June 11, 2002
Scientists last week announced the discovery of large quantities of subsurface water ice on Mars based upon gamma ray spectroscopy measurements which detect the presence of hydrogen in the soil to a depth of one meter. The big surprise has been how much water there is - some soils in the southern high latitudes appear to contain as much as 50 percent water ice by volume! Scientists have long suspected that water was once plentiful on the surface of Mars. The image of Mars above is of the floor of a crater at 72 degrees south latitude - well within the region where Mars Odyssey has mapped the presence of buried ice. The distinctive polygonal pattern, known as patterened ground, is suggestive of the work of periglacial processes where cold climate and possibly ice contributes to the evolution of landforms and landscapes. The lower image is an aerial photo taken by Sharon Johnson of patterned ground observed in the Northwest Territories of Canada between Inuvik and Tuktoyaktuk with a striking resemblance to the Martian features. On Earth, patterned ground polygons typically form from stresses induced by repeated freezing and thawing of water, contraction from stress induced by changing temperatures, and sorting of rocks brought to the surface along polygon boundaries by the freeze-thaw processes. Although not exclusively formed by freezing and thawing of water, that is often the dominant mechanism on Earth, and appears to be a likely candidate for the cause of the Martian polygons.