Mysterious Mass of Material Discovered Beneath Moon’s South Pole-Aitken Basin

The South Pole-Aitken basin — the largest crater in the Solar System — is a gigantic impact structure on the far side of the Moon. Data from NASA’s lunar spacecraft point to the existence of a large excess of mass — about 2.18*1018 kg — in the lunar mantle under the basin. According to new research, this mass anomaly may contain metal from a massive asteroid that crashed into the Moon and formed the crater.

The South Pole-Aitken Basin on the lunar far side is one of the largest and oldest impact features in the Solar System. It’s easily seen in the elevation data. The low center is dark blue and purple. Mountains on its edge, remnants of outer rings, are red and yellow. Image credit: NASA / GSFC / University of Arizona.

The South Pole-Aitken Basin on the lunar far side is one of the largest and oldest impact features in the Solar System. It’s easily seen in the elevation data. The low center is dark blue and purple. Mountains on its edge, remnants of outer rings, are red and yellow. Image credit: NASA / GSFC / University of Arizona.

“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 Dr. Peter B. James, a planetary researcher with Baylor University and the Lunar and Planetary Institute.

The South Pole-Aitken basin is oval-shaped, as wide as 1,600 miles (2,500 km) and 8.1 miles (13 km) 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, Dr. James and colleagues analyzed data from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission.

“When we combined that with lunar topography data from NASA’s Lunar Reconnaissance Orbiter, we discovered the unexpectedly large amount of mass hundreds of miles underneath the South Pole-Aitken basin,” Dr. 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.”

“The dense mass — whatever it is, wherever it came from — is weighing the basin floor downward by more than half a mile.”

Computer simulations of large asteroid impacts suggest that, under the right conditions, an iron-nickel core of an asteroid may be dispersed into the lunar upper mantle during an impact.

“We did the math and showed that a sufficiently dispersed core of the asteroid that made the impact could remain suspended in the Moon’s mantle until the present day, rather than sinking to the lunar core,” Dr. James said.

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

“The South Pole-Aitken basin — thought to have been created about 4 billion years ago — is the largest preserved crater in the Solar System,” Dr. James said.

“It is 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.”

The findings appear in the journal Geophysical Research Letters.

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Peter B. James et al. Deep Structure of the Lunar South Pole-Aitken Basin. Geophysical Research Letters, published online June 5, 2019; doi: 10.1029/2019GL082252

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