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China’s moon rover revealed what lies beneath the lunar farside

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.

The Yutu-2 rover is using radar to probe the lunar subsurface (illustrated; the rover’s path from January to March 2019 shown in white double lines). So far, the rover has found three different subsurface layers: fine soil down to about 12 meters, coarser rock with boulders embedded in it up to about 12 meters below that, and sublayers of coarse and fine materials down to about 40 meters. What’s below that is a mystery, for now.C. Li et al/Science Advances 2020

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.”

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