Ninth International Conference on Permafrost ... - IARC Research

Thermal Conditions in Martian Permafrost: Past and PresentMikhail A. KreslavskyUniversity of California – Santa CruzPresent ConditionsGlobal cryosphereThe temperature of the Martian surface has beenmonitored by several orbital thermal infrared sensors.These measurements, accompanied by careful and cautiousmodeling, give rather accurate knowledge of the thermalregime of the uppermost meters of the surface in the presentepoch.The year-average surface temperature on Mars is wellbelow 0°C everywhere on the planet, meaning a thick globalcryosphere. The day-average temperature also never exceedsthe ice melting point, meaning the absence of the active layerof Martian permafrost in the present epoch.Observations with orbital gamma-ray and neutron sensorssensitive to the presence of hydrogen in the uppermost meterof the surface have indicated that high latitudes (above ~60° in both hemispheres) contain much hydrogen, whichobviously means the presence of water ice. This ice isabundant, and its amount in the soil noticeably exceeds 50%by volume. At mid and low latitudes, ice in the upper meter isless abundant or absent. The hydrogen content in equatorialregions is spatially variable and in some regions exceeds 10wt% water-equivalent by weight. Deeper in the ground, icemay be present virtually everywhere.Calculations of the ground ice stability against diffusionof water vapor to the atmosphere (e.g., Mellon & Jakosky1995, Schorghofer & Aharonson 2005) show that the groundice is stable at high latitudes and unstable at low latitudes;in addition to latitude, surface albedo and thermal propertiesinfluence stability.Patterned groundHigh-resolution images reveal a great variety of polygonalpatterns (e.g., Mangold et al. 2004), totally coveringmore than one-quarter of the planet (predominantly athigh latitudes). The extent of massive polygonal patternoccurrence somewhat exceeds the limits of the observedhigh hydrogen content. Polygonal patterns are often forminghierarchical systems of different scales (Fig. 1). Thesepatterns were probably initiated by thermal cracking of icerichpermafrost. On Earth, thermal cracking of permafrostusually leads to formation of ice-wedge polygons due toseasonal thaw of the active layer. On Mars, seasonal thawdoes not occur, and the cracks evolve into sand-wedgepolygons and/or sublimation polygons, features, observed inextremely cold and dry terrestrial environments (Marchantet al. 2002).Thermal cracking of ice-rich frozen soils occurs due to ananomalously high bulk thermal expansion coefficient of theice-soil mixtures. However, the thermal expansion coefficientdecreases with the temperature decrease. Very high seasonalFigure 1. Two scales of polygonal pattern on Mars. HiRISE imagePSP_001404_2490, 69°N, 106°W.temperature amplitude at high latitudes on Mars favorsthermal cracking, while generally low temperatures are notfavorable.The absence of polygons in the low-latitude hydrogen-richregions has been interpreted as evidence for the absence ofwater ice in the soil (the hydrogen being bound in hydratedminerals). However, in these regions, the seasonal temperatureamplitudes are modest (typically, 20–30 K;