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[90-91] Figure 100
is one of the more instructive pictures taken at the Viking
2 site. A linear depression, or trench, can be traced across
the middle of the picture. The bottom of the trench is 10 to
15 cm lower than bordering lips. The trench can be traced
more than 10 m (figs. 103 to 106), trending generally east
west and descending slightly to the east. It is partly
filled with sediment finer than on adjacent surfaces. The
sediment apparently has been scalloped by the wind. Larger
rocks are relatively rare, and the few that are present are
partly buried by finer sediment.
The trench seen here is part of a
more extensive polygonal network that occurs near the
Lander. The origin is uncertain but it is possible that the
polygonal soil structure is the result of cyclic freezing
and thawing of groundwater. Analogous structures form in
periglacial regions on Earth. During spring, meltwater
segregates in the soil and freezes to form an ice wedge.
Expansion of permafrost during the summer causes an upward
bulge in the vicinity of the ice wedge. During the winter
the permafrost contracts and the wedge opens, initiating
another annual cycle of wedge growth. A depression forms
over the wedge and, in windswept terrains, is filled by wind
transported sand.
Viking 2 landed at 48°N,
25° north of the Viking 1 site. This difference in
latitude might account for "polar" landforms at one site but
not the other. If the trenches do reflect the subsurface
formation of ice wedges in the same way as on Earth, then
their formation must have occurred at some former time when
liquid water was stable. With the present thin atmosphere
only the vapor and ice phases are stable. Any liquid water
would either freeze or boil away.
Figures 101 and 102 are enlargements
of particularly significant regions in figure 100. The large
rock, the left side of which is shown in figure 101, is
approximately 1 m wide. Stratification in the block runs
from upper left to lower right. The left end of the boulder
is much more pitted than the central part. These pits, which
occur in the majority of the boulders, may be the result of
vesiculation, or frothing, in a gas charged lava during its
consolidation. The layering of pits may represent vertical
differentiation in the original volcanic deposit, with the
most highly vesiculated lava occurring near the top of the
flow.
Figure 102 shows one of the
windsculpted drifts seen at the Lander 2 site. The polygonal
shapes indicate that the drifts are probably being eroded by
wind activity.
Figures 103 to 106 show the
development of the trench as it is traced from left (west)
to right (east). The right side of figure 103 overlaps the
left side of figure 104 which is also displaced downward
10° in elevation. The right side of figure 104 overlaps
with the left side of figure 106.
Note that many of the features in
figures 104 to 106 are also identifiable in figure 100. A
series of crescent shaped drifts within the trench is
particularly well developed. The perspective is different
because the scene is photographed with two cameras, situated
0.8 m apart on the spacecraft.
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