Le pergélisol martien: Martian permafrost

Abstract

On planet Mars, with a mean annual temperature of −60°C and a dry periglacial-type climate, a permafrost extends all over the surface. The atmospheric pressure of 6 mbar is likely to affect the stability of ground-ice at depth. In order to quantify the characteristics of Martian ground-ice, 2600 rampart craters have been measured over the planet. Rampart craters have been proposed as indicators of the spatial concentration of volatiles. The latitudinal distribution of rampart craters indicates that type 1 rampart craters, together with low-mobility ejecta, are frequently observed in the equatorial region. Rampart craters of type 2, with high-mobility ejecta, occur at mid-latitudes. The increase of ejecta mobility with latitude accounts for a concentration of volatiles at high latitudes with near-surface ground-ice at high latitudes. The map shows a meridional distribution of Martian ground-ice with underground ground-ice at equatorial latitudes and subsurface ground-ice at mid and high latitudes. The top layer of ground-ice is found at a depth exceeding 0.3 km at equatorial latitudes, and 150 m for high latitudes, with a minimum value of 30 m in Acidalia Planitia and Utopia Planitia. There is a discontinuity in the depth of ground-ice near 35–40° latitudes, which might be related to the stability of near-surface ground-ice at those latitudes. At latitudes higher than 40°, the soil never reaches the frost point during the year and is therefore in equilibrium with the atmosphere. In such conditions water is always stable as ice. This observation confirms the theoretical distribution of volatiles.