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The topographic and geologic characteristics of grooves and groove-like features in the south polar layered deposits near the Mars Polar Lander/Deep Space 2 landing sites are evaluated using Mariner 9 images and their derived photoclinometry, normalized using Mars Orbiter Laser Altimeter data. Although both Mariner 9 and Viking images of the south polar layered deposits were available at the time of this study, Mariner 9 images of the grooves were selected because they were generally of higher resolution than Viking images. The dimensions and slopes of the grooves, together with orientations that nearly match the strongest winds predicted in the Martian Global Circulation Model and directions inferred from other wind indicators, suggest that they formed by aeolian scour of an easily erodible surface. Most grooves are symmetric and V-shaped in transverse profile, inconsistent with an origin involving extensional brittle deformation. Although the grooves strike along slopes and terraces of the south polar layered deposits, the variable depths and lack of terracing within the grooves themselves indicate that any stratigraphy in the uppermost 100 m of the polar layered deposits is composed of layers of similar, and relatively low, resistance. The grooves do not represent landing hazards at the scale of the Mariner 9 images (72-86 m/pixel) and therefore probably would not have affected Mars Polar Lander and Deep Space 2, had they successfully reached the surface.

We present two photoclinometric profiles across a trough in the martian northern polar layered terrain. Complications caused by albedo variations were avoided by using an early springtime Viking image with a thin cover of seasonal [CO.sub.2] frost. The topographic profiles were constrained with stereogrammetric elevations derived from summertime Viking images of the same region. We find that the photoclinometric profiles are consistent with a nearby MOLA (Mars Orbiter Laser Altimeter) track crossing the same polar trough. The trough is asymmetric, with higher relief and a steeper slope on the equatorward-facing wall. Individual layers are subdued and difficult to observe in the profiles. A decrease in both relief and elevation toward the eastern end of the trough suggests that layers become thinner to the east. Declining equatorward slopes in the eastern portion of the trough imply that erosion rates have varied along the trough. The variation in erosion rate may be linked to the change in layer thickness along the trough. Layers have an average thickness of 19 [+ or -] 8 m in the center of the trough and 59 [+ or -] 32 m on the northern wall. The northern wall is most likely composed of thinner layers that are obscured. To first order, we find that a 19-m layer requires 16,000 years of deposition to form. Although this timescale does not coincide with orbital variation periods of [10.sup.5] and [10.sup.6] years, deposition rates may not be constant and thus the 16,000-year layer formation time does not preclude layer formation during part of each orbital oscillation. [C] 2000 Academic Press Key Words: Mars; Mars, surface; Mars, climate; surfaces, planets.

As the planet's principal cold traps, the martian polar regions have accumulated extensive mantles of ice and dust that cover individual areas of ~[10.sup.6][km.sup.2] and total as much as 3-4 km thick. From the scarcity of superposed craters on their surface, these layered deposits are thought to be comparatively young--preserving a record of the seasonal and climatic cycling of atmospheric [CO.sub.2], [H.sub.2]O, and dust over the past ~[10.sup.5]-[10.sup.8] years. For this reason, the martian polar deposits may serve as a Rosetta Stone for understanding the geologic and climatic history of the planet--documenting variations in insolation (due to quasiperiodic oscillations in the planet's obliquity and orbital elements), volatile mass balance, atmospheric composition, dust storm activity, volcanic eruptions, large impacts, catastrophic floods, solar luminosity, supernovae, and perhaps even a record of microbial life. Beyond their scientific value, the polar regions may soon prove important for another reason--providing a valuable a accessible reservoir of water to support the long-term human exploration of Mars. In this paper we assess the current state of Mars polar research, identify the key questions that motivate the exploration of the polar regions, discuss the extent to which current missions will address these questions, and speculate about what additional capabilities and investigations may be required to address the issues that remain outstanding. Key Words: Mars, surface; Mars, atmosphere; Mars, climate; ice; exobiology.

Interpretation of the polar stratigraphy of Mars in terms of global climate changes is complicated by the significant difference in surface ages between the north and south polar layered terrains inferred from crater statistics. We have reassessed the cratering record in both polar regions using Viking Orbiter and Mariner 9 images. No craters have been found in the north polar layered terrain, but the surface of most of the south polar layered deposits appears to have been stable for many of the orbital/axial cycles that are thought to have induced global climate changes on Mars. The inferred surface age of the south polar layered deposits (about 10 Ma) is two orders of magnitude greater than the surface age of the north polar layered deposits and residual cap (at most 100 ka). Similarly, modeled resurfacing rates are at least 20 times greater in the north than in the south. These results are consistent with the hypotheses that polar layered deposit resurfacing rates are highest in areas covered by perennial ice and that the differences in polar resurfacing rates result from the 6.4 km difference in elevation between the polar regions. Deposition on the portion of the south polar layered deposits that is not covered by the perennial ice cap may have ceased about 5 million years ago when the obliquity of Mars no longer exceeded 40 [degrees]. Key Words: Mars, surface; cratering; Mars, climate.