Water loss and evolution of the upper atmosphere and exosphere over martian history

摘要

The loss of water from Mars can be evaluated by studying the evolution of the escape rate of atomic oxygen over time. Throughout martian history, the evolution of solar radiation has led to significant variations in the macroscopic parameters of the thermosphere/ionosphere, which in turn govern the hot species population of the exosphere and especially the atmospheric loss rates. In this study, the combination of our Direct Simulation Monte Carlo (DSMC) kinetic model and the 3D Mars Thermosphere General Circulation Model (MTGCM) [Valeille, A., Combi, M.R., Tenishev, V., Bougher, S.W., Nagy, A., 2009. Icarus. doi:10.1016/j.icarus.2008.08.018] is used to describe self-consistently ancient upper atmospheres of Mars for different solar inputs. 3D descriptions from the MTGCM of the ancient thermosphere/ionosphere are presented and discussed for the first time, including density profiles and temperature maps of the background neutrals and ions for the three epochs considered, which can be related to a solar EUV (Extreme Ultraviolet) flux enhancement of 1, 3 and 6 times the present values. Furthermore, solar cycle effects are quantified and discussed for both present and past conditions. Along with maps of ion production by photoionization (PI), charge exchange (CE) and electron impact (EI), the DSMC model provides density and temperature profiles, return fluxes and atmospheric loss rates of suprathermal exospheric oxygen as functions of the Solar Zenith Angle (SZA). This approach allows us to study the effects of dynamics on the ancient Mars upper atmosphere structure. Thermospheric variations are found to be not as large as previous 1D models predicted. The study of the evolution of the heat balance suggests that the ancient Mars thermosphere, of about 3.5 billion years (Gyr) ago, was relatively similar to the present Venus thermosphere. While O_2~+ dissociative recombination (DR) is by far the main source of atmospheric escape at present, its relative contribution is shown to be reduced in the past compared to secondary escape processes. Nevertheless, O_2~+ DR is found to remain the main driver of atmospheric escape over this period. The long-term decay of atmospheric loss throughout the martian history (until about 3.5 Gyr ago) may be comparable to the short-term periodic variations at present. A heuristic scaling law for evolution of water over time is proposed and a conservative estimate of about 10 m of water is found to have escaped globally to space over the last ～3.5 Gyr.