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A. P. Ingersoll, S. Byrne, B. C. Murray (Caltech), E. C. De Jong, G. E. Danielson (Caltech/JPL), K. E. Herkenhoff, H. H. Kieffer, L. A. Soderblom (USGS, Flagstaff)
Because the orbit of the Mars Global Surveyor (MGS) is inclined at an angle of 93 degrees relative to the equator, the spacecraft gets especially good coverage of the ring at 87 degrees latitude. The following is a list of phenomena that have been seen by the Mars Orbiter Camera (MOC) during the spring and summer at the South Pole. (1) Circular depressions, described by Thomas et al. [Nature 404, 161-164, 2000], which are ~10 meters deep and 100's of meters in diameter. (2) Layers several meters thick that are exposed on the walls of the mesas. (3) Late summer darkening of the walls of the mesas, which are half as bright as the flat tops. (4) Small dark features that appear in spring on the seasonal frost outside the residual cap. Some of the features have parallel tails that are clearly shaped by the wind. Others are more circular, like black spiders with multiple branching arms. After the CO2 frost has disappeared the arms are seen as troughs. (5) Polygons whose sides are dark troughs. Those that are outside the residual cap seem to disappear when the frost disappears. (6) Irregular depressions outside the residual cap. They look like degraded versions of the circular depressions inside the residual cap, and may be a remnant of the cap's changing location. (7) Areas of burial and exhumation of circular depressions. Thomas et al. give an example with a sharp boundary: On one side the depressions are buried and on the other side they are exposed. In other cases there are rounded troughs up to 1 kilometer wide, which are dark in summer and appear to have eroded down below the floor of the circular depressions.
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