Influence of the thermal properties of a regolith cover on the depth of the melting point of water and the sublimation rate of ice

Abstract

The thermal state of the immediate subsurface is mainly defined by the thermal properties of the regolith, which covers extensive areas of Mars. Although the exact vertical extend of the regolith, its porosity and therefore thermal conductivity are unknown, it is possible to deduce maximum values by using analogues to lunar material. Given a lunar regolith thickness of about 22 km, gravitational considerations lead to a maximum Martian regolith cover of approximately 10 km. At this depth the temperature will likely be above the melting temperature even if conservative values for surface temperature and thermal conductivity are assumed. The thermal conductivity of the regolith still poses a problem as only the upper and lower bounds are fairly well known. The upper limit is given by the thermal conductivity of solid crustal material which is in the range of about 2 W/(mK). The lower limit can be inferred from measurements of thermal interia which yields values of 0.001 to 0.05 W/(mK) for very fine grained unconsolidated regolith. Asumming an increase of the thermal conductivity with depth due to increased pressure and reduced porosity, it is possible to extrapolate reasonable models for the thermal conductivity. Based on these models the depth of the melting point of water can be calculated depending on surface temperatures and heat flow from below. Both parameters have changed considerably in certain areas (e.g. Olympus Mons) in the recent past due to changes in obliquity and volcanic activity. It is therefore necessary to understand the effect of these variations on the temperatures within the regolith to assess the stability of ground ice especially in low latitude regions. As new results of the HRSC indicate the occurrence of glaciers at equatorial latitudes on the flanks of volcanoes, the sublimation rate and long-term stability of these near-surface ice layers is analysed considering different burial depths, changes in the thermal properties of the regolith as well as variations in obliquity during the last 10 Ma.