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Ubiquitous etch pits occur in cobbles of nonvesicular dolerite (basalt) from many ice-free areas of Victoria Land, Antarctica. The pits (millimeters to several centimeters in diameter) appear to be formed by dissolution of the rock by rare snow meltwater during the austral summer. Wind erosion removes weathered material from the pits. A yellow precipitate lines the upper interior walls of the pits and a very thin zone of active weathering underlies the pit bottoms. The precipitate consists of illite and quartz in an intimate mixture. The pit bottoms contain alteration products including Fe-rich clay minerals and soluble salts. A model for rock pitting on Mars analogous to that described above for Antarctic dolerites is proposed. Water frost or snow deposited on rock surfaces during the winter, as observed at the Viking 2 lander site, occasionally melts, wetting the rocks for short periods of time. During these brief wetting episodes chemical weathering takes place in an extremely thin near-surface zone. Pit formation is initiated at sites of greatest weathering activity. Following evaporation of the water, salts and Fe-rich clay minerals crystallize in the pits. Illite crystallizes in adherent layers on the pit walls, segregating Al and K from the salts and clays. Subsequent wind erosion scours the pits, removing the salts and clays more efficiently than the illite-rich coatings. The results are Fe-rich aeolian fines with lower Al and K contents than their source rocks.