A heat-sensitive camera 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.

Very dark, nearly circular features ranging in diameter from about 100 to 250 meters (328 to 820 feet) puzzled researchers who found them in images taken by NASA's Mars Odyssey and Mars Global Surveyor orbiters. Using Mars Odyssey's infrared camera to check the daytime and nighttime temperatures of the circles, scientists concluded that they could be windows into underground spaces.

Seven very dark holes on the north slope of a Martian volcano have been proposed as possible cave skylights, based on day-night temperature patterns suggesting they are openings to subsurface spaces. These six excerpts of images taken in visible-wavelength light by the Thermal Emission Imaging System camera on NASA's Mars Odyssey orbiter show the seven openings. Solar illumination comes from the left in each frame. The volcano is Arsia Mons, at 9 degrees south latitude, 239 degrees east longitude.

The features have been given informal names to aid comparative discussion. They range in diameter from about 100 meters (328 feet) to about 225 meters (738 feet). The candidate cave skylights are (A) "Dena," (B) "Chloe," (C) "Wendy," (D) "Annie," (E) "Abby" (left) and "Nikki," and (F) "Jeanne." Arrows signify north and the direction of illumination. Image credit: NASA/JPL-Caltech/ASU/USGS

Philip Christensen, a Regents Professor of geological science Arizona State University, 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."