Lithospheric Loading by the Northern Polar Cap on Mars

Abstract

New topography data for the northern polar region on Mars, returned by the the Mars Orbiter Laser Altimeter (MOLA) during the aerobraking hiatus and science phasing orbits, allow characterization of the topography of the present northern polar cap and its environs. Models for loading of an elastic shell by an axisymmetric load approximating the present polar cap geometry indicate that the maximum deflection of the subice basement is in the range 1200 to 400 m for an elastic lithosphere of thickness 40 to 200 km overlain by a cap of pure H 2O ice. Corresponding model cap volumes increase from 1.5 to 1.8 × 10 6 km 3, as elastic lithosphere thickness decreases from 200 to 40 km. The presence of sediments in the polar cap increases the depth to basement and resulting cap volume for a given value of elastic lithosphere thickness. One-dimensional heat flow calculations indicate that the temperature at the base of the cap may approach the melting point of cap material if the lithosphere underlying the cap is thin. The basal temperature is 170 K for a 200-km-thick lithosphere overlain by pure ice but is as great as 234 K for a 40-km-thick lithosphere overlain by a cap with a high sediment/ice ratio. Constraints on elastic lithosphere thickness are weak, but geologic mapping and MOLA data suggest that a flexurally derived circumpolar depression filled with sediments is consistent with elastic lithosphere thickness values in the range 60–120 km. Gravity and topography over the whole cap are poorly correlated, possibly due to viscous relaxation of long-wavelength topography, but gravity and topography over the western portion of the main cap are consistent with an elastic lithosphere thickness of 120 km, for a crustal thickness of 50 km. Both MOLA data and geological information suggest a formerly larger northern polar cap. The relationship of time scales for changes in the polar cap volume and extent to time scales for viscous relaxation of topography has important implications for investigations of even present polar cap topography. Viscoelastic calculations show that the Maxwell time for an Earth-like mantle viscosity for Mars (10 21 Pa-s) is 10 5 yr. The Maxwell time scales directly with the martian mantle viscosity so that values as high as 10 7 yr are possible. Time scales for changes in polar cap volume are poorly constrained, but major changes in cap volume over periods of 10 6–10 8 yr are consistent with current understanding of polar cap processes.