Present Day Situation

Permanent and seasonal polar caps occupy vast territories, and are the obvious evidence of the Martian cryosphere (Hvidberg 2005). Seasonal caps represent the up to 2 m thick CO2 condensate, which drops out until approximately 60° latitude during the winter polar night in the corresponding hemisphere, and sublimates in spring and summer. In summertime at the poles, permanent caps remain consisting of water; but because of the ellipticity of the Martian orbit, the southern summer is shorter, and on the surface of the south cap a condensate of carbon dioxide partly remains (Mitrofanov 2005). Both caps together have a mass that is equivalent to a water layer of about 22-33 m spread over the planet's surface (Smith et al. 1999).

At present, spatiotemporal regularities of water distribution on the Martian surface and near subsurface horizons are being studied. According to the Inverse Square Law which is used to calculate the decrease in radiation intensity due to an increase in distance from the radiation source, Mars is located at 1.524 astronomic units and receives 2.32 times less solar radiation than the Earth. This fact determines the existence of a global frozen envelope, the cryosphere. Mean annual temperature on the Martian surface varies from -100°C at the poles to -50°C at the equator. The absence of atmosphere predetermines high temperature oscillations; for example, on Mars Pathfinder landing site temperature reached 2°C at noon, and fell to -80°C at night (Read and Lewis 2004).

The existence of permafrost appears in different relief forms, mainly in polygonal frost-cracking forms that are widespread in high latitudes (> 45°N, > 55°S) and cover the plains of different origin, flat hills and crater walls (Kuzmin 2005).

Morphological comparison of Martian and terrestrial polygons shows their similarities, but Martian polygons might be larger in size (up to 300 m).

The fluxes of neutrons and gamma rays affected by regolith have been measured on the Mars Odyssey mission since 2002 by two independent physical methods using the Gamma Subsystem, High Energy Neutron Detector and Neutron Spectrometer. Both methods, gamma-ray and neutron spectroscopy, indicate the presence of near-subsurface water-ice abundances on Mars (Boynton et al. 2002). According to these data, the 1-m thick surface ground really contains water-ice on any latitudes where thermodynamic parameters favor its existence

Martian average annual surface temperature is everywhere below the water triple point temperature. But due to the extremely low water vapor pressure in the atmosphere, the frost-point temperature is about -70°C. This means that within a latitude band of 40° ground ice may exist only at great depths (much below the accessibility depth for instruments currently searching for life and water on Mars) in unstable conditions.

Pole-ward from 40°, in both hemispheres the average annual surface temperatures are lower than the frost-point temperature, and stable ground ice exists under the thin dry regolith layer. The permafrost table occurs at depths from a few centimeters to 1 m. This dry layer protects the water ice either by reducing the sublimation rate of molecules (impeding the diffusion) and/or by attenuating the amplitudes of daily temperature oscillations above the permafrost table. The thickness of this armor regolith layer is determined by latitude, exposition, albedo, and thermal inertia of the dry layer, and corresponds to the depth where an equilibrium between steam pressure in the atmosphere and steam pressure above ice exists at the specific temperature. Such layering structure and surface distribution of water ice permafrost on Mars was predicted theoretically (Schorghofer and Aharonson 2006), and is empirically proven according to combined analysis of HEND/Odyssey and MOLA/MGS data (Mitrofanov et al. 2007).

The depth of the permafrost bottom is still not known. Estimations based on the solution of the equation on thermal conductivity with known boundary data (temperature on permafrost table) and an unknown value of heat flow from below (geological activity of Mars is lower than that of the Earth, so the value of Martian heat flow is assumed about 1/2.5 of terrestrial heat flow), indicate a thickness of permafrost of ~2 km on the equator and of 6 km on the poles (Clifford 1993). These estimations have a high degree of uncertainty due to the unknown value of heat flow from below. Mellon and Phillips (2001) calculated that the Martian subsurface temperature reaches 0°C at a depth between 150 m and 8 km, depending on soil thermal conductivity. According to empiric data from the MARSIS instrument aboard the ESA spacecraft Mars Express, the frozen sediments surrounding polar caps stretch to depths of at least 1.8 km in the north and 3.7 km in the south (Picardi et al. 2005).

From terrestrial experience, permafrost is underlain by groundwater, as a rule under pressure. This water lifts to the surface along the borehole to an elevation depending on the pressure value. The thermal groundwater decrements to the surface take place along the old faults, even in tectonically stable Arctic lowlands. For example, an outcrop of 20°C water was observed on Cape Chukochii (eastern Arctic lowlands) across the continuous permafrost (mean annual temperature -11°C) throughout the area, to depths of 600-800m.

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