TEMPE, Ariz. - A heat-sensitive camera designed at Arizona State University and flying on NASA's Mars Odyssey orbiter has led a team of Mars geologists to find seven small, deep holes on the flanks of Arsia Mons, a giant volcano on Mars. The holes may be openings, called skylights, in the ceilings of underground caves. The discovery is announced in a scientific paper published recently in Geophysical Research Letters.
The team of scientists includes Philip Christensen of ASU, plus Glen Cushing and Tim Titus of the U.S. Geological Survey in Flagstaff, and Judson Wynne of Northern Arizona University. Cushing is the lead author on the paper.
Christensen, a Regents Professor of geological science in ASU's School of Earth and Space Exploration, designed the Thermal Emission Imaging System (THEMIS), the instrument the team used to make the discovery. THEMIS has been photographing the Red Planet at five visual and 10 infrared wavelengths since February 2002.
Says Christensen, "THEMIS is the only heat-sensing imager currently orbiting Mars." Temperature data was the key in spotting the potential cave skylights, he notes.
The features the team found are dark, nearly circular holes in the ground with diameters ranging from 100 to 250 meters (yards). The holes appear in images of Arsia Mons taken by Mars Odyssey and Mars Global Surveyor orbiters. Located in the volcanic region of Tharsis, Arsia is one of the larger volcanoes on Mars, and like the rest of Tharsis, it has a heavy coating of dust.
"We examined the flanks of the volcano in nighttime infrared images, looking for temperature anomalies -- warm spots," explains Christensen. "Then when we re-examined the locations in daytime images, we saw the small, deep holes in the ground."
Dusty surfaces, he says, become hot during the day, both on Earth and Mars. But at night, dust and sand give up heat quickly, becoming very cold shortly before sunrise. The holes, however, changed temperature by only two-thirds as much as the surface.
Says Christensen, "We saw that we had dark holes that are warm at night, but cool by day. The best way to explain that is to have a deep hole with vertical walls, so you're looking at a rocky surface free from sand and dust."
The team suggests that the deep holes on Arsia Mons probably formed as faults created stresses that opened spaces underground. Some of the holes are in line with strings of bowl-shaped pits where the surface has collapsed.
The observations have been discussed at meetings with other Mars scientists earlier this year, and they have prompted researchers using Mars Odyssey and NASA's newer Mars Reconnaissance Orbiter to search for other openings to underground spaces.
Christensen adds, "The temperature data is what really separated these unique holes from millions of run-of-the-mill craters, volcanic vents, and collapse pits."