Using observations by NASA's Mars Odyssey orbiter, (Click on image to enlarge) scientists have discovered that water ice lies at variable depths over small-scale patches on Mars, an indication that Mars has a currently active water cycle—a major discovery.
The findings draw a much more detailed picture of underground ice on Mars than was previously available. They suggest that when NASA's next Mars mission, the Phoenix Mars Lander, starts digging to icy soil on an arctic plain in 2008, it might find the depth to the ice differs in trenches just a few feet apart. Phoenix is completing launch processing at the Kennedy Space Center in preparation for its scheduled Aug. 3 launch.
The new results appear in the May 3, 2007, issue of the journal Nature.
"We find the top layer of soil has a huge effect on the water ice in the ground," said Joshua Bandfield, a research specialist at Arizona State University, Tempe, and author of the paper. His findings come from data sent back to Earth by the Thermal Emission Imaging System camera on Mars Odyssey. The instrument takes images in five visual bands and 10 heat-sensing (infrared) ones.
The new results were made using infrared images of sites on far-northern and far-southern Mars, where buried water ice within an arm's length of the surface was found five years ago by the Gamma Ray Spectrometer suite of instruments on Mars Odyssey.
Color coding in this map of a far-northern site on Mars indicates the change in nighttime ground-surface temperature between summer and fall. This site, like most of high-latitude Mars, has water ice mixed with soil near the surface. The ice is probably in a rock-hard frozen layer beneath a few centimeters or inches of looser, dry soil. The amount of temperature change at the surface likely corresponds to how close to the surface the icy material lies. Phoenix will land in the north polar regon at about 70 degrees north latitude.
The dense, icy layer retains heat better than the looser soil above it, so where the icy layer is closer to the surface, the surface temperature changes more slowly than where the icy layer is buried deeper. On the map, areas of the surface that cooled more slowly between summer and autumn (interpreted as having the ice closer to the surface) are coded blue and green. Areas that cooled more quickly (interpreted as having more distance to the ice) are coded red and yellow.
The depth to the top of the icy layer estimated from these observations, as little as 5 centimeters (2 inches), matches modeling of where it would be if Mars has an active cycle of water being exchanged by diffusion between atmospheric water vapor and subsurface water ice.
--Craig Covault
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