Mysterious Mass Discovered under Moon's Largest Crater

The mass might be a remnant of the asteroid that initially caused the crater.

Researchers have discovered a large mass of a mysterious material underneath one of the moon’s biggest craters.

The material underneath the Moon's South Pole-Aitken basin might be the result of an asteroid crash that caused the crater in the beginning. “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 in Baylor’s College of Arts & Sciences. 

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Massive crater is an important research site

The oval-shaped crater is over 2000 kilometers across, at its widest point and is 3-5 kilometers deep. The massive crater can’t be seen from Earth as it is on the far side of the moon not visible to us.

The mass was almost discovered by accident as researchers were examining the moon's subtle changes in gravity by analyzing data from spacecraft used for the National Aeronautics and Space Administration (NASA) Gravity Recovery and Interior Laboratory (GRAIL) 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.”

Astroid embedded in moon's surface

The unknown dense mass is weighing down the carter's basin floor by half a mile.

While still unconfirmed, Baylor researchers suspect that the mass is iron-nickel that dispersed into the upper mantle of the moon's crust during 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 Moon’s core,” James said.

The crater is an important site for scientists who believe it to be the best-preserved example of a crater in the solar system.

The South Pole-Aitken basin is estimated to have been created about 4 billion years ago. While many bigger impacts have occurred in the solar system and even on Earth, evidence of those impacts have been lost.

The research into the crater and its mysterious mass is detailed in the article "Deep Structure of the Lunar South Pole-Aitken Basin," published in the journal Geophysical Research Letters.

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