Credit: NASA/JPL/Arizona State University
A week and a half ago (before my end-of-term workload crashed down on me), we were discussing THEMIS's cababilities to observe Mars in both visible light and infrared radiation. The previous post discussed THEMIS's visible light camera; this post and the next will focus on its infrared camera.
By making repeated observations on the same sites over a period of time, THEMIS is able to measure the ground temperatures and determine the depth (roughly) of ice below the surface, from shallow (less than one centimeter) to deep (up to 20 centimeters). The sensitivity of this method for estimating the depth is not good for depths greater than about 20 centimeters.
In the above photo, THEMIS observed the above site (67° South, 36.5° East, near Melea Planum) in infrared wavelengths during night time, providing surface-temperature information. It did so once on December 27, 2005, during late summer in Mars' southern hemisphere, and again on January 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, while red indicates areas with the most change. 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 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 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 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.