Formation and decomposition processes of CO2 hydrates at conditions relevant to Mars

Abstract

From a thermodynamic point of view there is no argument against the existence of CO2 hydrates in the Martian regolith close to the surface. It was postulated, that CO2 hydrates may occur in the ice layers of the north and the south polar caps. On this basis, many suggestions linking decomposition of CO2 hydrates to morphological features like chaotic terrains, some outflow channels or gullies have been put forward. Another group of theories discusses the possibility that releases gases may have an environmental impact such as causing a climate change (greenhouse effect) or alter the isotopic ratios in the atmosphere. At present days and possibly also in the past p-T conditions the most likely formation reaction to take place between gaseous CO2 and water ice. Both components are available on the surface. Lately, also H2O has been found to be abundant in Martian regolith. However, the discussions in a large number of publications didn‟t reach a final conclusion, because of the lack of elementary knowledge about the formation and decomposition kinetics of this particular gas hydrates, yet. The investigations presented here provide the required information. To achieve a physicochemical basis for these ideas, a series of CO2 hydrate formation and decomposition experiments at Martian surface and sub-surface conditions were performed, using p-V-T methods as well as in-situ neutron diffraction at ILL Grenoble. The experiments indicate that the formation time is directly related to the accessible surface area of the ice grains as well as temperature and CO2-pressure. At p-T conditions close to the Martian poles CO2 hydrates are thermodynamically stable at the surface. Despite this fact the results show that at these low temperatures slow kinetics and nucleation difficulties prevent any significant formation of clathrates. However, there is still a fair chance to find CO2 hydrates deeper in the regolith at different latitudes (given a pressure sealing of the overburden layers e.g. by water ice). Higher temperatures and pressures create much more favorable conditions. Additionally, climate variations on longer time-scales provide a conceivable scenario for hydrate decomposition and perhaps formation cycles as long as suitable conditions can be created. Gases from dissociating clathrates might be able to affect isotopic ratios in the atmosphere. Larger releases could also potentially cause episodes of warmer climate. The experimental decomposition runs in a temperature interval from about 240 to 273 K, have firmly established a behavior, called self-preservation (or anomalous preservation ), which may preserve CO2 hydrates for geologically long time scales. Self-preservation is a complex micro-structural process related to changes on the surface of decomposing hydrates. Small (up to 20μm) ice crystals formed upon decomposition create a layer, which due to annealing of ice defects and grain coarsening drastically slows down the out-diffusion of gas molecules and thus preventing decomposition. Below this temperature regime the self-preservation also occurs in the narrow p-T range. The sealing is less effective and is governed by the microstructure of an ice film. The destruction of this fragile, mechanically or by reaching the ice melting temperature achieved state may lead to the rapid gas release from decomposing clathrates. Sudden increase of pore pressure in the regolith may trigger the formation of large geomorphologic features like chaotic terrains thus letting pressurized liquids escape to the surface.