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What’s below the far side of the Moon?

Scientists using data from China’s Chang’E-4 mission are peering beneath the surface of the Moon with unprecedented clarity.

On 3 January 2019, Change’E-4 made history by becoming the first spacecraft to land on the far side of the Moon — a tricky process because that side permanently faces away from the Earth, requiring communications to be relayed via a specially deployed communication satellite.

Once on the surface, the lander wasted no time deploying a rover, called Yutu-2, which as of 18 February has travelled 367.25 metres, according to the China National Space Administration. 

Now, a team of Chinese and Italian scientists spearheaded by Chunlai Li of the Chinese Academy of Sciences reports in the journal Science Advances that ground-penetrating radar carried by the rover has allowed it to peer as deep as 40 metres beneath the surface at surprisingly high resolution.

“You can see details on the order of a few [centimetres] to 10 centimetres,” says co-author Elena Pettinelli, a geophysicist at the University of Roma Tre, Italy. 

This was not the first attempt to use radar to peer beneath the lunar surface. Similar efforts were made as far back as the Apollo program, and subsequently by orbiting spacecraft. But “this is the first image of the lunar subsurface with such resolution”, Pettinelli says.

The data, she says, reveal that to a depth of about 12 metres the area near the landing site is covered by a layer of fine regolith that is probably even finer than beach sand: the remains of rocks pulverised for billions of years by micrometeorite bombardment.

The regolith, Pettinelli says, may even be as fine-grained as clay or silt. “It’s very fine.”

Further down comes a 12-metre layer of coarser materials, including boulders as large as two to three metres, that probably represent rocks ejected from an ancient asteroid impact.

Below that, to the maximum depth the radar can reach, is a zone of alternating rocks and fine-grained materials akin to the surface regolith, suggesting that between impacts, ejecta lay undisturbed for long enough for micrometeorite bombardment to start building up a layer of regolith – until the next time, an asteroid impact showered the site with debris. 

“You can see the stratigraphy very clearly,” Pettinelli says — something she hopes can be used to help reconstruct the sequence of events that produced the layers seen by Yutu-2.

David Kring, a lunar geologist and senior staff scientist at the Lunar and Planetary Science Institute in Houston, Texas, who was not part of the study team, is impressed. To begin with, he says, the new results show that even though the region visited by Chang’E-4 lies atop an ancient lava flow, impacts have very much altered its surface.

“Rather than a lava flow, the mission encountered debris ejected from impact craters near and far,” he says. “This study shows that even missions targeting volcanic terrains must address the geological consequences of impact cratering.”

But, he says, the findings are important not just for science, but for hunting for resources for future Moon bases.

“The results indicate that radar can be used to image the subsurface, which supports the method we propose to locate subsurface ice deposits,” he says.