Vast lava tubes beneath the surface of Mars and the Moon could be large enough to host bases for visiting astronauts, scientists have said.
New research examined the similar tubes that can be found on Earth, and used them to estimate the size of similar – but much bigger – tunnels that are thought to exist underneath the surface of other worlds.
Living inside such tubes, beneath the surface, could theoretically provide some protection for the astronauts who are expected to visit other parts of the solar system in coming years.
On the Moon, the tubes are thought to be 1,000 times wider than those on Earth, where they already can be span around 30 meters across. The extra size is explained in large part by the lower gravity found on those worlds.
Despite their vast size, the tubes are still thought to be stable enough to serve as a home. The lower gravity also means that the forces that might collapse the tubes on Earth would not pull them down on the Moon or Mars, scientists said.
As part of the research, scientists from the European Space Agency as well as the universities of Bologna and Padua explored examples of such caves on Earth, where they can be found in Hawaii, the Canary Islands, Australia and Iceland.
They also measured the size of collapsed tubes on the Moon and Mars, using images and other data taken from visiting probes. They then compared that data with information about collapsed chains on the Earth's surface, allowing them to understand the relationship between those tubes that collapse and others that stay stable.
They found that the conditions on other worlds would allow them to grow much bigger before they fall in.
"Tubes as wide as these can be longer than 40 kilometres, making the Moon an extraordinary target for subsurface exploration and potential settlement in the wide protected and stable environments of lava tubes," said Riccardo Pozzobon, one of the researchers on the paper, and a scientist at the University of Padua. "The latter are so big they can contain Padua's entire city centre".
Such tubes on other worlds could form a key part of plans to go and stay on the Moon and Mars, where conditions would be much more dangerous than they are on Earth.
"Lava tubes could provide stable shields from cosmic and solar radiation and micrometeorite impacts which are often happening on the surfaces of planetary bodies," said Francesco Sauro, on of the authors of the new paper, who works on European Space Agency programmes.
"Moreover, they have great potential for providing an environment in which temperatures do not vary from day- to night-time. Space agencies are now interested in planetary caves and lava tubes, as they represent a first step towards future explorations of the lunar surface (see also NASA's project Artemis) and towards finding life (past or present) in Mars subsurface".
A paper discussing the findings, titled 'Lava tubes on Earth, Moon and Mars: A review on their size and morphology revealed by comparative planetology', is published in Earth-Science Reviews.