens to hundreds of kilometers across. Their main
characteristics are low albedo and relative abundance of craters. In
many areas the dark materials are cut by furrows and ridges of different
sizes and morphologies. These features and the patterns they form,
however, vary from one area to another. No distinct boundaries exist
between areas of different patterns.

In the southwestern part of Galileo Regio and the northwestern part of
Marius Regio in the map area, the dark cratered material characterized
by subparallel linear furrows is mapped as

furrowed material (unit df). The edges of the furrows are scalloped and
tend to be associated with regions of slightly higher albedo than the
surrounding terrain, especially in Galileo Regio. Furrows tend to be
more closely spaced in Marius Regio than in Galileo Regio.

The vermicular material (unit dv) occurs in the southeast corner of the
map area and is identified by the presence of sinuous troughs that are
randomly oriented. A third unit, termed scabrous (unit dc), has few
distinctive characteristics apart from small irregular hummocks giving a
rough appearance. The relative ages of dark cratered materials are
difficult to determine, and it is likely that the characteristics of
these units result as much from tectonism as from differences in
lithology.

Craters in the dark terrain range in size from the resolution limit to
about 100 km across and range in age from those that are contemporaneous
with the dark units to those that postdate all materials in the map
area. Some craters, especially larger ones, are considered palimpsests
(Smith and others, 1979a), which more commonly occur in the furrowed
facies of Marius Regio. In the furrowed material, all observed craters
are superposed on the furrows. This observation implies that the craters
postdate the furrows and confirms the view of Casacchia and Strom (1984)
that the furrows are some of the oldest features on Ganymede. The oldest
palimpsest material (unit p1) is, however, cut by furrows in places.

Within the dark cratered materials (especially in the furrowed unit) are
small areas of dark plains, one of which is large enough to be mapped as
dark smooth material (unit ds).

Along the boundaries between the dark cratered materials and the light
materials of the sulci, dark materials occur that have surface
characteristics in common with those of the light materials. The dark
grooved material (unit dg) has some patches of higher albedo and fewer
superposed craters than are seen on the dark cratered materials; most
craters are strongly degraded. In this unit the parallel grooves are
similar to those in the light grooved terrain, but the general pattern
is not as well developed, and there are no distinct domains of groove
sets.

The dark lineated material (unit dl) associated with Tiamat Sulcus, like
the dark grooved material, probably represents the partial breakup of
dark cratered materials by processes similar to those that formed the
light grooved material. Some areas of the southeastern extension of
Tiamat Sulcus contain dark reticulate material (unit dr) originally
defined by Ferguson and others (1982).

LIGHT MATERIALS

Light materials form two major northwest-trending sulci across the area.
Their widths range from 100 km to more than 500 km. In addition, narrow
zones of light material, some 10 km wide and as long as 80 km, cut the
dark cratered materials. Most of the light material is grooved to varied
degrees (unit g), and it is subdivided into domains of different groove
orientations. The grooves in Uruk Sulcus tend to be oriented either
parallel or perpendicular to the axis of the sulcus (Bianchi and others,
1984). Two domains in the southwestern part of the map area show a
reticulate pattern of grooves similar to that of the dark reticulate
material, but the grooves are more closely spaced. Most domains are
bounded by a deep trough interpreted to be a graben; these troughs are
not depicted on the map unless they cut across a domain.

Of lesser areal extent is light smooth material (unit s), which has an
albedo similar to that of the light grooved material and which occurs as
patches within it.

PALIMPSEST MATERIALS

These materials form subcircular areas 100 km to nearly 400 km across
that are generally brighter than the surrounding terrain. Crosscutting
relations indicate that palimpsests range in age from older to younger
than light grooved material. Ancient palimpsest material (unit pl) forms
the least circular outcrops of all the palimpsest materials and occurs
only within the dark cratered materials. The superposed impact-crater
densities are similar to those of the dark cratered materials. Unit 2 of
the palimpsest materials (unit p2) occurs within the dark cratered
terrain, and crosscutting relations suggest that most, if not all, of
this material is older than the light grooved terrain. Many of these
palimpsests contain areas of palimpsest smooth material (unit ps),
which, in some cases, is surrounded by a circular ridge. The youngest
palimpsest material (unit p3)

forms a circular feature surrounded by clusters of satellitic craters;
all are superposed on light grooved material as well as on dark
material. This superposition and the apparent freshness of the feature
indicate its relative youth.

CRATER MATERIALS

Impact craters in the Uruk Sulcus quadrangle range in morphology from
fresh, crisp craters that have well-defined ejecta and rays to features
that have low, narrow, battered rims. However, the degree of degradation
may not be a good indication of age on Ganymede: first, the initial
crater shape may be influenced by the physical characteristics of the
lithosphere and its thickness (Greeley and others, 1982), both of which
may have varied in time and place; and, second, the rate of degradation
may be controlled by subsequent impact and the state of the lithosphere
(Bianchi and Pozio, 1985).

The most degraded crater material (unit cl) is distorted and cut by
light grooved material, although in a few places the craters postdate
the light grooved material.

The most common type of crater material (unit c2) formed during nearly
the entire history of the map area and is found on almost all units. The
least degraded crater material (unit c3) postdates the light grooved and
light smooth materials.

Some small craters that have light central domes are floored by dark
materials. A few of the fresh craters appear to have produced low-albedo
ejecta, which indicates the presence of dark rocky layers below the
surface. Although most crater ejecta are featureless, some form a
concentric, shallow, outward-facing scarp similar to that of pedestal
craters on Mars (Horner and Greeley, 1982).

**STRUCTURAL AND GEOLOGIC HISTORY**

The interpretable geologic history within the Uruk Sulcus quadrangle
begins with the period of formation of the dark cratered materials.
These probably consist of a mixture of ice and silicate rocky material,
giving a relatively dark appearance, and may represent the original
planetary crust following accretion of the satellite. After (and
possibly during) its formation, some areas of the surface were deformed
to produce furrows (unit df), either by internal processes (Casacchia
and Strom, 1984) or as a result of basin-forming impact events (Smith
and others, 1979a; McKinnon and Melosh, 1980). Most of the preserved
craters were formed after the furrows, which suggests either that the
furrows were produced relatively quickly or that the impact rate was low
during their time of formation. However, the most ancient palimpsest
material in the area appears to have formed penecontemporaneously with
the furrows because it is cut by some of them. The furrows, therefore,
probably are the oldest structural features in the map area.

Areas of dark cratered materials other than the furrowed material
continued to be modified by internal processes, resulting in the
vermicular and scabrous textures. This modification was accompanied and
followed by the excavation of impact craters.

The next major event in the history of the map area was the breakup of
the lithosphere, which consisted of the dark cratered materials. This
breakup may have been caused by global expansion due to phase changes of
ice in the interior (Squyres, 1980; Parmentier and others, 1982) or by
mantle convection causing global tectonic stresses (Bianchi and others,
1986). As the lithosphere broke up, water or a slush of water and ice
welled up and froze at the surface, forming the light materials
(Lucchitta, 1980). During freezing and afterward, tectonic stresses,
together with stresses caused by volume changes during freezing,
resulted in the patterns of grooves and grabens in the light materials.
At the margins of the sulci, the dark cratered materials were probably
modified as a result of local stresses and temperature changes to form
the smooth, grooved, lineated, and reticulate units of the dark
matelials.

Probably because of lithospheric thickening, no other major tectonic
events occurred later in the history of the area. Minor eruptions of
liquid water may have formed some areas of light smooth material, and
occasional impacts produced c3 craters; one impact was large enough to
produce a palimpsest (unit p3).

**REFERENCES CITED**

Bianchi, Remo, Casacchia, Ruggero, Lanciano, Pasquale, Pozio, Stefania,
and Strom, R.G., 1986, Tectonic framework of grooved terrain on
Ganymede: Icarus, v. 67, p. 237--250.

Bianchi, Remo, Casacchia, Ruggero, and Pozio, Stefania, 1984, Tectonics
of the grooved terrain on Ganymede, *in* Abstracts of Papers Submitted
to the Fifteenth Lunar and Planetary Science Conference, Houston, March
1984, p. 54--55.

Bianchi, Remo, and Pozio, Stefania, 1985, Morphometrical analysis of
craters with domed central pit on Ganymede: Annales Geophysicae, v. 3,
p. 129--134.

Casacchia, Ruggero, and Strom, R.G., 1984, Geologic evolution of Galileo
Regio, Ganymede, *in* Lunar and Planetary Science Conference, 14th,
Houston, 1983, Proceedings, part 2, Journal of Geophysical Research, v.
89, Supplement, p. B419--428.

Ferguson, H.M., Lucchitta, B.K., Squyres, S.W., and Wilhelms, D.E.,
1982, Geologic map of Ganymede, *in* Morrison, David, ed., Satellites of
Jupiter: Tucson, University of Arizona Press, p. 938--939.

Greeley, Ronald, Fink, J.H., Gault, D.E., and Guest, J.E., 1982,
Experimental simulation of impact cratering on icy satellites, *in*
Morrison, David, ed., Satellites of Jupiter: Tucson, University of
Arizona Press, p. 340--378.

Horner, V.M., and Greeley, Ronald, 1982, Pedestal craters on Ganymede:
Icarus, v. 51, p. 549--562.

Lucchitta, B.K., 1980, Grooved terrain on Ganymede: Icarus, v. 44, p.
481--501.

McKinnon, W.B., and Melosh, M.J., 1980, Evolution of planetary
lithospheres: Evidence from multiringed structures on Ganymede and
Callisto: Icarus, v. 44, p. 454--471.

Morrison, David, 1982, Introduction to the satellites of Jupiter, *in*
Morrison, David, ed., Satellites of Jupiter: Tucson, University of
Arizona Press, p. 3--43.

Parmentier, E.M., Squyres, S.W., Head, J.W., and Allison, M.L., 1982,
The tectonics of Ganymede: Nature, v. 295, p. 290--293.

Shoemaker, E.M., Lucchitta, B.K., Plescia, J.B., Squyres, S.W., and
Wilhelms, D.E., 1982, The geology of Ganymede, *in* Morrison, David,
ed., Satellites of Jupiter: Tucson, University of Arizona Press, p.
435--520.

Smith, B.A., and the Voyager Team, 1979a, The Galilean satellites and
Jupiter: Voyager 2 imaging science results: Science, v. 206, p.
927--950.

\_\_\_\_\_\_1979b, The Jupiter system through the eyes of Voyager 1:
Science, v. 204, p. 951--972.

Squyres, S.W., 1980, Volume changes in Ganymede and Callisto and the
origin of grooved terrain: Geophysical Research Letters, v. 7, p.
593--596.

Wilhelms, D.E., 1972, Geological mapping of the second planet, *in* U.S.
Geological Survey Interagency Report: Astrogeology, v. 55, 39 p.