Evolution of the north-polar cap of Mars: a modelling study

摘要

Celestial-mechanical computations show that, even stronger than for Earth, Mars is subject to Milankovic cycles, that is, quasi-periodic variations of the orbital parameters obliquity, eccentricity and precession. Consequently, solar insolation varies on time-scales of 10~4-10~5 years. It has long been supposed that this entails climatic cycles like the terrestrial glacial-interglacial cycles. This hypothesis is supported by the light-dark layered deposits of the north- and south-polar caps indicating a strongly varying dust content of the ice due to varying climate conditions in the past. This study aims at simulating the dynamic and thermodynamic evolution of the north-polar cap (NPC) of Mars with the ice-sheet model SICOPOLIS. The boundary conditions of surface accumulation, ablation and temperature are derived directly from the solar-insolation history by applying the newly developed model MAIC. We consider steady-state scenarios under present climate conditions as well as transient scenarios over climatic cycles. It is found that the NPC is most likely not in steady state with the present climate. The topography of the NPC is mainly controlled by the history of the surface mass balance. Ice flow, which is of the order of 1 mm a~(-1), plays only a minor role. Fn order to build up the present cap during the last five million years of relatively low obliquities, a present accumulation rate of ≥ 0.25 mm water equiv a~(-1) is required. Computed basal temperatures are far below pressure melting for all simulations and all times.