Mars has polar ice caps at both poles. The south polar residual cap - the portion that persists through the Martian summer - is composed primarily of carbon dioxide ice, or dry ice, layered over a much larger deposit of water ice. It is a structure unlike anything on Earth, and its surface bears the marks of a seasonal process that has no terrestrial equivalent at the same scale.
How the cap forms and changes
Each Martian winter, carbon dioxide from the atmosphere freezes directly onto the south polar cap, adding a new layer of dry ice. Each Martian summer, this layer sublimes - converts directly from solid to gas - and returns to the atmosphere. The net result, over thousands of Martian years, is a layered deposit recording the history of Martian climate changes, analogous to ice cores on Earth.
The sublimation process is not uniform. Variations in the reflectivity of the surface cause some areas to sublime faster than others, producing the characteristic pits, ridges and curvilinear troughs visible in HiRISE images. These features have been called "Swiss cheese terrain" for their appearance - roughly circular pits with flat floors and steep walls, separated by higher ground.
What HiRISE reveals
The HiRISE image of the south polar cap shows the sublimation terrain at 25 centimetres per pixel. The pits and troughs are clearly resolved, and the edges between the flat-floored pits and the higher ground between them are sharp. The overall pattern is simultaneously geometric - the circular pits have a regularity that suggests a single underlying process - and complex, because that process operates at many scales simultaneously.
The temperature at the south polar cap surface during summer is approximately -125°C - the sublimation temperature of carbon dioxide at Martian atmospheric pressure.
The Axisophy print
The Southern Polar Ice Cap print from the Radiance collection shows the sublimation terrain in monochrome. The contrast between pit floors and the raised ground between them creates a strongly textured surface that rewards close examination.
