North polar dust storms in early spring on Mars

Observations of Mars during 1996-1997 by the Hubble Space Telescope covered the entire spring season in the northern hemisphere of Mars, which was fav orably positioned for viewing the north polar region. This paper reports ob servations of dust activity during this period. Local dust activity occurre d near the edge of the subliming north polar cap during early spring; this included local dust clouds, similar to those seen by Viking near the edge o f the south polar cap, and unusual, are-shaped dust features that seem to r epresent a new type of dust phenomenon. These dust arcs may explain the Rim a Tenuis rift, which has been occasionally seen by ground-based observers o f Mars. Analysis of numerical simulations from a Mars general circulation m odel for the early northern spring season have provided insights into the p lausible meteorological background at high latitudes in the vicinity of the retreating north polar cap. These Hubble observations extend the geographi c and seasonal ranges during which local dust activity seems to be common o n Mars. The combination of image analysis and model calculations support th e hypothesis that these dust storms are the result of winds associated with fronts moving across the seasonal cap that lift dust exposed on the cap du ring CO2 sublimation. (C) 1999 Academic Press.