**GEOLOGIC MAP OF THE**

**PERRINE (Jg-2)**

**AND**

**NUN SULCI (Jg-5)**

**QUADRANGLES OF GANYMEDE**

By George E. McGill, Steven W. Squyres,

and Stephen J. Uzman

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--2459, 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

ls **Smooth material**---High albedo. Surfaces smooth and grooves
generally absent. *Interpretation:* Dominantly ice magmatically emplace
on or adjacent to dark materials

lg **Grooved material**---Medium to high albedo. Grooves either
subparallel or radiating; surface between grooves has smooth texture.
Sparsely cratered. *Interpretation:* Similar to smooth material. Grooves
due to extensional faulting

lgi **Irregularly grooved material**---Similar to grooved light material
except grooves not parallel or grouped in sets. Density of grooves
highly varied. Includes small areas of grooveless light material.
*Interpretation:* Similar to smooth material

l **Undivided material**---High albedo. Surfaces appear smooth.
*Interpretation:* Light materials whose distinguishing details cannot be
resolved because of poor image resolution

lt **Transitional material**---Intermediate in albedo between other
light materials and lightest dark material; texture varied.
*Interpretation:* Dark material covered by thin veneer of light material
or possibly by polar frost

DARK MATERIALS

dg **Grooved material**---Surface texture lineated; average albedo
slightly higher than for cratered dark material; streaked appearance in
detail. *Interpretation:* Cratered dark material structurally modified
by grooves. Darkest grooves also may contain younger deposits

df **Furrowed material**---Albedo like that of grooved material:
abundant superposed furrows. *Interpretation:* Cratered dark material
structurally modified by furrows

dc **Cratered material**---Surface texture rough. Average albedo low,
but unit appears mottled, patchy, or streaked. Craters of all sizes
common. *Interpretation:* Mixture of ice and silicates

d **Undivided material**---In generally small patches. Smooth-appearing;
no resolvable surface detail. *Interpretation:* Dark materials whose
distinguishing details cannot be resolved because of poor image
resolution or small size of exposure

CRATER AND PALIMPSEST MATERIALS

\[Only craters and palimpsests greater than 15 km in diameter are
mapped. They are interpreted to be formed by impact\]

c3 **Material of bright craters**---Rim crests are sharp and complete,
ejecta blankets well preserved, and albedo high; craters generally have
associated bright rays

c2 **Material of somewhat degraded craters**---Rim crests well defined
and complete; ejecta blankets subdued or incomplete. Albedo distinctly
lower than that of bright craters. Common central peaks

c1 **Material of highly degraded craters**---Only rim segments remain
except at crater Sati, which also retains mappable ejecta

cs **Material of secondary craters**---Clusters or chains of small, oval
craters inferred to be secondaries

p **Palimpsest material**---Relatively bright, generally subcircular
patches inferred to be scars of relaxed craters

**INTRODUCTION**

Ganymede is the third of the four Galilean satellites of Jupiter. It is
also the largest (radius of 2,631 km). Models for the interior structure
that are constrained by distance from the Sun and a mean specific
gravity of 1.93 indicate that about half of Ganymede's mass consists of
water ices (Cassen and others, 1982). Spectral and photometric
properties suggest that perhaps as much as 90 percent of Ganymede's
surface is water ice (Pilcher and others, 1972); the remainder is
inferred to be silicates (Pollack and others, 1978; Sill and Clark,
1982; Clark and others, 1986).

Although the surface of Ganymede exhibits many complex details, it can,
to first order, be divided into two general types of terrains on the
basis of average albedo (Smith and others, 1979a,b; Squyres and Veverka,
1981; Helfenstein and others, 1984). The albedo difference is believed
related to the proportion of silicates mixed with the ice. Light terrain
is generally riven by abundant long, narrow grooves. Dark terrain is
generally more heavily cratered than light terrain and much less
commonly cut by grooves.

The map area is in the sub-Jupiter hemisphere of Ganymede. All images
used for mapping were obtained by Voyager 1. The terminator was located
at about long 305° when the images were obtained, and all coverage in
Jg--2 west of about long 60° was obtained with emission angles too large
to produce images usable for mapping. Resolution of the primary mapping
images ranges from 2.4 to 5.2 km/line pair; images covering Jg--5 are
generally superior to those covering Jg--2 (see resolution diagram). The
mapped part of Jg--2 is about equally divided between light and dark
material, whereas the mapped part of Jg--5 comprises approximately 25
percent dark material and 75 percent light material. An
intermediate-albedo transitional unit occurs locally; this unit is
grouped with light materials.

**STRATIGRAPHY**

The dark and light materials are placed in a time-stratigraphic sequence
primarily by means of embayment and crosscutting relations. The density
of impact craters also is an indicator of relative age, and dark
materials are generally more heavily cratered and thus older than light
materials. However, within the two groups, crater densities are
generally not reliable indicators of relative age, because all materials
contain too few craters to determine statistically meaningful age
differences.

Crater and palimpsest units range from material that is highly degraded
(palimpsest) to very fresh (craters having bright rays). This
classification is inferred to correspond approximately to a relative-age
sequence. Caution is necessay, however, because of size-dependent
effects on degradation: (1) micrometeorite bombardment, favoring
survival of large craters, and (2) viscous relaxation, favoring survival
of small craters. Nevertheless, if very large and very small craters and
palimpsests are ignored, one can infer with reasonable confidence that
morphologically degraded craters are older than fresh craters.

DARK MATERIALS

The oldest and most widespread dark material is cratered material.
Although the average albedo is relatively low, in detail this material
appears patchy, streaked, or mottled, because its surface is a montage
of high- and low-albedo areas at scales of a few to tens of kilometers.
In places, this appearance seems caused by grooves or fragments of
crater rims that are too small to map, but generally the surface
characteristics responsible cannot be identified. The surface texture is
rough at a scale similar to or smaller than that of the albedo
variations. Craters of all ages and all sizes, from the limits of
resolution to the largest in the mapped area, are superposed. Individual
grooves and troughs are locally present, many of which crosscut or
coincide with contacts between light and dark materials.

Furrowed dark material occurs only in the northern part of the
subrounded, western part of Perrine Regio in Jg--2; the material is part
of one of the furrowed dark-terrain groups defined by Murchie and Head
(1988). The albedo of the furrowed material is generally higher than
that of cratered dark material, possibly because the abundant furrows
commonly have bright margins (Guest and others, 1988). Similar densities
of superposed impact craters indicate no difference in age between
furrowed and cratered dark materials; the furrowed unit very likely is
cratered material that has been tectonically modified by the furrows.

Grooved dark material occurs generally in small areas adjacent to larger
areas of cratered dark material, commonly along their contacts with
light materials. The average albedo is somewhat higher than for cratered
dark material, and in detail the surface has a streaked appearance
consistent with the orientations of lineaments or grooves. In many
places, the grooves are clearly continuous with grooves in adjacent
light materials; hence, the grooves represent a relatively young
structural modification of cratered dark material. Grooved dark material
here includes what has been mapped as lineated dark material in other
quadrangles of this series, because image quality does not permit
separation into two map units.

Undivided dark material is mapped where neither structural features nor
craters can be resolved. Its generally smooth appearance and uniform,
low albedo are most likely due to poor resolution.

LIGHT MATERIALS

The largest area of transitional material is in the northern part of
Jg--2. The unit is probably dark material covered by a thin veneer of
light material or blanketed by the polar frost caps. These caps
apparently are thin deposits of water frost that cover the surface of
Ganymede down to a latitude of 40° to 50° in both hemispheres. The caps
may be either (1) the remnants of a formerly more extensive frost
blanket that has been removed from lower latitudes by sublimation, or
(2) an accumulation of frost that has built up over time by transport
from lower latitudes (Purves and Pilcher, 1980).

Light grooved material is perhaps the most distinctive geologic unit on
the satellite. It is characterized by very regular patterns of grooves
transecting the surface. The grooves are curvilinear depressions as much
as a few kilometers wide, a few hundred meters deep, and hundreds of
kilometers long (Shoemaker and others, 1982). Unfortunately, the
resolution of the best images is insufficient to reveal details of the
geometry of the grooves, but they appear to be roughtly U-shaped in
cross section. Groove slopes are gentle; at Voyager resolution, typical
slopes are less than 10° (Squyres, 1981). The grooves are arranged in
subparallel or fan-shaped groupings called sets. The sets intersect and
crosscut one another with a geometry that is locally very complex. The
range in morphology from one groove set to another is considerable: in
some sets the grooves are packed closely together, producing roughly
sinusoidal topographic profiles, while in others they are widely
separated by distinct smooth areas. Within sets, spacings between
adjacent grooves range from about 4 to 17 km, with a mean of about 8 km
(Grimm and Squyres, 1985). Where the smooth areas are extensive, they
are considered to be a separate unit, smooth material.

Irregularly grooved material shares several characteristics with grooved
material: it has low crater density and fairly high albedo, and its
surface is dominated by grooves. Tlhe distinction is that the grooves
are not arranged in sets. Instead, they can have a variety of complex
geometries. Individual grooves may be straight, arcuate, or sinusoidal,
but the trends of adjacent grooves are not correlated as they are for
grooves that are arranged in sets. Individual grooves may crosscut and
truncate one another, and they may split or merge at acute angles,
forming a crudely anastomosing pattern.

Smooth material is similar in albedo to most other light materials, but
it is distinguished by a paucity of grooves and subdued topography at
the best available image

resolution. In particular, we observe no flow fronts or other features
that might provide morphologic clues to the emplacement mechanism.
Albedo is commonly fairly uniform across individual areas of smooth
material. Some exposures, however, appear mottled, suggesting local
variations in silicate content. Smooth material is seen in a variety of
geometric relations with other light units: it occurs as irregularly
shaped, groove-free patches in areas that are otherwise dominated by a
complex pattern of grooves, and it also forms long, curvilinear swaths
whose geometry is similar to that of groove sets; some swaths are
bounded by grooves. Smooth swaths and other exposures commonly have
sharp boundaries with all other units. Locally, however, boundaries are
indistinct, and smooth material is transitional to dark or other light
materials.

Undivided light material has an albedo similar to that of other light
materials, but it is observed at an image resolution, viewing geometry,
or illumination geometry that prevents the recognition of
characteristics that would identify it as one of the other light units.
It is mapped primarily in the poorly imaged central portion of Jg--2.

CRATER AND PALIMPSEST MATERIALS

Overall, the most degraded crater structures mapped on Ganymede are the
palimpsests (Passey and Shoemaker, 1982)---roughly circular areas that
are smoother and generally brighter than their surroundings. Their
contacts are not sharply defined. Most palimpsests mapped on Ganymede
are older than light materials, but the larger of the two palimpsests
mapped in Jg--5 is an exception; it overlies the light materials and is
therefore younger. It forms a distinct dome, rising more that 2 km above
the surrounding plains (Squyres, 1981). Its topography is roughly
concentric and gently undulatory except for a smooth central region
about 60 km across (mapped as smooth light material). No vestige of the
original crater morphology is preserved, but its well-developed field of
secondary craters (unmapped) clearly shows an impact origin. Palimpsests
have been suggested to form when viscous subsurface material is extruded
in response to an impact (Thomas and Squyres, 1990), and this palimpsest
may be a particularly late and massive example of such an extrusion.

Highly degraded craters (unit c1) are generally only rim remnants. An
exception is the 95-km-diameter, two-ringed crater Sati in Jg--2.
Because both of its rings are incomplete, this crater is classified as
highly degraded even though mappable ejecta are present, which is very
likely due to the large size of this feature. The ejecta blanket is
clearly truncated by light grooved material, indicating that Sati is
older.

Somewhat degraded craters (unit c2) are the most abundant class. Many
have central peaks or pits, and most retain at least some mappable
ejecta. In general, ejecta limits are difficult to map because the
contacts with surrounding materials are gradational, and the albedo and
texture of the underlying light or dark materials show through. Somewhat
degraded craters are present on both dark and light materials, but
superposition relations indicate that some of these craters are clearly
older than light materials.

Most bright craters (unit c3) have extensive bright rays, and generally
their ejecta completely mask the albedo and texture of older underlying
materials. Bright crater ejecta are everywhere younger than both light
and dark materials.

The floors of large craters of all types are bowed upward, providing
evidence of some viscous relaxation of their topography (Johnson and
McGetchin, 1973; Parmentier and Head, 1981; Passey and Shoemaker, 1982).

Mappable secondary craters are present locally in small clusters or
chains. No source crater has been identified for any of these secondary
craters, but they are presumed to be relatively young because of their
small size and fresh appearance.

**STRUCTURAL GEOLOGY**

Grooves are the dominant structural features on Ganymede. While single
grooves are found in many areas, they are more commonly grouped in sets
with common structural trends. Many grooves in sets are nearly parallel,
but fan-shaped sets also are observed. Groove spacing can differ
substantially from one groove set to another, but within a given set the
spacing tends to be more nearly constant. Sets may intersect in complex
crosscutting relationships; the central region of Jg--5 is one of the
most complicated on Ganymede in this regard. Where one groove set
crosscuts another, no trace of the cut groove set is generally observed
within the crosscutting set. One of the few exceptions is seen at lat
40° N., long 314°. The boundaries of groove sets are in places marked by
particularly long and deep grooves, and the boundary between light and
dark materials is commonly marked by a parallel single groove or groove
set in the light material.

Ganymede's grooves are generally thought to be extensional features
(Smith and others, 1979a,b; Squyres, 1980; Parmentier and others, 1982,
Squyres, 1982; Golombek and Banerdt, 1986). They could plausibly have
formed as a result of global expansion (Squyres, 1980; Shoemaker and
others, 1982), although expansion would have been somewhat limited
(McKinnon, 1981). Thermal stresses (Zuber and Parmentier, 1984) and
convective stresses (Squyres and Croft, 1986) may also have played a
significant role in groove formation. The subsurface geometry and origin
of grooves cannot be determined from Voyager images, however. The
grooves appear to be grabens, but the images are insufficiently resolved
to rule out the possibility that they are modified extension fractures
or some kind of ductile necking features. The interpretation of grooves
(and groove sets) as extensional features leads to the use of
relative-age criteria similar to those generally inferred for truncating
relations among sets of rock joints of Earth, as discussed by Golombek
and Allison (1981).

Some direct but sparse evidence for structural shear in the map region
is seen where groove sets appear to be offset by several tens of
kilometers along narrow shear zones (Lucchita, 1980). Compelling
evidence for compression is scarce, although the feature at lat 40° N.,
long 314° cited above appears to possess parallel ridges rather than
grooves, and thus it could be compressional.

**GEOLOGIC HISTORY**

Crater densities within the cratered dark material---the oldest unit in
the map area---are consistent with an age on the order of several
billion years (Smith and others, 1979b). Thus, cratered dark material is
probably crust that has survived from the end of the Solar System's
primordial intense bombardment. The other dark materials most likely are
cratered dark material that has been tectonically modified by furrows or
grooves, and the materials are probably the same age as dark cratered
material.

After formation and cratering of the dark materials, the crust was
fractured and dismembered, and light materials were emplaced between or
on top of the surviving crustal fragments. The process of emplacement is
inferred to be extrusion of magmas relatively rich in ice (for example,
Parmentier and others, 1982). The thickness of the light materials in
the map area is unknown, but in other parts of Ganymede it has been
estimated at 1 to 2 km (Schenk and McKinnon, 1985). The light materials
were then extensively stretched, resulting in the intricate pattern of
grooves that characterizes much of the light terrain in these
quadrangles. Complex crosscutting and truncation relations indicate that
the grooves were formed over a protracted period, which almost certainly
overlapped in time with the extrusion of ice-rich magmas. Futhermore, in
some places clear truncation relations indicate that smooth light
material is younger than the grooves in adjacent grooved light and dark
materials, indicating that ice volcanism continued after local cessation
of groove formation.

Impacts occurred throughout the time of emplacement of light and dark
materials. The oldest impact structure in the map area is probably the
partial palimpsest at lat 29° N., long 356°. Highly degraded craters
(unit c1) are younger than some cratered dark material and perhaps
younger than furrowed and grooved dark materials; these craters may once
have existed on the furrowed and grooved materials but later were
modified structurally so that they are unrecognizable on the available
images.

Most of the abundant somewhat degraded craters (unit c2) are younger
than all light materials except possibly smooth light material, but a
few appear to be older than all light materials. The large palimpsest at
lat 30° N., long 335° is probably the same age as somewhat degraded
craters. Bright craters (unit c3) appear younger than all light and dark
units.

This brief history is almost certainly greatly oversimplified as a
result of the poor resolution and unfavorable viewing geometry of most
the images covering the two quadrangles. Textural features that are just
at the resolution limit of the best images suggest that a much more
complex history can be inferred when better images are returned by
future missions.

**REFERENCES CITED**

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