A series of newly discovered depressions on the Martian surface could be the entrances to a cave system on the red planet.
Hints of subsurface tunnels have been found in images of Mars before, but the new evidence is more suggestive, said Glen Cushing, a physicist with the U.S. Geological Survey who discovered the possible caves.
Such a subsurface system could provide shelter to future Mars-visiting astronauts, as well as a protective habitat to any potential past or present Martian microbes, Cushing said.
Cushing presented his findings recently at a meeting of the Geological Society of America.
Lava tubes and skylights
Cushing found signs of a series of "collapse depressions" in extinct lava flows from the Martian volcano Arsia Mons in high-resolution images taken by spacecraft orbiting the red planet.
The depressions appear to be a set of long grooves in the surface with distinct features that look like skylight entrances into tunnel-like structures. The grooves are more than 62 miles (100 km) long and up to 330 feet (100 meters) across; the apparent skylights look to be up to 160 to 200 feet (50 to 60 m) across.
Cushing said that the grooves likely formed when a solid ceiling of cooled material formed over a lava channel during an ancient eruption. When the eruption was over, a tunnel, or "lava tube," was left behind. Sections of the ceiling likely collapsed at some point to form the skylight entrances.
These cave-suggesting features are different than a series of seven dark spots ? dubbed "the seven sisters" ? found in 2007 and also thought to potentially be the entrances to a cave system.
"What's special about these [newfound features] is that they are closer to the surface and smaller," Cushing told SPACE.com.
Having these smaller tunnels closer to the surface would make them easier to explore and possibly use in future human missions, he added.
Shelter for life
If the depressions turn out to indeed be the entrances to cave systems, they could be both interesting for any future Mars astronauts to explore and provide a safe shelter.
"Caves can protect human explorers from a range of dangerous conditions that exist on Mars' surface," Cushing said. "If caves are not used for long-term human habitation, then explorers must either transport substantial shelters of their own or build them on site."
The caves also have the potential for holding signs of microbial life, for much the same reason that they would be good shelter for humans.
"There's numerous hazards on Mars' surface," for struggling life, including radiation, extreme temperatures and dust storms, Cushing said. "Caves really protect from all of these things."
For these same reasons, caves are more likely to preserve any evidence of past life. "Caves are probably among the only places on Mars where you can actually look and see if there's possible evidence" of past life, Cushing said.
While the rovers currently on the surface of Mars are too far away to try to get an up-close look at these possible caves, Cushing thinks they will be an obvious destination for future missions.
"Someday robot explorers will probably visit caves such as these and show us a whole new hidden world," Cushing said.
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