**GEOLOGIC MAP OF THE**

**ETANA REGION (Jg-1)**

**OF GANYMEDE**

By Roland J. Wagner, Ralf Jaumann

and Gerhard P. Neukum

1995

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--2497, 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

\[Interpreted as differentiated materials, probably water ice relatively
free of silica, extruded as low-viscosity fluids\]

**lsg Light, slightly grooved material**---Smooth surface with pattern
of very subdued grooves. *Interpretation:* Faint structural imprint
caused by underlying topography of older, grooved material

**lg Light grooved material---**Light material with structural imprint
of grooves. Exposures arranged in domains characterized by parallel,
roughly evenly spaced grooves and ridges. Groove sets may be long,
narrow groove zones, called lanes by Murchie and others (1986), or short
and wide. Boundaries with dark materials mostly sharp; boundaries with
other light material may be indistinct. Grooves roughly linear; locally
slightly curved or sharply angled. *Interpretation:* Grooves due to
extensional stress. Orientation changes caused by complicated stress
regimes

**ls Light smooth material---**Sparsely cratered. Tectonic features such
as conspicuous grooves or troughs are single or in pairs, randomly
located. Faint, groovelike striations in places. *Interpretation:* Unit
may be precursor to grooved and slightly grooved materials

DARK MATERIALS

**db Material of dark bands---**Forms narrow, roughly linear swaths in
light materials. *Interpretation:* May be remains of dark material no
longer present, dikelike intrusions, or traces of faint troughs

**dg Dark grooved material---**Densely spaced grooves of low albedo
similar in shape to faint grooves in light material. *Interpretation:*
Silicate-ice mixture; grooves may have formed by shear failure due to
strike-slip movements along zones of breakup of old dark crust

**df Dark furrowed material---**Rough, hummocky, densely cratered
surface. Occurs mostly in Galileo Regio and its surroundings. Boundaries
with light materials generally sharp, in places marked by troughs. Most
superposed craters partly degraded or degraded. *Interpretation:* Oldest
crust visible in map region, composed of silicate-ice mixture with
imprint of extensional tectonics from early in Ganymede\'s history

**f2 Material of younger furrows---**Rims and floors of linear
depressions, as much as 500 km long (mostly located outside map region),
in dark furrowed material. Rims bright. *Interpretation:* Depressions
are extensional tectonic features, probably grabens. Radial to impacts
or due to endogenic processes. Albedo contrasts of bright rims with
surrounding dark materials probably due to material differences

**f1 Material of older furrows---**Rims and floors of linear, slightly
arcuate depressions in dark furrowed material. Margins kinked and bent.
Furrows 10

to 25 km wide, as much as 200 km long. *Interpretation:* Same as
material of younger furrows, but grabens formed concentric to impacts

**d Dark material, undivided---**Includes other dark materials whose
characteristics are difficult to identify because of poor resolution of
images. Lacks furrows, but has linear depressions of different lengths
resembling grooves in light materials. Small elliptical depressions may
be present in places. *Interpretation:* Silicate-rich ice mixture.
Grooves and depressions probably extensional tectonic features;
elliptical depressions may also be deformed impact craters or volcanic
vents

CRATER AND PALIMPSEST MATERIALS

\[Only craters greater than 20 km in diameter are mapped\]

**c3 Material of fresh craters---**Observed in map region only in
anti-Jovian hemisphere. Craters have complete, sharp rims and bright
ejecta. Bright rays rare; some extend northward into map region between
long 25° and 28°. *Interpretation:* Fresh impact craters whose ejecta
are composed of relatively clean ice. Superposed on all other materials

**c2 Material of partly degraded craters---**Rims complete, distinct;
diameters less than 20 to 80 km (crater Kishar); slightly hummocky
ejecta discernible in places. Central peaks indistinct; central pits,
locally with bright raised rims, common in craters greater than 40 km in
diameter. Includes forms having scarplike terminations surrounding
ejecta materials (pedestal craters), observed mostly on light smooth
material. *Interpretation:* Impact craters whose ages span much of
Ganymede\'s history

**c1 Material of degraded craters---**Found only in dark materials;
albedos similar to that of dark materials. Craters have subdued or
partial rims, no recognizable ejecta; diameters 20 to 60 km. Crater
floors mostly flat, some bowl-shaped, some have internal domes or
ring-shaped depressions forming moats (lat 82° N., long 75°). Central
pits barely discernible in places (lat 78° N., long 88°).
*Interpretation:* Old impact craters degraded by viscous relaxation
processes and meteorite bombardment

**p Palimpsest material---**Forms bright circular patch in dark furrowed
material. *Interpretation:* Remnant of old impact crater whose original
morphology has been erased by viscous relaxation

> **Contact---**Dashed where approximately located. Includes domain
> boundaries within light, slightly grooved; light grooved; dark
> furrowed; and undivided dark materials
>
> **Deep linear trough---**Mostly in dark materials
>
> **Deep linear trough---**Mostly in light materials; dashed where
> approximately located
>
> **Vague linear trough---**Mostly in light materials; dashed where
> approximately located
>
> **Trend of sharp grooves---**Schematic
>
> **Trend of subdued grooves---**Schematic
>
> **Furrow---**Dashed where approximately located
>
> **Lineament---**Arrows indicate possible sense of strike-slip movement
>
> **Irregular depression**
>
> **Crater rim crest---**Dashed where approximately located
>
> **Peak on crater floor**
>
> **Pit on crater floor**
>
> **Dome on crater floor**
>
> **Circular scarp---**Hachures point downslope
>
> **Bright ejecta rays**
>
> **Secondary craters---**Faint and irregular, around crater Kishar

**INTRODUCTION**

Ganymede, the largest of the Jovian satellites discovered by Galileo, is
a Mercury-sized object having a diameter of 5,268 km. Ganymede\'s
Jupiter-facing side is always the same. The low density of 1.93.103 kg
m-3 suggests that Ganymede has a large component of water ice; its
component of rock is unknown (Smith and others, 1979a). Absorption bands
at 1.55 and 2.0 µm are evidence for water-ice abundance at its surface
(Pilcher and others, 1972).

Ganymede\'s two major terrain types are dark and light materials, each
covering about 50 percent of the surface. Also, many of the impact
craters dispersed across the map region have bright ejecta or bright
rays; some have dark rays.

The dark material is heavily cratered and, therefore, known to be older
than the light material (Smith and others, 1979a,b; Shoemaker and
others, 1982). The global average albedo of dark material is 0.35
(Squyres and Veverka, 1981). In some places, dark material is cut by
linear furrows or by sets of curvilinear, roughly concentric furrows.
Light material, which has a global average albedo of 0.44 (Squyres and
Veverka, 1981)*,* is dissected by many linear or slightly arcuate,
locally sinusoidal grooves. Most of the grooves are arranged in groove
domains or \"structural cells\" (Smith and others, 1979a; Lucchitta,
1980). Both furrows and grooves are of tectonic origin, probably
extensional features (Smith and others, 1979a; Shoemaker and others,
1982).

Overall coverage of the map region is poor (see resolution diagram).
Only about a quarter, which is located in the sub-Jovian hemisphere
(long 270° to 0° to 90°) imaged by Voyager 1, is covered by images of
less than 2.0 km/pixel resolution. The area between long 120° and 240°
is covered by Voyager 2 images with poor resolution due to
foreshortening. Part of Galileo Regio, the largest expanse of dark
material on Ganymede, is in this area.

The map region and surrounding areas are covered by a light-colored
polar cap extending down to a mean latitude of about 48° N. (Squyres,
1980a). The cap is visible on far-encounter images only and does not
affect the contrast on the images used for geologic mapping.

**STRATIGRAPHY**

Albedo differences and morphology are the basic criteria for mapping
geologic units on Ganymede. Albedo differences are used for major
divisions, morphologic differences for further subdivision.

DARK MATERIALS

Dark materials are characterized by a relatively low albedo and commonly
by a high density of impact craters. They occur in areas as large as
1,000 km2 or in smaller, roughly polygonal areas. Dark materials consist
of dark furrowed material, materials of older and younger furrows, dark
grooved material, material of dark bands, and dark material, undivided.

Most of the dark material in the quadrangle is part of Galileo Regio
(roughly long 10° to 170°), a vast, dark, polygonal feature thousands of
kilometers across. The regio consists mainly of dark furrowed material
(unit **df**), but, due to foreshortening effects, only a few furrows
can be discriminated in the map region. Between individual furrows, dark
furrowed material is characterized by a rough, densely cratered surface.

Furrow materials differ in morphology and, as shown by crosscutting
relations in Galileo Regio (Casacchia and Strom, 1984), they also differ
in age. Material of younger furrows (unit **f2**) in the map region is
found exclusively in the area imaged by Voyager 2, whereas material of
older furrows (unit **f1**) is known to occur in regions imaged by both
Voyager 1 and 2, but it cannot be identified positively in Voyager 2
images due to their foreshortening. Older furrows that are curvilinear
elsewhere on Ganymede are roughly linear depressions, but their overall
length cannot be determined exactly due to

information loss at the limb. Furthermore, the kinked and bent margins
are bright and seem to form raised rims less than 100 m above the
surrounding terrain, as observed by Shoemaker and others (1982). The
bright albedo of furrow rims may be due to different lithologies,
photometric effects on slopes, or both. The possible lithologic
difference justifies the mapping of furrow materials as separate units.
Younger furrows are narrower and straighter than older furrows, and
their edges are less kinked and bent. In the map region they appear to
be longer than older furrows. Younger furrows in the map area are the
terminations of a furrow zone in Galileo Regio that trends north to
north-northeast and is more than 1,000 km long (Murchie and Head, 1989).

Dark grooved material (unit **dg**) is characterized by a low albedo and
by densely spaced grooves. It occurs in slivers within undivided dark
material (lat 78° N., long 20°) or light smooth material and is locally
associated with lineaments.

Material of dark bands (unit **db**) is characterized by dark, Iinear or
slightly arcuate stripes as much as 10 km wide and 50 to more than 100
km long. Some stripes are nearly parallel to grooves in dark grooved
material. Detailed mapping of this unit is not possible due to low
resolution of the images.

Dark material, undivided (unit **d**) occurs in patches surrounding
Galileo Regio. It appears similar to dark furrowed material, but it is
devoid of furrows. Boundaries between furrowed material and undivided
material cannot be sharply defined. Undivided material may occur in
slivers within light material (for example, at lat 69° N., long 51°).
The contact between dark and light materials is locally characterized by
narrow troughs in light material (for example, at lat 70° N., long
225°).

Tectonic features in undivided material are different from those in
furrowed material. In some places, undivided material is characterized
by deep, narrow, linear or sinuous troughs. Several types of troughs and
trough zones may be distinguished, the best examples being in the
elliptical area between lat 68° and 73° N., long 12° and 32°. Troughs
may be less than 10 km wide. The troughs, probably extensional features,
are single or in pairs and nearly parallel at a spacing of less than
about 10 km. They show raised, bright rims or intercalated ridges.
Parallel troughs in places merge to form a single trough. The long,
sinuous, parallel troughs at about lat 71° N., long 15°--30° may be
offset by a few kilometers in a left-lateral sense along a poorly
defined lineament. Also, short, roughly elliptical depressions some 10
km long and wide occur near lat 80.5° N., long 8° and may be of tectonic
origin.

LIGHT MATERIALS

Light materials are arranged in broad stripes called sulci less than 100
to 200 km wide, or in less regular areas. The sulci separate polygons of
dark material. Light material has a lower crater density than dark
material, which implies a younger age.

Light materials are characterized by their tectonic pattern of linear
depressions (grooves). Generally, grooves are subparallel, linear to
arcuate, several tens of kilometers long, 5 to 10 km wide, and 3 to 10
km apart. Photoclinometric measurements yield mean depths of 300 to 400
m and mean slopes of 5°, concave upward (Squyres, 1981). Grooves are
arranged in groove sets (domains) or structural cells, some wide and
short, others narrow and long (Lucchitta, 1980). Grooves in cells may be
truncated by other cells. Linear depressions that are deeper than
grooves are called troughs, which, singly or in pairs, cut through
structural cells and, in places, transect dark materials. The troughs
were termed \"prominent structures\" or \"through-going grooves\" by
Bianchi and others (1986). Both grooves and troughs are thought to be
tectonic features, probably caused by surface extension.

In the map region, three different units of light material can be
distinguished by the presence and arrangement of tectonic features. A
light-colored, smooth, sparsely cratered surface almost devoid of
tectonic features characterizes light smooth material (unit **ls**).
Grooves are randomly located on this unit. Light smooth material is of
two different types

in the map region. One type, at lat 75°--90° N., long 40°--90°, has an
albedo that is either light or intermediate between those of light and
dark materials; superposed tectonic features may be single troughs, some
20 to 150 km long. A second, less common type is cut by one central
trough. This type occurs about 100 km south of the north pole (long
300°--330°).

Light grooved material (unit **lg**) is arranged in domains with nearly
parallel grooves and troughs. Three different types of this unit may be
distinguished. The first type is characterized by linear to slightly
arcuate groove sets whose grooves parallel the boundary of the set. Sets
may be as wide as 200 km, as in Dukug Sulcus, and several 100 km long;
groove spacing is less than 10 km. Also, elliptical depressions several
tens of kilometers long are seen in Dukug Sulcus. East of crater Etana,
conspicuous troughs cut both groove sets and dark material. Some of
these troughs are arranged en echelon, as at lat 72° N., long 325°, from
which shear failure may be inferred. Troughs in the unit are oriented
dominantly northeast, but some trend north or northwest. Structures
oriented north are transected by those trending northwest or northeast.
A second type of light grooved material has no sharp boundary with light
smooth materials, as at lat 65°--70° N., long 0°--22°. Grooves,
generally less than about 200 km long, appear subdued, which may be a
consequence of poor image resolution. The third type of light grooved
material is characterized by short, wide groove sets, as at lat 65°--71°
N., long 325°--341°; these grooves are slightly curved and oriented
north to east northeast.

Light, slightly grooved material (unit **lsg**) embays or seems to cover
other light materials. Its surface is smooth, but a pattern of highly
subdued, groovelike features spaced more than 10 km apart may be
recognized. The east contact of the unit with light smooth material
between lat 70° and 78° N. appears curved and resembles a flow front.
East of crater Kishar, this unit embayed and apparently flooded older
light grooved material. Like the light grooved material, the unit
locally contains elliptical depressions, here less than 30 km long.

Light materials are thought to be materials differentiated from the
mantle and probably composed of water ice with minor silicate
contamination. They may have been emplaced as low-viscosity fluids
(Smith and others, 1979a,b; Lucchitta, 1980; Shoemaker and others,
1982). No features unequivocally indicative of volcanism can be
identified. The small elliptical depressions in some parts of light
materials may be volcanic vents, but they may also be tectonic features
such as small grabens or tectonically deformed craters, or even
secondary craters.

CRATER AND PALIMPSEST MATERIALS

Craters in the map region range in morphology from that of a palimpsest
to those of degraded and partly degraded craters to fresh craters. Some
craters have central pits, moats, or scarplike terminations of ejecta.
However, morphologic information is lost in the highly foreshortened
regions at the limbs.

Age relations of degraded craters (unit **c1**) cannot be established
easily. No degraded craters are observed to overlie furrows in dark
material in the map area. Because degraded craters are almost entirely
restricted to dark materials, these craters probably predate the
emplacement of light materials. Partly degraded craters (unit **c2**)
constitute the most common type throughout the map region. The large
central pit of crater Kishar is very likely caused by tectonic
modification, as evidenced by a faint trough that extends northward from
the center of the pit. The ejecta of many partly degraded craters have
scarplike terminations similar to those of the inner lobes of Martian
rampart craters. Most of these terminations are sharp, for example,
those at lat 80.5° N., long 330° and lat 73° N., long 342° (Etana).
These craters are termed pedestal craters (Horner and Greeley, 1982),
and they were likely caused by an impact into an icy target forming
flow-lobe ejecta. Partly degraded craters apparently span a wide age
range. Most are younger than the dark materials, and, as observed by
Casacchia and Strom (1984), they postdate

furrows. Some of the craters are transected by younger troughs in dark
materials, for example, craters Neheh (lat 71° N., long 58°) and Adapa
(lat 72° N., long 30°). The rim and ejecta of crater Aya (lat 67° N.,
long 325°), in light grooved material, are interrupted by grooves and
troughs, but the crater floor is undisturbed by tectonic structures,
which indicates that the interior may have been resurfaced subsequent to
tectonism. Other partly degraded craters are superposed on grooves in
light grooved material, for example, at lat 70.5° N., long 320° and lat
79° N., long 311°.

**GEOLOGIC AND TECTONIC HISTORY**

The oldest recognizable event in the map region was the formation of a
dark crust, now densely cratered. Part of the previous crater population
was obliterated by resurfacing, viscous relaxation of the crust, and
tectonic events (Passey and Shoemaker, 1982; Murchie and Head, 1987,
1988); therefore, the overall crater population is not the original one.
Prior to the formation of most craters that are now visible on dark
materials, the hemispheric-scale, rimmed-furrow system was created that
is roughly concentric to a point (at about lat 15° S., long 165°) in
eastern Marius Regio. The furrows may be grabens formed by a giant
impact (McKinnon and Melosh, 1980; Schenk and McKinnon, 1987) or
extensional tectonic features that reactivated primary impact structures
(Murchie and Head, 1988). Alternatively, they may have been formed by an
endogenic event such as a rising mantle plume (Casacchia and Strom,
1984). Younger furrows apparently spread radially, also from the eastern
part of Marius Regio. Their occurrence is attributed to internal
activity; they might be radial extensions caused by doming (Murchie and
Head, 1987).

Furrows then developed in a crust whose strength was not sufficient to
retain structures with long wavelengths, such as impact craters greater
than 100 km in diameter (Passey and Shoemaker, 1982). Therefore, in the
old dark areas, viscous relaxation transformed old large craters into
palimpsests, and it degraded craters having central domes and interior
moats.

After the formation of furrows, linear or sinuous troughs of differing
origins evolved in dark materials. Some troughs are younger than some
impact craters, but they are cut by and are therefore older than light
materials that formed later in the history of Ganymede. The troughs are
spatially restricted and may have been caused by local extensional
stress fields.

Global expansion, possibly due to phase changes in the ice mantle
(Shoemaker and others, 1982), to differentiation processes (Squyres,
1980b), to global thermal activity (Zuber and Parmentier, 1984), or to
mantle convection (Shoemaker and others, 1982), led to extensional
stresses that caused a breakup of the old dark materials and the
emplacement of light materials (Parmentier and others, 1982; Squyres,
1982). Fractures widened and rift zones were resurfaced by smooth light
material. In many parts of the map region, this material remained
structurally unaffected, or it may have resurfaced and thereby smoothed
older grooved light materials; elsewhere, sets of closely spaced,
parallel grooves formed within light materials. Associated with the
global breakup of the crust were strike slip movements along lineaments,
which created shear fractures (mapped in dark grooved material). Grooves
may have formed contemporaneously with light-material emplacement, or
they may have developed subsequently. Some conspicuous troughs cut
through both groove sets and dark materials.

The light, slightly grooved material probably resurfaced older light
grooved material. On the other hand, we cannot exclude the possibility
that this unit is as old as the smooth unit but has undergone a
groove-forming process that created only faint grooves.

Partly degraded craters formed before, during, and after light-material
emplacement. The latest events in the map region were impacts, which
generated fresh craters with bright rims, bright ejecta, and locally
bright rays.

**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.

Casacchia, Ruggero, and Strom, R.G., 1984, Geologic evolution of the
Galileo Regio, Ganymede: Journal of Geophysical Research, v. 89, suppl.,
p. B419--B428.

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, H.J., 1980, Evolution of planetary
lithospheres: Evidence from multiring structures on Ganymede and
Callisto: Icarus, v. 44, p. 454--471.

Murchie, S.L., and Head, J.W., 1987, Origin and evolution of furrows in
the dark terrain of Ganymede, *in* Abstracts of papers submitted to the
Eighteenth Lunar and Planetary Science Conference, Houston, March
16--20, 1987: Houston, Lunar and Planetary Institute, p. 682--683.

\_\_\_\_\_1988, Possible breakup of dark terrain on Ganymede by
large-scale shear faulting: Journal of Geophysical Research, v. 93, no.
B8, p. 8795--8824.

\_\_\_\_\_1989, Geologic map of the Philus Sulcus quadrangle of Ganymede
(Jg4): U.S. Geological Survey Miscellaneous Investigations Map I--1966,
scale 1:5,000,000.

Murchie, S.L., Head, J.W., Helfenstein, Paul and Plescia, J. B., 1986,
Terrrain types and local-scale stratigraphy of grooved terrain on
Ganymede: Journal of Geophysical Research, v. 91, no. B13, p.
E222--E328.

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

Passey, Q.R., and Shoemaker, E.M., 1982, Craters and basins on Ganymede
and Callisto: Morphological indicators of crustal evolution, *in*
Satellites of Jupiter (David Morrison, ed.), Tucson, University of
Arizona Press, p. 379--434.

Pilcher, C.B., Ridgway, S.T., and McCord, T.B., 1972, Galilean
satellites: Identification of water frost: Science, v. 178, p.
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Schenk, P.M., and McKinnon, W.B., 1987, Ring geometry on Ganymede and
Callisto: Icarus, v. 72, p. 209--234.

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

Smith, B.A., and 21 others, 1979a, The Jupiter system through the eyes
of Voyager 1: Science, v. 204, p. 951--972.

Smith, B.A., and 21 others, 1979b, The Galilean satellites and Jupiter:
Voyager 2 imaging results: Science, v. 206, p. 927--950.

Squyres, S.W., 1980a, Surface temperatures and retention of H2O-frost on
Ganymede and Callisto: Icarus, v. 44, p. 502--510.

\_\_\_\_\_1980b, Volume changes in Ganymede and Callisto and the origin
of grooved terrain: Geophysical Research Letters, v. 7, no. 8, p.
593--596.

\_\_\_\_\_1981, The topography of Ganymede\'s grooved terrain: Icarus,
v. 46, p. 156--168.

\_\_\_\_\_1982, The evolution of tectonic features on Ganymede: Icarus,
v. 52, p. 545--559.

Squyres, S.W., and Veverka, Joseph, 1981, Voyager photometry of surface
features on Ganymede and Callisto: Icarus, v. 46, p. 137--155.

U.S. Geological Survey, 1987, Shaded relief and surface markings of the
Etana region of Ganymede: U.S. Geological Survey Miscellaneous
Investigations Series Map I--1810.

Zuber, M.T., and Parmentier, E. M., 1984, Lithospheric stresses due to
radiogenic heating of an ice-silicate body: Implications for Ganymede\'s
tectonic evolution, *in* Lunar and Planetary Science Conference, 14th,
Houston, March 14--18, 1983, Proceedings: Journal of Geophysical
Research, v. 89, p. B429--B437.