CO2 jets formed by sublimation beneath translucent slab ice in Mars' seasonal south polar ice cap

A levitating ice cap Two papers in this issue discuss the remarkable seasonal changes seen at the martian south pole. The ice caps on Mars are composed mainly of frozen carbon dioxide, with unusual dark spots that have been attributed to a clear slab of nearly pure CO 2 ice. Langevin et al . present infrared and visible images from the THEMIS camera on Mars Odyssey that are not consistent with the presence of a thick slab of transparent ice. In the companion paper Kieffer et al . report infrared and visible data that show that the features stay at CO 2 ice temperatures well into summer, and must be granular materials brought up to the surface of the ice. They propose a model involving a translucent, impermeable CO 2 cap in which sublimation occurs at the base, producing high-velocity CO 2 gas flow beneath the ice, levitation of the cap and jets that erupt sand-sized grains through vents. Infrared and visible observations show that features on the martian south polar cap remain at CO 2 ice temperatures well into summer, and must be granular materials that have been brought up to the surface of the ice. The martian polar caps are among the most dynamic regions on Mars, growing substantially in winter as a significant fraction of the atmosphere freezes out in the form of CO 2 ice. Unusual dark spots, fans and blotches form as the south-polar seasonal CO 2 ice cap retreats during spring and summer. Small radial channel networks are often associated with the location of spots once the ice disappears. The spots have been proposed to be simply bare, defrosted ground 1 , 2 , 3 ; the formation of the channels has remained uncertain. Here we report infrared and visible observations that show that the spots and fans remain at CO 2 ice temperatures well into summer, and must be granular materials that have been brought up to the surface of the ice, requiring a complex suite of processes to get them there. We propose that the seasonal ice cap forms an impermeable, translucent slab of CO 2 ice that sublimates from the base, building up high-pressure gas beneath the slab. This gas levitates the ice, which eventually ruptures, producing high-velocity CO 2 vents that erupt sand-sized grains in jets to form the spots and erode the channels. These processes are unlike any observed on Earth.