Polar wandering of Mars

Abstract

Most interpretations of Martian geology implicitly invoke a globe locked to its present spin axis. Thick (4–5 km) uncomformable deposits near the equator (the Mesogaea area southwest of Olympus, Mons), however, bear a striking resemblance to the present-day polar deposits in extent, appearance, and inferred processes. We propose that these and a similar antipodal deposit are remnants of a previous geographic polar location. Although crater statistics of the surface of these deposits may indicate a very young age, such a date reflects active aeolian reworking. Relative ages based on superposed craters in peripheral deposits suggest that these deposits predate the early stages of Tharsis volcanism but postdate Lunae Planum. Similar crater-count age dates of adjacent more eroded deposits indicate a sequence in formation, thereby revealing a possible polar wandering path. The youngest near-equatorial deposits occur southwest of Olympus Mons (15°W, 0°S); more eroded deposits with a slightly greater age occur east of Apollinaris Patera (180°W, 6°S). In the opposite hemisphere, these deposits are mirrored by similar deposits in Arabia and northwest of Schiaparelli, respectively. Very old and heavily eroded deposits also occur south of Elysium (210°W, 0°S) and are mirrored in the opposite hemisphere by the chaotic terrains of eastern Valles Marineris. The proposed polar wandering path provides a different perspective for interpreting the origin and evolution of volatile reservoirs such as the inferred source regions of major outflow channels. The more recent uncomformable deposits were eroded principally by sublimation of trapped volatiles and aeolian processes. Frozen volatiles in more ancient deposits (pre-Lunae Planum) were absorbed in the underlying regolith when higher lithospheric/atmospheric temperatures favored melting. Early polar wandering is proposed to be related to changes in the principal moments of inertia resulting from early epochs of widespread basin-filling flood basalts. However, the last major shifts are related to the formation of the Tharsis volcanic constructs and Olympus Mons. Although the total polar wandering path exceeded 120°, it was not a continuous process but occurred in short-lived shifts. These shifts are reasonably consistent with the observed distribution of compressional and extensional tectonic features, but the detailed record is complicated by preexisting impact basins, later volcanic/ tectonic events, and the global thermal history.