What lunar caves tell us about the shared origins of the Earth and the Moon

By Robin Smith, Sheffield Hallam University

Using radar, a Nasa spacecraft, Lunar Reconnaissance Orbiter (LRO), has confirmed the existence of caves beneath the lunar surface. Here’s why such geological features will be key for establishing a base on the Moon, and what they can tell us about Earth and our Moon’s shared cosmological origins.

Lunar orbiting satellites first spotted pits on the Moon’s surface decades ago. Many of these were thought to be openings that connected to substantial underground tunnels that form through volcanic processes, but only now has this been confirmed through the analysis of radar data.

Some of the tunnels thought to exist on the Moon are expected to be lava tubes, which are also found on Earth. When molten lava flows out of the ground, the lava stream eventually cools and hardens into a crust. The lava inside is still molten, and continues to flow. Once the lava has flowed away, it leaves an empty tunnel called a lava tube. These formation processes are thought to be be very similar on the Earth and the Moon.

The data used in the latest study was collected in 2010 by LRO but only recently analysed using state of the art signal processing techniques. Radar (electromagnetic waves of 12.6cm wavelength) fired at acute angles towards these lunar pits, partially illuminated the shadowed subterranean areas to generate measurable radar echo signals.

The timing and amplitude of the reflected signals allowed researchers to compare with simulations and build up a picture of the underground terrain. Data indicate that the largest “Mare Tranquillitatis” pit leads to a cave 80 metres long and 45 metres wide: an area equivalent to around half a football pitch.

It is likely that the lunar surface is home to hundreds of such caves. It is widely thought that around 4.5 billion years ago, a young Earth violently collided with a Mars-sized proto-planet, splitting our youthful planet into the Earth and Moon system we have today.

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