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SP-425 The Martian Landscape


Enigmatic Troughs



Figure 100


Figures 101 & 102


[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.


Figure 103

Figure 104

Figure 105

Figure 106