Astronomers Make Massive Discovery on the Far Side of the Moon

The heavy core of a giant asteroid may be buried beneath the moon’s South Pole-Aitken basin

Mass Anomaly
NASA/Goddard Space Flight Center/University of Arizona

Planetary scientists have identified an abnormally massive area located deep below a crater on the moon’s far side. The lunar feature has a mass five times the size of Hawaii’s Big Island, but the exact reason why this anomaly exists in unclear, according to a new study in the journal Geophysical Research Letters.

The massive blob was discovered by researchers using data from NASA’s 2011 Gravity Recovery and Interior Laboratory, or GRAIL, mission and mapping information from the Lunar Reconnaissance Orbiter. By combining both datasets, researchers found that the abnormal mass is located 180 miles underneath the South Pole-Aitken basin, a huge four billion-year-old crater.

“[The South Pole-Aitken 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,” says study co-author Peter James, a planetary scientist at Baylor University, in a statement.

The 1,200-mile-wide crater was formed when some large space rock with a heavy metal core smashed into the lunar surface billions of years ago, as Maya Wei-Haas at National Geographic describes. When that happened, the asteroid drilled through layers of the moon's crust while losing mass of its own. Molten rock partially refilled the impact area, melting chunks of the asteroid's busted metal core along the way. James explains that today, metal from the asteroid’s core could still be embedded in the lunar mantle, causing the extra mass.

Providing more evidence for this theory, there appears to be what’s called a central depression on the basin's floor. The oval-shaped depression is about half a mile deeper than the rest of the crater, suggesting that something beneath it has enough gravitational pull to tug the area inward.

“That’s a huge result,” lunar geologist Daniel Moriarty of NASA’s Goddard Space Flight Center tells Wei-Haas. “It really gives us a hint of what’s going on in the lunar interior.”

James and his team are gearing up to continue analyzing the crater. Others are excited as well. “As an impact modeler, it’s very exciting,” says Brandon Johnson, a planetary scientist at Brown University who was not involved in the new study, tells Wei-Haas. “I can’t wait to possibly get started working on this.”

We’re already getting a few clues about the ancient goings-on at the South Pole-Aitken basin from other sources, too. Just last month, researchers released data showing that China’s Chang’e-4 mission to the far side, which explored part of the basin in January, may have found rocks from the moon’s mantle on the surface, which could give scientists new insights into the processes that formed the moon.

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