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A huge mass, potentially the metallic core of an asteroid, is hidden under a sprawling crater on the dark side of the moon, according to a team of researchers that included two people from MIT.

The mystery object has been detected under the moon’s South Pole-Aitken basin, which is the largest crater in our solar system, thought to have been created about 4 billon years ago, Baylor University said this week in a statement.

“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,” Peter B. James, assistant professor of planetary geophysics in Baylor’s College of Arts & Sciences, said in the statement.

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James was the lead author of a study describing the discovery that was published recently in the journal Geophysical Research Letters.

The data “point to the existence of a large excess of mass in the moon’s mantle under the South Pole-Aitken basin,” the study said. “Plausible sources for this anomaly include metal from the core of a differentiated impactor.”

The oval crater is as wide as about 1,200 miles. The mysterious mass is estimated in the paper as weighing at least 4.8 quintillion pounds (10 to the 18th power), extending to a depth of more than 190 miles.

Crucial data for the mission came from the NASA GRAIL (Gravity Recovery and Interior Laboratory) mission.

“When we combined that with lunar topography data from the Lunar Reconnaissance Orbiter, we discovered the unexpectedly large amount of mass hundreds of miles underneath the South Pole-Aitken basin,” James said. “One of the explanations of this extra mass is that the metal from the asteroid that formed this crater is still embedded in the Moon’s mantle.”

Maria T. Zuber, vice president for research and professor of geophysics at MIT, led the GRAIL mission, where James began his research as an MIT graduate student. She is a co-author on the paper.

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“The results of this study provide new information on the violent history of our nearest celestial neighbor,” she said.

“With advanced processing of the GRAIL data, our group was able to isolate a large mass within the Moon’s mantle, beneath the deepest part of the basin depression. There is no correlation with the composition of the surface. Our analysis indicates that the size of the surface depression is consistent with a downward deflection that would be expected by the weight of the mass pulling the surface downward,” she said in an e-mail.

“Our team, led by Peter, came up with two possibilities to explain the mass. The first is a concentration of metallic oxides left over from the last stages of cooling and crystallization of a global ocean of magma that once covered the moon, and which formed the lunar crust (the bright white part of the moon). But there is no obvious reason why these oxides should concentrate in one place on the moon’s farside. The other possibility is that the mass represents the metallic core of the impactor that produced the basin, a very exciting possibility that we will be investigating further,” she said.

MIT research scientist David Smith, who led the Lunar Orbiter Laser Altimeter mission, was also a co-author of the paper.

“It’s just so mysterious,” Sara Mazrouei of Western University’s Center for Planetary Science and Exploration, who was not involved in the work, told National Geographic.

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“As an impact modeler, it’s very exciting,” Brandon Johnson, a planetary scientist at Brown University, who also was not involved in the work, told the magazine. “I can’t wait to possibly get started working on this.”


Martin Finucane can be reached at martin.finucane@globe.com