Lava tubes on the Moon, if they exist, might be big enough to contain permanent bases or settlements safely under the lunar surface, researchers say.

Lava tubes, common on Earth, are tunnels created when rivers of lava from volcanic eruptions cool around their edges, creating a pipe-like hard crust around the flowing molten rock.

As the eruption ceases, the hardened pipes empty, leaving behind hollow tunnels.

If such features exist under the lunar surface, they could be structurally stable enough and large enough to house entire cities, the researchers suggest.

"There has been some discussion of whether lava tubes might exist on the Moon," says Jay Melosh, a Purdue University professor of earth, atmospheric and planetary science. "Some evidence, like the sinuous rilles observed on the surface, suggest that if lunar lava tubes exist they might be really big."

Sinuous rilles are channels visible on the Moon's surface scientists believe were formed by flowing lava. They can be as wide as 6 miles across, and Purdue researchers are looking for evidence to see if lava tubes at the same scale might exist — and whether they would be structurally stable.

"We found that if lunar lava tubes existed with a strong arched shape like those on Earth, they would be stable at sizes up to 5,000 meters, or several miles wide, on the Moon," says study leader David Blair, a Purdue graduate student.

While tubes of such size aren't possible on Earth, they could exist on the Moon given its lower gravity and the fact that lunar rock isn't subject to the erosion and weathering common on Earth, the researchers said in a presentation of their findings at the Lunar and Planetary Science Conference in Texas.

"In theory, huge lava tubes — big enough to easily house a city — could be structurally sound on the moon," Blair says.

An underground location could offer protection against challenges presented by the lunar environment.

The Moon does not have the thick atmosphere and magnetic field that protects the Earth from the dangerous effects of cosmic radiation, the researchers point out, and the lunar surface is subject to more extreme temperatures, with variations of hundreds of degrees during a lunar day.

Locating a base or city in a subterranean lava tube would protect it from such hazards, the researchers suggest.

While no definitive evidence of the existence of lunar lava tubes has yet been found, spacecraft orbiting the moon have detected cave entrances.

These entrances, knows as skylights, could be signs of a lava tube underneath, the researchers say.

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