The Mars Odyssey mission may not be the latest or most glamorous Martian explorer, but it’s the longest-running, and it does boast an impressive thermal imaging system. Orbiting the planet as it does, Odyssey’s scientific packages continue to provide a very rich picture of the fourth planet’s aerology (see the figure).
Last September 30, Odyssey was directed to alter its orbit to gain even better sensitivity for its infrared mineral mapping of Martian minerals. The adjustment will allow THEMIS to look down at sites in mid-afternoon, rather than late afternoon, collecting infrared radiation when the rocks are warmer. Previously, its orbit was a compromise between THEMIS and the mission’s Gamma Ray Spectrometer. Part of the Gamma Ray Spectrometer is being turned off. In addition to the increase in time, THEMIS will now occasionally be aimed obliquely, rather than straight down, allowing the team to do some 3D imaging.
A set of composite photos from the Mars Odyssey THEMIS cameras shows how IR is being used to study the Red Planet. Figure A shows two infrared frames of the same area side by side. The black and white image represents a single wavelength. The false color version represents three different bands. It was created with a technique called Decorrelation Stretch (DCS), in which individual bands are stretched and overlaid in different false colors. In this case, the magenta colors usually represent basaltic content, cyan often indicates the presence of water ice clouds, and green may represent dust.
It’s interesting to compare this image with shots of the same area taken by the Viking mission in the 1970s. The Viking images are more suggestive of dust, and one interpretation of these images is that there’s a thin veneer of dust above which basaltic material emerges along the edges of the valley and the nearby knobby terrain. The wind streak in the lee of the small crater is also likely to represent basalt. Part of composite (b) superimposes simultaneous THEMIS infrared and visible images. It shows an area approximately 32 km wide near the south polar cap in late summer.
“The black areas in the infrared image are near –125°C and correspond to solid carbon-dioxide ice,” says NASA. “The purple regions are areas of exposed water ice at a temperature near –95°C. The warmest (red) areas are classic ‘dark lanes’ of frost-free soil at a temperature near –55°C.” The right panel shows the same infrared image with a THEMIS visible image superimposed. Sweeping an area of the Martian surface measuring 207 by 335 km, a large monochromatic mosaic (c) of infrared images is centered on Arsia Mons, the southernmost of the Tharsis volcanoes. It’s almost 12 miles high, nearly twice the height of Hawaii’s Mauna Loa, and the summit caldera is 72 miles wide.
NASA’s commentary notes that “The indentations on the SW and NE sides align with the Pavonis Mons and Ascreaus Mons to the NE. This may indicate a large fracture/vent system was responsible for the eruptions that formed all three volcanoes.”