Caves and lava tunnels on the Moon are crucial for exploring the moon. They offer natural shelters from the harsh lunar environment, protecting from cosmic radiation, micrometeorite impacts, and extreme temperature fluctuations. These subsurface structures could serve as potential sites for temporary or even permanent lunar bases, ensuring a stable and safer habitat for astronauts.

Over 200 pits have been discovered on the moon, some of which are likely to be linked with underlying lava tubes. However, whether such pits provide access to large caves was uncertain; until now. A new study concludes that an area called Mare Tranquillitatis contains a subsurface cave conduit that could potentially be used as a base.

Lava tubes on the moon

For decades, scientists have speculated about the presence of subsurface structures beneath its surface. We know that the moon had a lot of volcanism in the past. In fact, dark spots on the moon (called lunar maria) are dark basaltic plains that were formed by ancient eruptions — and where there are volcanic eruptions, there could also be lava tubes.

A lava tube is a type of cave formed when the surface of a flowing lava stream cools and solidifies while the molten lava beneath continues to flow, eventually draining away and leaving a hollow tunnel. It’s essentially a cave, but most caves are created by water erosion, whereas this type of cave is created by volcanic activity. Lava tubes exist on Earth, and researchers are pretty sure they exist on the moon too.

This is where the lunar pits come in. Lunar pits are depressions or holes on the Moon’s surface. They are often formed by the collapse of surface material into underlying voids, such as lava tubes or magma chambers. Mare Tranquillitatis (the Sea of Tranquility is one such pit; it’s also where the Apollo 11 moon landing took place.

Radar and the moon

Leonardo Carrer, Lorenzo Bruzzone, and colleagues from Italy and NASA analyzed radar data from the Lunar Reconnaissance Orbiter (LRO). The LRO has a radar instrument called the Mini-RF instrument. Its radar waves reach the lunar surface and then bounce back. Researchers then analyze this bounce and try to infer not only what’s above the surface, but also what’s beneath.

In this case, the radar reflections originating from the Sea of Tranquility indicated an anomalous increase in radar echo power beyond the pit, suggesting the presence of a subsurface conduit.

To confirm this hypothesis, the researchers conducted a series of 3D radar simulations. These simulations compared the observed radar data with various geometric models of the MTP and its subsurface features. The best-fitting models indicated that the anomalous reflections were consistent with a subsurface cave conduit.

The scientists estimate that the conduit lies at a depth of 130–170 metres and is 30–80 metres long and around 45 metres wide. The cave is also potentially flat, but it may also be inclined by up to 45 degrees. But, most importantly, the cave is likely accessible.

This discovery has significant implications for future lunar exploration. Such subsurface structures are ideal candidates for lunar bases. Additionally, exploring these conduits could yield valuable insights into the Moon’s geological history and volcanic activity.

However, there’s still a great deal of uncertainty when it comes to lunar caves. More data is needed to fully understand the extent and morphology of these conduits.

Robotic missions equipped with advanced radar and imaging technology could explore these conduits, mapping their extent and assessing their suitability for human habitation.

The study was published in Nature Astronomy. https://doi.org/10.1038/s41550-024-02302-y