Moon pits could be portals to 'kilometres' of underground tunnels

Hundreds of pits discovered on the Moon could be portals to underground tunnels formed by lava flows, a new paper published in the journal Icarus has suggested.

Ranging from between five to 900 metres in diameter, the pits could offer shelter to explorers in future and also help us peek beneath the lunar surface.

Detected using a computer algorithm that analysed the way shadows fall across the Moon's surface in photos from Nasa's Lunar Reconnaissance Orbiter. Just 40 percent of the Moon has been photographed with the correct level of lighting for the algorithm to work, meaning there could be hundreds more yet undiscovered.

"A habitat placed in a pit would provide a very safe location for astronauts: no radiation, no micrometeorites, possibly very little dust, and no wild day-night temperature swings," said the paper's lead author, Robert Wagner of Arizona State University.

On Earth, underground lava flows sometimes leave behind extensive tunnel networks that can become accessible if a section of the roof caves in. Similar processes on the Moon could have caused by what are called "impact melt ponds".

In essence, the energy from a meteoroid impact can heat and melt rock, which then takes thousands of years to cool. Before it cools, the molten rock can flow, creating features similar to those created by volcanoes on Earth.

One feature could be underground tunnels stretching for kilometres, the paper suggests.

"Pits represent evidence of subsurface voids of unknown extents. By analogy with terrestrial counterparts, the voids associated with mare pits may extend for hundreds of metres to kilometres in length, thereby providing extensive potential habitats and access to subsurface geology," it reads.

The majority of the 231 pits are found in impact craters, but a small number were found in the "maria", which are the Moon's large dark patches.

The maria, once thought to be the Moon's seas, were created by large ancient lava flows that occurred before the Moon cooled, rather than by meteorite impacts.

Pits found in the maria could help us better understand how the maria formed, said Wagner: "We've taken images from orbit looking at the walls of these pits, which show that they cut through dozens of layers, confirming that the maria formed from lots of thin flows, rather than a few big ones. Ground-level exploration could determine the ages of these layers, and might even find solar wind particles that were trapped in the lunar surface billions of years ago."