Comparison of the North and South Polar Caps of Mars:New Observations from MOLA Data and Discussionof Some Outstanding Questions

New high-resolution data from the Mars Orbiter Laser Altimeter (MOLA) have provided detailed topographic maps for the north and south polar regions. These new data allow one to compare the overall topography and geologic histories of the two polar regions and to highlight some specific outstanding questions in Mars polar studies, following earlier comparisons using Viking and Mars Global Surveyor data. The new data show that the centers of symmetry of the polar cap deposits in map view (which include both the layered terrain and residual ice) are offset from the current rotational pole in antipodal directions. Offset in the north appears to be due to retreat of the polar materials from predominantly one direction (180◦ W). Lines of evidence for movement and melting in different forms (e.g., lobes in young craters, kettle-like depressions, candidate residual mantles overlying polar layered terrain, and possible eskers) have been seen at both poles. The exact timing and causes of movement and melting is yet unknown. Differences in underlying topography (large, low, flat depression in the north; broad, cratered high and edge of a large impact basin in the south) may influence the accumulation, flow, and movement of polar material and the storage and movement of meltwater. The small number of superposed craters has been interpreted to indicate a Late Amazonian age for both caps, with the southern cap being somewhat older (7– 15 × 106 years) than the northern cap (<100 × 103 years). The Late Amazonian-aged caps are surrounded and underlain by Hesperianaged material, indicating an apparent hiatus almost 3-byr in duration. This apparent hiatus in the geologic record from the Late Hesperian to Late Amazonian at both poles may be accounted for in one of three ways. (1) Polar caps are recent events in the history of Mars: This scenario requires that conditions in the Late Amazonian changed to produce environments favorable for cap formation late in the history of Mars, or that polar wander brought the caps to their present position very recently. (2) The polar caps are oscillating: In this scenario, caps are periodically deposited and then retreat and disappear, possibly due to extremes in the obliquity cycle. In this model, the present caps are the latest example of cap deposition and represent the type of deposit that has come and gone numerous times in the Amazonian. (3) The present caps are old, but have been renewed: In this scenario, the present caps have been in their current position for much of the Amazonian, but some process (e.g., melting, flow) is periodically destroying preexisting craters to produce cap surfaces that appear very young in terms of crater retention ages. Documenting the nature of the emplacement history of the present polar deposits and distinguishing among these disparate scenarios for their history are among major challenges facing Mars polar studies in the coming years. c © 2001 Elsevier Science