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From an estate agent’s point of view, the lunar surface has little to recommend it. Its lack of any noticeable atmosphere means it is bombarded by meteorites, cosmic radiation is incredibly harsh and temperatures range from -246 °C to a maximum of 121 °C. Subsurface lunar caves, on the other hand, would be more attractive habitats, with stable temperatures and cover from projectiles. The only question is, does one exist?
In a paper published in Nature Astronomy, Lorenzo Bruzon and Leonardo Carrer of the University of Trento and their colleagues provide a definitive answer in the affirmative. They provide evidence of a cave approximately 80 meters long and 45 meters wide at the bottom of a 135 meter deep crater in the Tranquility Sea. The neighborhood is a historic one: the plains around the crater are the site of humanity’s first lunar landing. Although the site’s selenological (geological equivalent of lunar equivalent) origins are unclear, it may be the remains of a subsurface tube formed by a river of lava. When the roof of the tube collapsed – probably after a meteor impact – all that was left was a huge crater (with associated caves).
Researchers discovered Annecy by analyzing data from the Lunar Reconnaissance Orbiter (LRO), which has been orbiting the moon since 2009. There is a radar instrument on LRO primarily designed to hunt for water ice beneath the Moon’s surface. The instrument also collected data about the size of craters such as those in the Tranquility Sea. Dr. Bruzon says that although images made by radar equipment have been available since the malfunction in 2011, no one had previously looked at them in a good enough way to be able to see the cave.
This changed when the team compared the photos with satellite images of Earth made using the same radar technology as LRO. This helped them identify the telltale signs of a cave, as well as work out the different radar signals given off by different subsurface geometries. The team then ran simulations to compare LRO’s data with expected data from craters of different shapes and sizes. It turned out that the best match for the Sea of Tranquility data was a crater with a huge hidden cave. The important thing is that its entrance is accessible. “We found a front door,” says Dr. Carr. “This means it could be explored in future lunar missions.”
This kind of search can be much more useful than house-hunting. It is believed that subsurface lunar caves contain valuable resources that future settlers may need to establish a permanent base on the Moon. For example, water can be drunk as well as used as a potential source of fuel and oxygen. It has already been found in both solid and liquid forms on the moon’s surface, but there may be more hidden beneath.
The caves will also possibly shed light on how the Moon formed. Just as studying Earth’s interior layers provides clues about the conditions on the planet when each layer formed, studying ancient lunar cave walls can reveal a complete picture of the Moon’s formation. Scientists could potentially use these to explore the extent of volcanic activity on the Moon or determine the composition of the lunar core. All it takes is a visionary tenant.
© 2024, The Economist Newspaper Limited. All rights reserved. From The Economist, published under license. Original content can be found at www.economist.com
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