Is Mars hiding some ice in Terra Arabia?

Kurzfassung

Only a few years ago the common notion was that Mars today is a dry place. With the excellent dataset of the Gamma and Neutron spectrometer (GRS and HEND) on board of Mars Odyssey this view had to be corrected. The instrument detected water abundance of at least 8wt% in the equatorial regions. There are three main explanations for this observed amount of water which are not mutually exclusive. Some of the water measured is most likely adsorbed water. However this mechanism can not explain the high abundances measured in some place. We might see highly hydrated minerals, which are capable of holding large quantities of water. The last and maybe most exciting possibility are near surface ice deposits. The question is, how did they survive close to the surface under the hyper-arid conditions we encounter on present day Mars? We do not have any direct evidence for ice at lower latitudes. From the GRS and HEND measurements we know that the polar caps extend under the surface. The near surface ice deposits one see at low latitudes might be only the tip of a global ice reservoir in shallow depths. The ice deposits at low latitudes could also be the remnants of the last Martian ice age, caused by obliquity changes. In this case one only sees the dwindling remains of large equatorial glaciers covering these regions during the last ice age. Both ice related scenarios would imply that Mars has, or at least had in the very recent past, large quantities of ice on or close to the surface. While working on model calculations for the stability of ice on Mars today we discovered a possible third scenario. We have studied cases where the soil consists of layers with very different thermo-physical properties. One of the scenarios we have looked at is a low thermal conductivity dust layer on top of a sand layer with a significantly higher thermal conductivity. Such configurations can be found for example in the Terra Arabia region. For this case we observed the formation of an ice lens at the boundary between the dust/sand material, effectively closing the pore space and significantly reducing downward diffusion. This leads to an actual enrichment of ice compared to the initially assumed mass of more than 50% within the dust layer. Results depend on the thickness and the parameters used of the layers. We will discuss the implication of such a scenario on our understanding of ice on Mars. The ice enriched layer might lead to an overestimation of the global ice inventory based on GRS and HEND measurements. The process of enriching the ice significantly slows the movement of the ice table to greater depth and can therefore stabilize ice over several thousands of years close to the surface. This implicates that we might indeed observe today the remains of the last Martian ice ages, but the assumed amount of ice moved across the planet can be significantly smaller than previously thought. Furthermore some of the "young" glacial feature one see today would have been formed not during, but after the last ice age and might even exist until today.