A gaping hole on the Moon’s surface could provide access to a vast underground cavity.
This opening is known as Marius Hills Pit. It lies in a region marked by ancient volcanic activity. Since its discovery through orbital observations, scientists have wondered whether it is merely a local collapse or opens onto a long hollow space beneath the surface.
A natural opening in the lunar ground, interpreted as a possible access point to an ancient underground lava tube.
Credit: NASA/GSFC/Arizona State University, via ESA.
The second possibility is that it is a lava tube. On Earth, this type of tunnel forms when a volcanic flow solidifies at the surface while lava continues to flow beneath it. When the lava drains away, it can leave behind an empty passage. Comparable phenomena may have occurred on the Moon.
To find out what is really hidden beneath Marius Hills, NASA has selected the GIMLI project. Instruments are to be deployed directly in the region. The goal is therefore no longer just to observe the hole from space, but to probe the lunar subsurface nearby.
Several methods will be combined. A radar will send waves beneath the surface and record their reflections. Sensors will also measure the propagation of vibrations through the ground. Finally, a gravimeter will search for very small variations in gravity.
Cameras will also examine the surface and the visible walls of the pit. If a cavity extends beneath the ground, all these measurements should make it possible to estimate its dimensions. Scientists will then be able to determine whether the hole truly forms an entrance to an ancient volcanic tunnel.
Such a cavity would also be of interest for future crewed missions. The lunar surface is directly exposed to radiation and experiences very large temperature variations. A thick layer of rock above a tunnel could provide natural protection, although using such a cavity as a habitat would still require further study.
The project would nevertheless remain scientifically valuable even if no large cave is discovered. The walls of Marius Hills expose layers of lava and dust that are usually buried. Studying them can reconstruct part of the local volcanic history and clarify how the pit itself formed.