The Chinese Chang’e-4 rover may have confirmed a longstanding idea about the origin of a vast crater on the Moon’s far side.
The rover’s landing site lies within a vast impact depression created by an asteroid strike billions of years ago.
Now, mission scientists have found evidence that impact was so powerful it punched through the Moon’s crust and into the layer below called the mantle.
Chang’e-4 has identified what appear to be mantle rocks on the surface.
It’s something the rover was sent to the far side to find out.
Chunlai Li, from the Chinese Academy of Sciences in Beijing, and colleagues have presented their findings in the journal Nature.
The lunar far side, which is turned away from Earth, is more rugged than the familiar near side and has fewer “maria” – dark plains formed by ancient volcanic eruptions.
The Chinese spacecraft touched down on 3 January, becoming the first spacecraft to perform a soft landing on the lunar far side. The rover then rolled off the lander to explore its surroundings.
The rover landed inside a 180km-wide impact bowl called Von Kármán crater. But that smaller crater lies within the 2,300km-wide South Pole Aitken (SPA) Basin, which covers nearly a quarter of the Moon’s circumference.
It’s not known exactly how old the SPA Basin is, but it’s thought to be at least 3.9 billion years old. The asteroid that carved it out is thought to have been about 170km wide.
The Yutu-2 rover has now identified rocks with a very different chemical make-up to those found elsewhere on the Moon.
Early results from the rover’s Visible and Near Infrared Spectrometer (VNIS) suggest the rocks contain minerals known as low-calcium (ortho)pyroxene and olivine.
They fit the profile of rocks from the lunar mantle and suggest that the ancient impact that created the SPA drove right through the 50km-deep crust into the mantle.
Observational data taken by Moon-orbiting spacecraft have been inconclusive as to the presence of mantle rocks on the surface.
The authors of the paper want to continue their examination of these rocks and find others. They have also raised the possibility of sending another mission to deliver some of them to Earth for study in laboratories.
The results could now help scientists understand the chemical and mineralogical composition of the mantle, which could shed light on the origins and evolution of the Moon itself.
The team members also want to find out more about what happened after the asteroid collided with the Moon and formed the SPA Basin. Scientists predict that the hole in the surface may have been filled by molten rock – forming a “melt sheet” within the impact bowl, which complicates the picture of this region’s geology.
Patrick Pinet, from the Research Institute in Astrophysics and Planetology (IRAP) in Toulouse, France, called the results “thrilling” and said they “could have considerable implications for characterising the composition of the Moon’s upper mantle”.
He added: “It is of the utmost importance to make progress towards unpacking the geology of the lunar far side, expanding our fundamental knowledge of the Moon’s formation and the origin of the crustal asymmetry that exists between its near and far sides, and preparing future sample-return missions.”
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