Ice flow and isostasy of the north polar cap of Mars

Abstract

The flow of the north polar cap of Mars, which is assumed to consist mainly of H 2O ice, is investigated with the three-dimensional ice-sheet model SICOPOLIS. We consider a simplified topography and a climatic forcing varying between the present state and warmer, more humid conditions in the past with an obliquity cycle of 1.3 million earth years (yr). Furthermore, SICOPOLIS is coupled with the three-layer viscoelastic ground model displace to simulate the isostatic response of the underlying lithosphere/mantle system. Likely ice-flow velocities at present are a few mm/yr, along with surface accumulation/ablation rates of the order of 0.1 mm water equiv./yr and basal temperatures more than 50°C below pressure melting. Thicknesses of the rheological lithosphere of 50– 400 km are consistent with geothermal heat fluxes of 15– 36 mW m −2 , with some evidence for values of ca. 80– 120 km and 20– 30 mW m −2 , respectively. For an Earth-like upper-mantle viscosity of O(10 21 Pa s ) , relaxation times of the lithosphere/mantle system are much shorter than significant load changes, so that the mantle behaves as an inviscid fluid. The poor correlation between the computed and measured free-air gravity signal indicates that it is determined by mass anomalies of a different origin.