At the south pole of the moon is a giant crater called the South Pole-Aitken basin, measuring about 2500 kilometres across. It is thought to have been created by a large asteroid striking the moon 4 billion years ago, and is among the largest craters in the solar system. Now, researchers say the remains of that asteroid may have been found under the lunar surface.
Using gravity data from NASA’s , which orbited the moon from 2011 to 2012, Peter James at Baylor University in Texas and his colleagues spotted a huge mass hiding about 300 kilometres beneath the South Pole-Aitken basin. They calculate its mass to be 2 million billion tonnes, spread across an area hundreds of kilometres wide.
“We found that there is a special anomaly under the South Pole-Aitken basin,” says James. “We propose two mechanisms that would plausibly explain the origin of the mass anomaly, and the idea that it comes from the core of the asteroid that impacted the moon to make this huge basin is maybe the most compelling of them.”
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The impact quite literally shaped much of the moon as we know it today, including forming mountains nearby, known as massifs. “It decimated the whole surface of the moon,” says James.
The asteroid would have blasted a huge and deep hole in the moon, with material then mixing and sinking into the hole. This would have included the iron-nickel core of the asteroid itself, which is thought to have been about 95 kilometres across. As the moon cooled, this core was left suspended underground, a large chunk of iron within the upper lunar mantle.
“The deep density anomaly is fascinating,” says Linda Elkins-Tanton at Arizona State University. “It’s such a large observation.”
The other mechanism that could explain the large mass is that it is a remnant of the moon’s early magma ocean, formed not long after the satellite’s birth about 4.5 billion years ago.
But the fact that it is below such a large crater favours the asteroid explanation over the magma ocean one. “I am not aware of a reason why we would expect that to happen under the crater specifically, as opposed to somewhere else on the moon,” says James.
There are still some mysteries to resolve to work out if this truly is the asteroid’s core. “The location of the mass is not quite in the middle of the crater, but also not near the edge,” says Gareth Collins at Imperial College London. “To me, that suggests that the impact was at a relatively steep angle, and that might not be compatible with the fact that the basin is clearly quite elliptical.”
The mass is too deep for us to access directly, but a network of placed around the moon could reveal more details. And with the US hoping to send humans to the south pole of the moon in 2024, we might be able to get a close look at the material ejected by the asteroid onto the lunar surface and learn its composition and origin.
“That would be an interesting opportunity for future geological study on the moon,” says James.
Geophysical Research Letters
