Researchers Discover Mysterious Material Under Moon's Largest Crater

Olive Hawkins
June 12, 2019

Not only is it the biggest impact crater in the solar system, but its towering rim and deep basin also contain the moon's highest and lowest elevations. "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.

This false-color graphic shows the topography of the far side of the Moon.

These oxides have a great deal of mass, which somehow could have been concentrated beneath the South Pole-Aitken Basin (although that "somehow" is yet to be explored). (It's also where China landed its Chang'e 4 lunar rover in January.) Publishing in Geophysical Research Letters, the Baylor scientists have two theories for the origin of the huge subterranean blob.

The mass was found when the team merged data from NASA's Gravity Recovery and Interior Laboratory mission, known as GRAIL, with topography gathered from the Lunar Reconnaissance Orbiter, a robotic spacecraft launched from Cape Canaveral in 2009. Deep below the moon's South Pole-Aitken basin (the largest preserved impact crater anywhere in the solar system), researchers have detected a gargantuan "anomaly" of heavy metal lodged in the mantle that is apparently altering the moon's gravitational field. The mass is located under a crater in the Moon's South Pole-Aitken basin, and researchers believe it could be metallic remnants from the asteroid that caused the crater in the first place.

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The South Pole-Aitken Basin is believed to be the site of a horrendous collision that occurred about 500 million years after the moon formed. Beneath this basin lies a odd anomaly-an excess of mass extending at least 300 kilometers down, more than 10 times the depth of the Earth's crust.

Whatever this thing is, it's dense enough to make the crater floor sink over half a mile. Meanwhile, the LRO has been mapping the lunar surface for a decade.

"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. If the mass was produced by an impact, its location around 400 kilometers southeast of the crater's center could help improve our knowledge of how impacts form craters, according to the paper.

Using computer simulations of large asteroid impacts, researchers could recreate conditions where an iron-nickel core of an asteroid could be dispersed into the layer between the Moon's crust and core during an impact. Or, intriguingly, the extra mass could suggest the presence of an enormous metal core deposited in the Moon mantle, left over from the impact. Essentially, something caused a giant hole on the Moon billions of years ago, and astronomers have just discovered that there's something big - really big - buried underneath the surface.

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