Seven circular pits on the surface of Mars appear to be openings to underground caverns, researchers have announced.
The discovery of potential caves is exciting, the scientists said, because such underground formations may be the most promising places to look for signs of life.
Researchers were able to peer into the openings from far above, using visual and infrared imaging instruments aboard the Mars orbiter Odyssey.
No bottom is visible in six of the chambers. In the seventh, a section of cave floor illuminated by direct sunlight suggests a minimum depth of about 425 feet (130 meters).
Thermal scans helped establish that the holes are probably "skylight" openings to an underground cave system. Each skylight is 330 to 820 feet (100 to 250 meters) across.
A research team presented the discovery at a meeting of the Lunar and Planetary Institute last week in Houston.
Phil Christensen, of Arizona State University in Tempe, heads the thermal imaging project on Odyssey.
He noted that temperatures at the openings remained more constant than at surrounding areas exposed to Mars' bitter nighttime temperatures.
"These pits stay relatively warm at night," Christensen said. "That suggests we're looking down into a cavern that is trapping daytime heat."
To an observer on the Martian surface, he added, "it would be a pretty spectacular view. You could stand on the edge and look in, but I'm not sure you could see the bottom."
Pits and Tubes
The openings are scattered across several hundred kilometers on the side of Mars' second highest mountain, known as Arsia Mons, near Valles Marineris (see map of Mars).
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