Some caves are situated high on the massive volcano, where the atmosphere is thin even by Martian standards.
Others lie on the mountain's lower flanks, where conditions may be more hospitable for life.
The surface of Mars is strewn with craters from meteor impacts and depressions formed by the collapse of underground chambers formed by flowing lava, the experts said.
Similar "collapse pits" and lava tubes with skylight openings are found in volcanic cave systems on Earth, in places such as Hawaii, the team pointed out.
The Martian caves may be similar in structure to Hawaii's lava tubes, Christensen said, albeit on a larger scale.
Smaller skylights may also be present, he added. Odyssey's thermal imager can only detect openings larger than about 330 feet (100 meters) across.
New details may come soon from additional imaging, the researchers said, as well as from the use of the high-resolution camera aboard the Mars Reconnaissance Orbiter.
Shelter From the Storm
In addition to insulation from cold temperatures, Mars' caverns may provide shelter from the barrages of dust, ultraviolet radiation, charged particles, and small meteorites that whip the planet's surface.
Some researchers have suggested that Martian caverns in low-lying areas could hold reservoirs of water, which would make the existence of microbial life much more likely.
(Read related story: "Mars Has Liquid Water, New Photos Suggest" [December 6, 2006].)
At the elevations where the caves are located, the presence of water or ice is doubtful, the Odyssey scientists said. But there remains an intriguing possibility of ongoing volcanic and hydrothermal activity in the region.
Dirk Schulze-Makuch of Washington State University headed a recent study of potential Martian hydrothermal sites including Arsia Mons.
"If there is still volcanic activity at or near the [cave] sites, the chances for life are much higher," he said.
"Hydrothermal water and associated nutrient-containing compounds could be released periodically and sustain life."
Whatever secrets the newfound caves may hold, they are likely to remain mysterious for some time. The caves' location makes them difficult if not impossible to reach with robotic rovers, the scientists said.
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