What’s Causing the Huge Mass Anomaly Beneath the Moon?

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The far side of the Moon contains an enormous and very weird structure at its South Pole. And scientists think they might know the source of this anomaly.

The South Pole-Aitken basin is a huge crater produced by an ancient impact on the Moon, whose longest axis would span from New York City to Omaha, Nebraska. Beneath this basin lies a strange anomaly-an excess of mass extending at least 300 kilometers down, more than 10 times the depth of the Earth’s crust. Scientists think this might be a remnant of whatever impactor created the crater.

Two datasets contributed to the research: topography data from the Lunar Orbiter Laser Altimeter (LOLA) onboard the Lunar Reconnaissance Orbiter (LRO) and global gravity data from the pair of small Gravity Recovery And Interior Laboratory (GRAIL) spacecraft. Analysis of the two taken together revealed “a conspicuous mass excess in the mantle” under the basin with around 2.18 quintillion (that’s 10^18) kilograms extra mass, according to the paper published in Geophysical Research Letters. That’s about the mass of a decently sized asteroid.

How did so much extra mass end up there? The team presented two ideas. Perhaps some process caused certain materials to concentrate beneath the basin as the lunar mantle cooled. Or, intriguingly, the extra mass could suggest the presence of an enormous metal core deposited in the Moon mantle, left over from the impact.

Either way, should this mass anomaly continue to exist under further scrutiny, it’s going to have important implications for scientists’ understanding of the Moon’s history. If the mass was produced by an impact, its location around 400 kilometers southeast of the crater’s center could help improve our knowledge of how impacts form craters, according to the paper. If it was caused by uneven crystallizing of a magma ocean, then scientists will want to understand why and how that occurred.

There’s lots of lunar excitement recently, thanks in part to the fact that China has landed its Chang’e 4 mission on the Moon’s far side, and because the U.S. is working toward returning humans to the lunar surface. And research has made it clear that the South Pole-Aitken basin is a very interesting place. Just recently, the Chang’e 4 mission scientists found evidence of mantle material in the basin, potentially kicked up by another impactor.

The Moon is a very strange place. Who knows what we’ll find when we return?