A mysterious large mass of material has been discovered beneath the Moon's South Pole-Aitken basin, the largest crater in our solar system.

The crater, thought to have been created about 4 billion years ago, is oval-shaped and as wide as 2,000 kilometers (roughly the distance between Waco, Texas, and Washington, D.C.) and several miles deep. Despite its size, it cannot be seen from Earth because it is on the far side of the Moon. 

To measure subtle changes in the strength of gravity around the Moon, researchers analyzed data from spacecrafts used for the National Aeronautics and Space Administration (NASA) Gravity Recovery and Interior Laboratory (GRAIL) mission. Then they combined it with lunar topography data from the Lunar Reconnaissance Orbiter and discovered the unexpectedly large amount of mass hundreds of miles underneath the South Pole-Aitken basin.

False-color graphic showing topography of the far side of the Moon. Warmer colors indicate high topography and the bluer colors indicate low topography. Credit: (NASA/Goddard Space Flight Center/University of Arizona

The authors believe this extra mass is metal from an asteroid that formed the crater, which is still embedded in the Moon's mantle. 

"Imagine taking a pile of metal five times larger than the Big Island of Hawaii and burying it underground. That's roughly how much unexpected mass we detected," said lead author Peter B. James,  Ph.D., assistant professor of planetary geophysics at Baylor University.

The dense mass -- "whatever it is, wherever it came from" -- is weighing the basin floor downward by more than half a mile, he said. Computer simulations of large asteroid impacts suggest that, under the right conditions, an iron-nickel core of an asteroid may be dispersed into the upper mantle (the layer between the Moon's crust and core) during an impact.

Another possibility is that the large mass might be a concentration of dense oxides associated with the last stage of lunar magma ocean solidification.

James said that the South Pole-Aitken basin -- thought to have been created about 4 billion years ago -- is the largest preserved crater in the solar system. While larger impacts may have occurred throughout the solar system, including on Earth, most traces of those have been lost.

James called the basin "one of the best natural laboratories for studying catastrophic impact events, an ancient process that shaped all of the rocky planets and moons we see today."