Dark and light terrain materials in the Perrine and Nun Sulci quadrangles are divided into nine map units, four dark, and five light. These are placed in time-stratigraphic sequence primarily by means of embayment and cross-cutting relationships. Dark terrain is generally more heavily cratered and thus older that light terrain but, at least in these quadrangles, crater densities are not reliable indicators of relative ages among the four dark material units. The four mapped material units within dark terrain are: cratered dark materials (dc), grooved dark materials (dg), transitional dark materials (di), and dark materials, undivided (d). The five mapped units within light terrain are: intermediate light materials (li), grooved light materials (lg), irregularly grooved light materials (lgl), smooth light materials (ls), and light materials, undivided

Mcgill, George E.

Squyres, Steven W.

English

NASA Technical Reports Server

N9 -19911: 
STRATIGRAPHY OF THE PERRINE AND NUN SULCI QUADRANGLES (Jg-2 AND Jg-5),
GANYMEDE.
George E. McGII1, Department of Geology and Geography, University of Massachusetts,
Amherst, MA, and Steven W. Squyres, Center for Radlophysics and Space Research,
Cornell University, Ithaca, NY.
Dark and light terrain materials in the Perrine and Nun Sulcl quadangles are
divided into 9 map units, 4 dark and 5 light. These are placed in tlme-stratigraphlc
sequence primarily by means of embayment and cross-cutting relationships. Dark
terrain is generally more heavily cratered and thus older than light terrain but, at
least in these quadrangles, crater densities are not reliable indicators of relative
ages among the 4 dark material units.
Dark Materials: The 4 mapped material units within dark terrain are: cratered dark
materials (dc), grooved dark materials (dg), transitional dark materials (di), and dark
materials, undivided (d).
The most widespread dark terrain unit consists of cratered dark materials (dc).
Although the average albedo is relatively low, these materials have a patchy,
streaked, or mottled appearance in detail because the surface is actually a montage
of high and low albedo areas at km to 10's of km scale. In places, this appearance
seems due to the presence of grooves or fragments of crater rims that are too small to
map, but generally it is not possible to identify the surface characteristics
responsible. The surface texture is rough at a scale similar to the albedo variations
or smaller. Craters and palimpsests of all sizes from the limits of resolution to the
largest present in the mapped area are superposed on this unit. These span the full
range of crater ages present. Individual large grooves are present in places, many
of which cross-cut or coincide with contacts between light and dark terrains.
Grooved dark materials (dg) occur generally as small areas adjacent to larger areas
of cratered dark materials, commonly along contacts between the latter and light
materials. The average albedo is somewhat higher than for cratered dark materials,
and in detail the surface has a linear or streaked appearance consistent with the
groove topography. The only large area of grooved dark materials occurs in the
northern third of the large, sub-circular portion of Perrine Regio on Jg-2. Here the
crater density appears to be less than in the southern two-thirds of this region,
which is underlain by cratered dark materials. However, this impression is based on
a grand total of 20 mapped craters and crater fragments, three in the northern third,
17 in the southern two-thirds, and thus should be only provisionally accepted as
evidence for a real age difference. In many places, grooves in dark materials are
clearly continuous with grooves in adjacent light materials, hence the grooves
represent a relatively young structural modification of the dark materials. Grooved
dark materials may thus be simply cratered dark materials that have been
structurally modified, and older craters would logically be less recognizable where
they are cut by grooves. Consequently, the apparent difference in crater ages
between cratered and grooved dark materials is possibly unrelated to the true
formation ages of the materials. The large area of grooved terrain mapped in Perrine
Regio is included in the group of "system III" furrowed dark-terrain polygons by
Murchie and Head (1988). We believe that the materials of this area are more
correctly mapped as grooved than as furrowed because the troughs are
morphologically similar to grooves, and not similar to furrows, and because at least
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McGill and Squyres, Stratigraphy of Ganymede Jg-2, Jg-5
some of them are continuous with typical grooves in the adjacent light terrain
materials. However, the quality of the available images is so poor that it is not really
possible to resolve this issue with any confidence.
Transitional dark materials (di) occur only in small patches adjacent to light
materials. The distinguishing characteristic is an albedo intermediate between
typical dark materials and typical light materials. Surface appearance may be similar
to grooved dark materials or to dark materials, undivided. The intermediate albedo
is presumably due to a smaller fraction of dark silicate(?) material near the surface
than is the case for normal dark materials. The age relationships with cratered and
grooved dark materials are unknown.
Dark materials, undivided (d) occur in small patches or in areas of very poor image
resolution. Neither grooves nor craters can be resolved, and the surfaces generally
appear to be smooth and of a uniform low albedo, an appearance most likely due to
poor resolution in most instances. Undivided dark materials are inferred to be similar
to either grooved or cratered dark materials.
Light Materials: The 5 mapped units within light terrain are: intermediate light
materials (ll), grooved light materials (lg), irregularly grooved light materials (lgl),
smooth light materials (Is), and light materials, undivided. Intermediate light
materials (ll) are materials that are intermediate in albedo between other light
materials (lg, lgl, is, 1) and the lightest dark material (dl). Usage of this unit Is
restricted to a region in the northern portion of Jg-2. Texturally, this material
appears similar to dark materials, but it is distinctly brighter than any of them. A
possible explanation for the appearance of this unit is that the underlying material
is in fact dark terrain that is blanketed by Ganymede's polar frost caps. These caps
apparently are thin deposits of H20 frost that cover the surface of Ganymede down
to a latitude of 40°-50 ° in both hemispheres. They may be either the remnants of a
formerly more extensive frost blanket that has been removed from lower latitudes by
sublimation, or may be an accumulation of frost that has built up over time by
transport from lower latitudes.
Grooved light materials (lg) are common, and are perhaps the most distinctive
geologic unit on the satellite. Like irregularly grooved light and smooth light
materials, they have a markedly lower crater density than is observed in dark
terrains. Their distinguishing characteristic is the presence of very regular patterns
of grooves transectlng their surface. Grooves are curvlllnear topographic depressions
up to a few km wide, a few hundred m deep, and hundreds of km long. Grooves appear
to be roughly U-shaped in cross-section, but unfortunately the resolution of the best
images is Insufficient to reveal the details of their geometry. Groove slopes are
gentle; at Voyager resolution, typical slopes are less than 10 ° (Squyres, 1981). The
grooves are arranged in subparallel or fan-shaped sets that Intersect and crosscut
one another with geometry that can be very complex. The grooves in some sets are
packed closely together, producing roughly sinusoidal topographic profiles, while in
others they are widely separated, with distinct smooth areas between them. Where
these smooth units become extensive, they are mapped as a distinct unit, Is.
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McGill and Squyres, Stratigraphy of Ganymede Jg-2, Jg-5
Irregularly grooved light materials (lgi) are similar to grooved light materials in
having low crater density, high albedo, and a surface dominated by grooves. The
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 in
grooves that are arranged in sets. Individual grooves may crosscut and truncate one
another, and it is not uncommon for grooves to split and merge at acute angles,
forming a crudely anastomoslng pattern.
The simplest light terrain is composed of smooth light materials (Is). These
materials are similar in albedo to most other light terrains, but are distinguished by
being free of grooves at the best available image resolution. Except for impact
craters, topography is very subdued. In particular, no volcanic flows or other
features that might provide morphologic clues to the emplacement mechanism are
seen. Albedo is commonly fairly uniform across individual regions of smooth light
material. Some regions, however, have a mottled appearance suggesting local
variations in silicate content. Smooth light materials are seen in a variety of
geometric relationships with other light units. Some occur as irregularly-shaped
groove-free patches in areas that are otherwise dominated by a complex pattern of
grooves. They also occur as long curvilinear swaths with geometry similar to groove
sets. Some of these have bounding grooves. Smooth swaths and regions commonly
have very sharp boundaries, both with other light units and with dark units. In other
cases, however, the boundaries are indistinct and transitional to dark materials or
to other light materials.
Undivided light materials (1) are simply materials that appear to have an albedo
similar to those of other light terrains, but that are observed at an image resolution,
viewing geometry, or illumination geometry that prevents recognizing characteristics
that would identify them as belonging to one of the other mapped light units. They
are mapped primarily in the poorly-imaged central portion of Jg-2.
Crater and B_|r_ Matgrla]_: Crater and palimpsest materials are divided into 4
morphologlc classes from highly degraded (palimpsests) to very fresh (bright ray
craters). This classification is inferred to correspond approximately to a relative age
sequence. Caution is necessary, however, because of size-dependent effects on
degradation by mlcrometeorlte bombardment, favoring survival of large craters, and
size-dependent effects on degradation by 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.
References cited:
Murchie, S.L., and Head, J.W., 1988, Possible breakup of dark terrain on Ganymede by
large-scale shear faulting: Journal of Geophysical Research, vol. 93, p. 8795-8824.
Squyres, S.W., 1981, The topography of Ganymede's grooved terrain: Icarus, vol. 46,
p. 156-168.
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Stratigraphy of the Perrine and Nun Sulci quadrangles (Jg-2 and Jg-5), Ganymede

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