By Bill O’Neill
A recent issue of Science (April 11, 234-237) reports a conference in Houston about the latest evidence from Mars-orbiting spacecraft on the history of water on the red planet. As we noted last summer (BPAA Newsletter July, 2002), instruments on Mars Odyssey detected water near the surface in both hemispheres. The latest measurements appear to confirm that a lot of ice is buried beneath a few centimeters of dry soil from the Martian poles down to about 60 degrees latitude. In the high southern hemisphere, ice is now reported to constitute 4073% of the soil (by volume) averaged over hundreds of kilometers. (Terrestrial permafrost contains up to 30% ice.)
The data from Odyssey’s Gamma Ray Spectrometer suite coincide nicely with photos taken by the Mars Global Surveyor, presented at the Houston “Microsymposium.” Pole-ward of about 60 degrees (the terrestrial latitude of Greenland’s southern tip), wherever the Mars Orbiter Camera scanned, the surface looks “as if meters of heavy snow lie upon the land. But when viewed at highest resolution, this smooth mantling… [shows] a pattern of closely spaced, meter-scale knobs that give a stippled or ‘basketball’ look to the surface.” The texturing of this high latitude mantling could result from partial loss of the shallowest ice. Permafrost on earth can get similarly lumpy with warming. In the mid-latitudes between 30 and 60 degrees there appear to be scraps of this thin mantling scattered across the Mars-scape.
In the latest photo analysis, Brown University researchers find a progressive disruption as they approach the equator. “In places, the mantling (appears) partially stripped way, revealing multiple layers that total a few to ten meters in thickness. This dissected mantling is most abundant at about 40 degrees latitude.”
The photographic evidence is supported by topographic data from Global Surveyor’s Laser Altimeter, which “shows high-latitude terrain, smooth at a scale of tens to hundreds of meters, extending to 60 degrees, where it roughens…. topography and imagery both give the impression of a thin layer or layers of dirty ice that were once continuous above 30 degrees, but now look different because warming has driven out the ice up to a latitude of 60 degrees.”
Inspection of 13,000 Images from the Mars Orbiter Camera provides many examples of “viscous flow features,” resembling the effects of rock-laden glacier flows on Earth. The evidence of viscous flows is greatest at 40 degrees, where the “dissected mantling” is most abundant. “Furthermore, the mysterious gullies – where liquid water seems to have flowed down steep slopes in the recent past – follow the same latitudinal distribution…they even tend to cluster in the same three or four places as the viscous flow features do.” Lots of controversy remains about icy Mars, but the picture seems to be getting clearer.