Eolian intracrater deposits on Mars: Physical properties and global distribution

An investigation of Martian intracrater materials has been made using their thermophysical properties as derived from Viking IRTM observations. Over one-fourth of all craters larger than 25 km in diameter between -50°S and 50°N have localized deposits of coarse material on the floor which are associated with dark "splotches" seen visually. Assuming homogeneous, unconsolidated materials, the measured thermal inertias of these deposits (I = 0.003 × 10^-3 to 0.026 × 10^-3 cal cm^-2 sec^{- 1 2}°K^-1) imply effective grain sizes ranging from 0.1 mm to 1 cm, with a modal value of 0.9 mm. These deposits are coarser and darker than the surrounding terrains and the majority of the Martian surface, but are not compositionally distinct from materials with similar albedos. They occur more frequently in the south, in regions of relatively coarse material (0.2 to 2 mm), and in relatively dark areas. These features most likely formed by entrapment of marginally mobile material which can be transported into, but not out of, crater depressions by the wind. Very few have recognizable dune forms: those that do have effective grain sizes less than 0.5 mm. The majority of the "splotch" deposits are coarser than the dune-forming materials found in the north polar region and inside extreme southern latitude craters and probably form low, broad zibar dunes or lag deposits. Intracrater deposits are noticeably lacking from the interior of the large, northern hemisphere low-inertia region of Arabia (-10°S to 30°N, 300° to 360°W), interpreted to be a sink for suspended dust, but do occur around the perimeter of this region. This distribution suggests that the intracrater features have been buried in the interior of Arabia and that the dust deposit is less extensive at the margins and may currently be expanding. The occurrence of regional dust deposits in the north may be related to the maximum wind activity currently occurring in the southern hemisphere and suggests that the location of regional sinks may migrate with time as the solar insolation maximum migrates.