Scientists studying the moon have made an unexpected discovery. While we have good data on the surface topography, there’s still a lot we don’t know about what lies beneath the craggy craters and dunes. A large crater in the southern polar region appears to contain a large deposit of dense material, possibly the remains of an ancient metallic asteroid.
The region, known as the South Pole-Aitken basin, is one of the largest known impact craters measuring about 1,600 miles (2,500 kilometers) in diameter. While larger impacts have occurred (including some on Earth) the moon’s unchanging environment is much better at preserving the evidence of impacts. This distinction makes the South Pole-Aitken basin a subject of frequent research. The Chinese Chang’e 4 lander landed in the basin, specifically inside a smaller crater called Von Kármán.
Researchers from Baylor University in Texas used data from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) and Lunar Reconnaissance Orbiter (LRO) missions to develop this new hypothesis on the origins of the basin. The twin GRAIL spacecraft mapped the moon’s gravity in 2011 and 2012. Meanwhile, the LRO has been mapping the lunar surface for a decade. The two data sets seem to show a mismatch, according to the new study. Where you would expect gravity to drop slightly over the crater, it actually increases.
The leading explanation for the gravitational anomaly, according to the researchers, is that the object responsible for the crater is still mostly intact beneath the surface. So, some 4 billion years ago, a mostly metallic asteroid hit the moon and remains embedded in the mantle to this day. Another potential explanation is that the region is naturally rich in oxides that formed as the moon cooled in the distant past. However, the overlap of the crater and increased gravity seems a bit too convenient.
If there is a large metallic object buried under the South Pole-Aitken basin, it could tell us something about the moon’s interior. After 4 billion years, the iron-nickel remains of the asteroid would have been dispersed throughout the mantle if the moon was geologically active for any significant period of time.
Additional research and missions will be needed to confirm the presence of asteroid deposits beneath the South Pole-Aitken basin. Getting a sample isn’t very likely, though. Data from GRAIL suggests the asteroid core is about 186 miles (300 km) below the surface.