The following are from today's Nature Contents,
Al.
Editor's Summary
17 August 2006
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 CO2 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 CO2 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 CO2 cap in which sublimation occurs at the base, producing high-velocity CO2 gas flow beneath the ice, levitation of the cap and jets that erupt sand-sized grains through vents.
Letter
Nature 442, 790-792(17 August 2006) | doi:10.1038/nature05012; Received 17 February 2006; Accepted 13 June 2006
No signature of clear CO2 ice from the 'cryptic' regions in Mars' south seasonal polar cap
Yves Langevin
1, Sylvain Douté
2, Mathieu Vincendon
1, François Poulet
1, Jean-Pierre Bibring
1, Brigitte Gondet
1, Bernard Schmitt
2 and F. Forget
3
Top of page The seasonal polar ice caps of Mars are composed mainly of CO2 ice1,2. A region of low (< 30%) albedo has been observed within the south seasonal cap during early to mid-spring3,4. The low temperature of this 'cryptic region' has been attributed to a clear slab of nearly pure CO2 ice, with the low albedo resulting from absorption by the underlying surface4. Here we report near-infrared imaging spectroscopy of the south seasonal cap. The deep and broad CO2 absorption bands that are expected in the near-infrared with a thick transparent slab of CO2 ice are not observed. Models of the observed spectra indicate that the low albedo results from extensive dust contamination close to the surface of a CO2 ice layer, which could be linked to atmospheric circulation patterns5,6. The strength of the CO2 absorption increases after mid-spring, so part of the dust is either carried away or buried more deeply in the ice layer during the CO2 ice sublimation process.
Letter
Nature 442, 793-796(17 August 2006) | doi:10.1038/nature04945; Received 4 April 2006; Accepted 30 May 2006
CO2 jets formed by sublimation beneath translucent slab ice in Mars' seasonal south polar ice cap
Hugh H. Kieffer
1,
2, Philip R. Christensen
3 and Timothy N. Titus
2
Top of page 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 CO2 ice. Unusual dark spots, fans and blotches form as the south-polar seasonal CO2 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 ground1,2,3; the formation of the channels has remained uncertain. Here we report infrared and visible observations that show that the spots and fans remain at CO2 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 CO2 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 CO2 vents that erupt sand-sized grains in jets to form the spots and erode the channels. These processes are unlike any observed on Earth.
