North polar region of Mars: Topography of circumpolar deposits from Mars Orbiter Laser Altimeter (MOLA) data and evidence for asymmetric retreat of the polar cap

We have used high-resolution Mars Orbiter Laser Altimeter (MOLA) data to analyze the topography, morphology, stratigraphy, and geologic history of the Martian north circumpolar deposits. The present polar deposits are offset about toward 0 degrees W from the rotational pole. An are of irregular topography, concentric to Olympia Planitia and the cap, consists of polar material remnants, depressions which we interpret to be kettles, frost-covered and residual ice-filled craters, and frost patches. Olympia Planitia, originally thought to be a flat, sand-covered plain, is characterized by a convex-upward topography, contiguous with the polar cap. We interpret Olympia Planitia to represent a now dune-covered extension of the polar materials. Together, Olympia Planitia and the outlying deposits delineate a former extent of the polar cap. Topographic data have clarified relationships among the circumpolar deposits. Contributors to these deposits include local volcanics, fluvial and aqueous sediments (from outflow channels and a possible standing body of water), pyroclastic ash, sublimation lag from the Olympia Lobe, and eolian-reworked materials. Significant events in the history of the region include (1) formation of the northern lowlands; (2) emplacement of volcanic plains, fluvial and aqueous sedimentation, and subsequent desiccation, forming polygonal patterns which in part underlie the present polar layered deposits; (3) formation of the polar cap, composed primarily of layered deposits; (4) asymmetric retreat of the Olympia Lobe, resulting in sublimation lag deposits, polar remnants, and kettles; and (5) continued collection and reworking of sediments by eolian processes. The cause of the asymmetrical retreat of the Olympia Lobe is unknown.