AstroAlert: Something big under the Moon’s surface

Hi everyone,

Astronomers have discovered something big buried beneath the Moon’s surface – probably the relic of an ancient asteroid.

NASA’s GRAIL (Gravity Recovery and Interior Laboratory) mission used twin spacecraft to map the gravitational tug at different locations on the Moon. As the two spacecraft orbited, they continuously measured the distance between them. This distance changed slightly whenever they flew over regions of more or less mass – and hence more or less gravity – as their relative speed changed. This allowed scientists to map the local gravitational pull, and hence mass concentration, at different locations. The result is the topological map shown above.

The large blue region, known as the Aitken basin, is the largest impact crater on the Moon, more than a thousand miles across. The circle in the map shows an area within the basin where the local gravitational tug reveals a large object buried nearly 200 miles below the surface. The mass and size of the object indicate it must be very dense, leading scientists to speculate that it could be the remains of an iron-rich asteroid that smashed into this region billions of years ago.

“Imagine taking a pile of metal five times larger than the Big Island of Hawaii and burying it underground,” says Peter James of Baylor University. “That’s roughly how much-unexpected mass we detected.”

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