The moon hasn’t had it easy over the years. Since the dawn of the solar system 4.5 billion years ago, its gray and lifeless surface has been repeatedly pummeled by incoming space rocks, leaving behind a pockmarked landscape strewn with rubble. Beneath this surface, however, hide the moon’s most tantalizing secrets for human explorers, from possible reservoirs of ice for producing potable water and rocket fuel to hollow lava tubes that are suitable for harboring habitats. More fundamentally, mapping the moon’s subsurface can reveal otherwise-hidden epochs of solar system history written by impacts, buried craters and associated debris—as demonstrated by fresh results from a Chinese rover on the little-explored lunar far side.

In a paper published in the journal Science Advances today, a collaboration of Chinese and European researchers describes the latest results from the Chang’e-4 mission, run by the China National Space Administration. Launched in December 2018 and reaching the moon in early January 2019, the mission became the first to land on the far side of the natural satellite, targeting an intriguing region near the lunar south pole called the South Pole–Aitken Basin. Formed 3.9 billion years ago and stretching some 2,500 kilometers across, it is the biggest impact basin in the solar system—and perhaps a key to understanding how great impacts have shaped Earth and other inner planets. The Chang’e-4 rover is still operational today and has been slowly trundling across this region, traveling a few hundred meters since it landed.

Chang’e-4’s landing site is within the 186-kilometer-wide Von Kármán crater, which lies inside the basin. Nearby are several other craters, such as the 72-kilometer-wide Finsen crater, thought to be about 3.2 billion years old. Using a ground-penetrating radar instrument on Chang’e-4, researchers have found that the rover is likely sitting on different layers of ejecta—debris from multiple impacts over time that rained down at high velocities to blanket the lunar surface and now fill the crater.  “[We] see a very clear sequence of [layers],” says Elena Pettinelli of Roma Tre University in Italy, one of the paper’s co-authors.

The rover’s radar instrument was able to penetrate up to 40 meters below the surface of the moon, more than twice the distance achieved by its predecessor, the Chang’e-3 mission, which landed on the lunar near side in December 2013. Data from the latest mission show three distinct layers beneath the rover: one made of lunar regolith, or soil, down to 12 meters; another made of a mix of smaller and larger rocks down to 24 meters; and a third with both coarse and fine materials extending the rest of the 40-meter depth.

Schematic illustration of the Chang’e-4 rover’s traverse across the lunar surface and the three distinct layers of subsurface debris revealed by its ground-penetrating radar. Credit: From “The Moon’s Farside Shallow Subsurface Structure Unveiled by Chang’E-4 Lunar Penetrating Radar,” by Chunlai Li et al., in Science Advances. Published online February 26, 2020

It is not currently possible to definitively date the layers under Chang’e-4 and assign them to nearby craters. But they do provide some clues into lunar history stretching back about four billion years. Pettinelli notes that smaller rocks in the layers likely come from more distant craters, because they would have been able to travel farther across the moon, while the larger rocks hint at closer impacts. “If the blocks are big, you’re probably close to the source of the ejecta,” she says. Debris from at least four or five impacts is thought to be beneath the rover, extending down perhaps 80 meters or more to the basin’s floor.

While the moon was the focus of the American Apollo and Soviet Luna missions in the 1960s and 1970s, they mostly lacked the ground-penetrating-radar capabilities of the Chang’e-3 and Chang’e-4 missions—and of course, none of those earlier efforts ventured to the surface of the far side. As such, China’s two rovers have provided some of our first glimpses into the upper reaches of the moon’s subsurface. Other missions—such as NASA’s twin GRAIL (Gravity Recovery and Interior Laboratory) spacecraft, which orbited the moon from 2011 to 2012—have been able to peer much deeper beneath the surface but only in a limited way: using lunar-gravity data, they have provided relatively low-resolution glimpses of large features at depths of hundreds of kilometers.

Lunar scientist Daniel Moriarty of NASA’s Goddard Space Flight Center, who was not involved in the new paper, says the researchers’ results are interesting because those findings provide a look at how the moon has evolved over time. “The surface of the moon is very different from Earth,” he says. “The only two real large-scale processes that occur on the surface of the moon are impact cratering and volcanic activity, and they’re seeing evidence for both of those things here. The place they landed is a big volcanic floodplain. And then that floodplain was affected by impacts itself.”

Moriarty notes that the floodplain and the impact debris likely mixed together, which could suggest that some of the larger boulder-sized objects that were observed were from volcanic material being broken down rather than the result of debris from nearby impacts. It might also be that material from the lunar mantle, exposed by the initial impact that created the South Pole–Aitken Basin, has mixed in with the debris, something hinted at in earlier results from the Chang’e-4 mission.

The rover is continuing to move across the surface, making regular stops to take measurements and use its instruments. And as it does so, researchers are hoping that it might see the subsurface layers of debris change in size, revealing more subtle details of the moon’s vast, violent and ancient impact history. “We are asking [for the rover] to go toward [places where researchers] can say the [debris] is changing in thickness,” says Pettinelli. “That will be important.”