Polar Sediments Ground Ice and Glaciers

At each pole is a stack of finely layered water-ice-rich sediments about 3 km (2 miles) thick and only a few tens of millions of years old. The layering is exposed around the periphery of the sediments and in valleys that spiral out from the poles. In winter the sediments are covered with carbon dioxide frost, but they are exposed in summer. At the north pole they extend southward to 80° latitude. At the south pole their extent is less clearly defined, but they appear to extend farther from the pole than in the north. The layering is believed to result from variations in the proportion of dust and ice, probably caused by changes in the tilt of the rotational axis (obliquity). At high obliquities water ice is driven off from the poles, probably causing the residual water-ice caps to disappear entirely and the ice to be deposited at lower latitudes. At low obliquities the water-ice caps are at their maximum. Obliquity variations also affect the incidence of dust storms and deposition of dust at the poles. The deposits have a young age because they have all accumulated since the last period of high obliquity when the previous sediments were removed. One peculiarity of the sediments at the north pole is that they are surrounded by, and perhaps rest upon, a vast dune field rich in the sulfate mineral gypsum.

Under present conditions, at latitudes higher than 40°, ground ice is permanently stable at depths roughly 1 metre (3 feet) or more below the surface because temperatures there never get above the frost point. Above 60° latitude the ice is shallow enough to have been detected from orbit. Ice was also found just below the surface by the Phoenix lander at 68° N. At latitudes higher than

40° are numerous surface features suggestive of the presence of abundant ground ice. These include polygonally fractured ground similar to that found in terrestrial permafrost regions and a general softening of the terrain probably caused by ice-abetted flow of the near-surface materials. Possibly the most striking characteristic of the 40°-60°-lat-itude bands indicative of ice is the presence of debris aprons at the base of most steep slopes. Materials shed from the slopes appear to have flowed tens of kilometres away from the slopes, and ground-penetrating radar shows that the aprons contain large fractions of ice.

During periods of high obliquity, ice driven from the poles accumulated on the surface at lower latitudes, possibly to form glaciers. Modeling of atmospheric circulation suggests that the preferred sites for ice accumulation during these periods are the western slopes of the Tharsis volcanoes and northeast of the Hellas basin. All these locations are rich in flow features and morainelike land-forms, which suggests that glaciers were indeed formerly present.

The north polar region also contains the largest area of sand dunes on Mars. The dunes, which occupy the northern part of the plain known as Vastitas Borealis, form a band that almost completely encircles the north polar remnant cap. Interlayering of sand and seasonal carbon dioxide snow can be seen in some locations, indicating that the dunes are active on at least a seasonal timescale.

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