The farside of the moon is a lunar layer cake. New data from China’s Chang’e-4 lander and Yutu-2 rover reveal alternating layers of coarse rock and fine soil down to a depth of 40 meters, suggesting a history of violent impacts, scientists report February 26 in Science Advances.
“We know much of the moon’s nearside” from the Soviet Lunokhod and American Apollo programs, but little about the farside, says lunar scientist Yan Su of the Chinese Academy of Sciences in Beijing. “The Chang’e-4 mission revealed the first ‘ground-truth’ detailed subsurface stratigraphy … on the farside of the moon.”
Chang’e-4 and Yutu-2 became the first spacecraft to land on the farside in January 2019, touching down inside the 186-kilometer-wide Von Kármán crater (SN: 1/3/19). As Yutu-2 explored the crater, which lies within the 2,500-kilometer-wide South Pole–Aitken basin, the rover sent radar pulses into the ground to probe the material beneath its wheels.
Lunar scientist Chunlai Li, also of the Chinese Academy of Sciences, and colleagues analyzed the 106-meter path that the rover took in its first two lunar days (about two Earth months) of collecting data. The team discovered a layer about 12 meters thick of fine soil, or regolith, closest to the surface.
“It’s like being on very clean sand,” says study coauthor Elena Pettinelli of Roma Tre University in Italy. “It’s like you’re on the beach.”
Below that fine soil, the rover found another layer of about 12 meters containing coarser material embedded with larger rocks, like cherries in a fruitcake. And lower still was a series of alternating coarse and fine materials, spanning depths of about 24 meters down to roughly 40 meters — the limit of the rover’s radar.
Those layers were probably created by material ejected by successive impacts, the researchers say. The floor of Von Kármán crater is a smooth sheet of cooled lava from long-ago volcanic activity. But that lava has been pummeled repeatedly and covered up by material, called ejecta, that is scattered when objects like meteorites slam into the lunar surface and leave craters behind.
“That’s a really violent process,” says lunar geologist Daniel Moriarty of NASA’s Goddard Space Flight Center in Greenbelt, Md., who was not involved in the study. Some of the ejecta could have come from as deep as the moon’s mantle (SN: 5/15/19).
The top layer of smooth sand is probably the result of the surface being pulverized by micrometeorites and cracked by extreme temperature shifts over time.
Chang’e-4’s view of the moon’s subsurface is different from its predecessors’. Chang’e-3 and its Yutu rover landed in Mare Imbrium on the nearside of the moon in 2013, and that rover’s radar was blocked by dense volcanic rock at a depth of just 10 meters or so (SN: 12/16/13). That’s probably because the nearside’s volcanic floodplains are closer to the surface than those on the farside.
“The subsurface structure at Chang’e-4’s landing site is more complex … and suggests a totally different geological context,” Su says. In fact, the lava basement of the Von Kármán crater may be too deep for Yutu-2 to sense at all, the researchers speculate.
Future work could help figure out why the moon’s nearside is awash in smooth plains of volcanic rock called mare, while the farside is more rugged and cratered.
“One of the biggest driving questions in lunar science for a while has been, why does the nearside look so different from the farside?” Moriarty says. “If people can use what they found to unravel some of the volcanic history of the farside, that would be helpful.”