Sharp views show ground ice on Mars is patchy and variable
Thu May 3, 2007 at 18:50 UTC
Using observations by NASA's Mars Odyssey orbiter, scientists have discovered that water ice lies at variable depths over small-scale patches on Mars.
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. 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. The smallest patches detectable by those instruments are several hundred times larger than details detectable by the new method of mapping depth-to-ice, which sees differences over scales of a few hundred yards or meters.
The new approach uses thermal imaging as a thermometer to measure how fast the ground changes temperature during local spring, summer and fall. 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.
The resulting maps show that the nature of the surface soil makes a difference in how close to the surface the ice lies. Areas with many rocks at the surface, Bandfield explained, "pump a lot of heat into the ground and increase the depth where you'll find stable ice." In contrast, dusty areas tend to insulate the ice, allowing it to survive closer to the surface. "These two surface materials -- rock and dust -- vary widely across the ground, giving underground ice a patchy distribution," he said.
Image Credit: NASA/JPL/ASU
High resolution image
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.
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.
This map and its interpretation are in a May 3, 2007, report in the journal Nature by Joshua Bandfield of Arizona State University, Tempe. The Thermal Emission Imaging System camera on NASA's Mars Odyssey orbiter collected the data presented in the map. The site is centered near 67.5 degrees north latitude, 132 degrees east longitude, in the Martian arctic plains called Vastitas Borealis. It was formerly a candidate landing site for NASA's Phoenix Mars Lander mission. This site is within the portion of the planet where, in 2002, the Gamma Ray Spectrometer suite of instruments on Mars Odyssey found evidence for water ice lying just below the surface. The information from the Gamma Ray Spectrometer is averaged over patches of ground hundreds of kilometers or miles wide. The information from the Thermal Emission Imaging System allows more than 100-fold higher resolution in mapping variations in the depth to ice.
The Thermal Emission Imaging System observed the site in infrared wavelengths during night time, providing surface-temperature information, once on March 13, 2005, during summer in Mars' northern hemisphere, and again on April 8, 2005, during autumn there. The colors on this map signify relative differences in how much the surface temperature changed between those two observations. Blue indicates the locations with the least change. Red indicates areas with most change. Modeling provides estimates that the range of temperature changes shown in this map corresponds to a range in depth-to-ice of 5 centimeters (2 inches) to more than 18 centimeters (more than 7 inches). The sensitivity of this method for estimating the depth is not good for depths greater than about 20 centimeters (8 inches).
The temperature-change data are overlaid on a mosaic of black-and-white, daytime images taken in visible-light wavelengths by the same camera, providing information about shapes in the landscape. The 10-kilometer scale bar is 6.2 miles long.
Image Credit: NASA/JPL/ASU
High resolution image
The depth to the top of the icy layer estimated from these observations suggests that in some areas, but not others, water is being exchanged by diffusion between atmospheric water vapor and subsurface water ice. Differences in what type of material lies above the ice appear to affect the depth to the ice. The area in this image with the greatest seasonal change in surface temperature corresponds to an area of sand dunes.
The site is centered near 67 degrees south latitude, 36.5 degrees east longitude, near a plain named Melea Planum. This site is within the portion of the planet where, in 2002, the Gamma Ray Spectrometer suite of instruments on Mars Odyssey found evidence for water ice lying just below the surface.
The Thermal Emission Imaging System observed the site in infrared wavelengths during night time, providing surface-temperature information. It did so once on Dec. 27, 2005, during late summer in Mars' southern hemisphere, and again on Jan. 22, 2006, the first day of autumn there. The colors on this map signify relative differences in how much the surface temperature changed between those two observations. Blue indicates the locations with the least change. Red indicates areas with most change. Modeling provides estimates that the range of temperature changes shown in this map corresponds to a range in depth-to-ice of less than 1 centimeter (0.4 inch) to more than 19 centimeters (more than 7.5 inches). The sensitivity of this method for estimating the depth is not good for depths greater than about 20 centimeters (8 inches).
The temperature-change data are overlaid on a mosaic of black-and-white, daytime images taken in infrared wavelengths by the same camera, providing information about shapes in the landscape. The 20-kilometer scale bar is 12.4 miles long.
Computer models helped him interpret the temperature observations, he said. "They show areas where water ice would be only an inch or so under the soil, while in other areas ice could lie many feet below the surface."
The results fit long-term climatic models for Mars. These show the planet has been both warmer and colder in the past, similar to glacial cycles on Earth. Bandfield said, "The fact that ice is present near the depth of stability in the current Martian climate shows that the ground ice is responding to climate cycles." In turn, he added, this implies that water ice in the ground can swap places with water vapor in the atmosphere as the climate changes.
Philip Christensen of Arizona State University, Tempe, principal investigator for the Thermal Emission Imaging System, said, "Scientists have known for more than a decade that water is on Mars, mostly in the form of ice. What's exciting is finding out where the ice is in detail and how it got there. We've reached the next level of sophistication in our questions."
Jet Propulsion Laboratory News Release
