Natural tunnels known as lava tubes that exist on the Moon could be sufficiently large and structurally stable to house cities and protect humans residing inside from cosmic radiation, meteorite impacts and extreme temperatures on the lunar surface, say scientists.

Lava tubes, which are commonly found on Earth, are tunnels created by rivers of lava spewing out from volcanic eruptions that cool around their edges to create a pipe-like hard crust around the flowing molten rock. The hardened pipes are left empty after the volcanic eruptions end and leave behind hollow tunnels. Scientists believe that these underground structures were formed during such basaltic lava flows on the Moon.

Unlike Earth, the Moon does not have a thick atmosphere and magnetic field for protecting it against cosmic radiation. As a result of the lack of an atmospheric buffer, the Moon's surface is impacted by regular meteorite impacts and exposed to extreme temperatures. But scientists now believe that if human bases are housed underground inside the lava tubes, they could offer protection against these dangerous elements.

Using computer modeling, David Blair and his colleagues from Purdue University in West Lafayette determined the stability of lunar lava tubes with different widths, roof shapes and roof thicknesses. The researchers discovered that tunnels with a width of 1km could exist on the Moon that would be structurally sound enough to protect human settlers residing inside from the challenges posed by the lunar environment.

It is not possible for tubes of similar size to exist on Earth, but they can exist on the Moon due to its comparatively lower gravity, and the lower erosion as well as weathering experienced by lunar rock, according to the researchers who presented their findings at the Lunar and Planetary Science Conference in Texas.

While there have been no definitive discoveries of such lava tubes on the Moon, spacecraft missions have revealed cave entrances called skylights that could open into lava tubes. Interestingly, it is possible that similar lava tubes could exist on Mars as well.