GLACIAL RETREAT FEATURES IN THE NORTH POLAR REGION OF MARS

Building upon previous studies of the north polar region of Mars [e.g., Dial, A. (1984) USGS Misc. Invest. Ser. Map I-1640; Tanaka, K. & D. Scott (1987) USGS Misc. Invest. Ser. Map I-1802-C; Thomas, P., S. Squyres, K. Herkenhoff, A. Howard, & B. Murray, (1992) in Mars, H. Kieffer et al., eds., U. of Ariz. Press, 767-795; Zuber, M. et al. (1998), Science 282, 2053-2060.], we [K. Fishbaugh & J. Head (2000), JGR 105, 22455-22486] outlined evidence for retreat of the polar cap, including the presence of polar material remnants and kettle-like features in the Olympia Depression concentric to the cap. In this study, we further characterize and categorize features in this region using Viking and MOC images and high-resolution MOLA data. Within the Olympia Depression are large, deep, irregularly-shaped depressions and associated higher-standing topography. Together, these constitute what we term rough terrain. These features appear to have undergone significant removal of volatiles and subsequent collapse. Evidence of moraines, sedimentation at the margins, and drainage channels has also been found. Dome-like features, which could be volatile and sediment-rich, lie just to the south of the rough terrain. Their morphology and dimensions are dissimilar from terrestrial and Martian south polar volcanic domes [Ghatan, G. & J. Head (2001), LPSC 32, Abs. 1039] and from impact-related central peak structures. On Mars, the high concentration of polar deposit inter-layered debris (about 40%) will favor the development of kames as ablation occurs. The morphologic features within the Olympia Depression are transitional to Olympia Planitia, a partially ablated lobe of the polar cap, and to the main polar cap. We interpret the rough terrain as armored ice remnants and large, proximal kame and kettle features, formed primarily by stagnation of ice blocks. The dome-like features are interpreted as distal, smaller, armored ice remnants and kame features which may represent an earlier stage of retreat, possibly involving more meltwater. Evidence for melting has also been found in the form of a large reentrant, Chasma Boreale, which may have formed by a sub-cap melting event. Retreat of the polar cap has apparently occurred geologically relatively recently (in Late Amazonian times), and melting may have contributed to this retreat.