The Moon’s Largest Crater Holds A Secret

The Moon’s largest crater, the South Pole-Aitken Basin, holds a secret. The lunar crater has a massive lump of metal beneath it. Find out more here.

The Moon’s Largest Crater Has a Lump of Metal Embedded.

One of the biggest craters in our Solar System is on the Moon. It’s named the South Pole-Aitken basin (SPA), and it’s 2,500 km or 1,600 miles in diameter and 13 km or 8.1 miles deep. And a brand-new study says that the South Pole-Aitken basin may hold a huge chunk of metal that’s bigger than Hawaii’s Big Island.

The new study, titled – Deep Structure of the Lunar South Pole-Aitken Basin, is published in the famous Geophysical Research Letters. Its main author is Peter B. James. He is an Assistant Professor of Planetary Geophysics in Baylor’s College of Arts & Sciences. And the study is based on data from NASA’s GRAIL-mission or Gravity Recovery and Interior Laboratory mission.

The South Pole-Aitken basin is the most massive universally-recognized impact crater in our Solar System. The big crater can’t be seen from Earth because it’s on the far side of our Moon. Furthermore, it’s oval-shaped, and if it were on Earth, it would reach from Waco, Texas, to Washington, DC. And with a depth of 13 km or 8.1 miles, the vast crater is approximately six times deeper than it is wide. It’s standing as one of the oldest, well-preserved formations on the Moon, making the South Pole-Aitken basin a subject of much scientific attention.

Picture showing a false-color graphic of the far side of the Moon showing the impact crater. Credit: NASA/University of Arizona/Goddard Space Flight Center.
Picture showing a false-color graphic of the far side of the Moon showing the impact crater. Credit: NASA/University of Arizona/Goddard Space Flight Center.

Beneath This Massive Crater is an Enormous Chunk of Metal.

Picture taking a quantity of metal five times larger than the Big Island of Hawaii and burying it underground. That’s about how much-unexpected volume we detected,” said lead author Peter B. James.

The mass is possible hundreds of miles below the Moon’s surface, but as the paper says, It could be a substantial density anomaly distributed over a modest range of depths, or it could be a subtle density exception distributed throughout the thickness of the mantle.

GRAIL-A, and GRAIL-B, or Ebb and Flow.

NASA’s Gravity Recovery and Interior Laboratory mission were launched in 2011 and spent nearly a year mapping the lunar’s gravity before being plunged into the Moon in one last maneuver. Gravity Recovery and Interior Laboratory used two spacecraft in the same lunar orbit. They were named GRAIL-A, and GRAIL-B, or Ebb and Flow.

As they flew over the Moon with differing masses, Ebb and Flow moved a little bit away from each other. 

Precision instruments on the Gravity Recovery and Interior Laboratory spacecraft measure these changes, and the data was used to produce a high-resolution image of the lunar’s gravitational field.

Picture showing the gravity map of the Moon created by GRAIL. Red represents mass excesses, and blue represents mass deficiencies. Image Credit: By NASA/JPL-Caltech/MIT/GSFC – GRAIL’s Gravity Map of the Moon, Public Domain.
Picture showing the gravity map of the Moon created by GRAIL. Red represents mass excesses, and blue represents mass deficiencies. Image Credit: By NASA/JPL-Caltech/MIT/GSFC – GRAIL’s Gravity Map of the Moon, Public Domain.

One of Gravity Recovery and Interior Laboratory science goals was to determine the subsurface structure of impact basins and the source of Moon’s mascons or mass concentrations. This investigation derives from that effort and combines Gravity Recovery and Interior Laboratory data with data from the LRO or the Lunar Reconnaissance Orbiter.

LOR or The Lunar Reconnaissance Orbiter.

When we combined Gravity Recovery and Interior Laboratory data with the Moon’s topography data from the LOR or the Lunar Reconnaissance Orbiter, we found the surprisingly large amount of mass hundreds of miles beneath the South Pole-Aitken basin, James said.

The picture showing a side by side comparison of SPA (right) and the Moon’s Orientale Basin (left.) Note the bullseye pattern in the Orientale impact basin,a nd the absence of a bullseye pattern in the SPA basin. Image Credit: NASA/GRAIL.
The picture showing a side by side comparison of SPA (right) and the Moon’s Orientale Basin (left.) Note the bullseye pattern in the Orientale impact basin,a nd the absence of a bullseye pattern in the SPA basin. Image Credit: NASA/GRAIL.

And according to James, whatever that solid matter is, it’s pulling the floor of the huge crater down by more than a half-mile. Furthermore, computer simulations imply that the mass could be an immense chunk of iron and nickel and that it was the nucleus of an asteroid that hit the Moon and stuck in the upper mantle.

The Metal From The Asteroid is Still Embedded.

One of the explanations of this additional mass is that the metal from the giant asteroid that formed this massive crater is still embedded in the lunar’s mantle, James said in an earlier press release. And according to the new paper, the excess mass seen in the lunar mantle is approximately equal to a 95-km-diameter iron-nickel core in the lunar’s mantle.

The scientists did the math and determined that an enough dispersed core of the asteroid that made the impact could still remain suspended in the lunar’s mantle till the present day, rather than sinking to the lunar core, according to James.

However, that’s not the only possibility— just the obvious one.

Picture showing a schematic of the structure of the Moon. Image Credit: By Bryan Derksen at English Wikipedia.
Picture showing a schematic of the structure of the Moon. Image Credit: By Bryan Derksen at English Wikipedia.

The bulk of material could be remnants of the Moon’s formation. It may be a concentration of dense oxides from the last stage of the lunar magma ocean setting. Our Moon was created approximately 4.51 billion years ago, presumably due to Earth’s colliding with an ancient asteroid called Theia. Shortly after it was created, about 4.5 billion years ago, its magma oceans chilled and hardened. As they cooled and hardened, one of the last oxides to form would’ve been FeTiO3, likewise known as ilmenite. And it is very dense and could explain this anomaly.

It’s Hard to Determine The Nature of The Material Beneath The South Pole-Aitken Basin.

Furthermore, it’s hard to determine the precise nature of the material underneath the South Pole-Aitken Basin. It seems very unlikely that a massive concentration of nickel and iron, if that’s what it really is, right beneath one of the Solar System’s most significant impact craters, without the two being connected.

The picture showing another image of the South Pole-Aitken basin from Japan’s Kaguya probe. Credit: Ittiz – Own work, CC BY-SA 3.0, wikimedia.
The picture showing another image of the South Pole-Aitken basin from Japan’s Kaguya probe.
Credit: Ittiz – Own work, CC BY-SA 3.0, wikimedia.

So, if it really was the aftermath of a huge asteroid impact, when did it occur? And as James mentioned in Universe today, – We don’t know, but if our Moon was too hot at the time of the asteroid impact, the dense metal from the asteroid core presumably should have sunk all the way down to the lunar’s core, in which case we wouldn’t see it.

The massive preserved crater rim around the South Pole-Aitken basin provides important confirmation for establishing the time of the asteroid impact. And as James further told Universe Today, – We too wouldn’t see a preserved rim if the South Pole-Aitken basin formed in a magma ocean. Consequently, the Moon likely had a chance to cool slightly before this asteroid impact happened.

Picture showing a mantle mass excess per unit area. A huge excess of mass in the southern interior of the South Pole-Aitken basin corresponds with the central depression, outlined here with a dashed gray circle. Black lines mark the best fit ellipses for the South Pole-Aitken basin’s inner ring, outer ring, and exterior scarp. Credit: James et. al., 2019.
Picture showing a mantle mass excess per unit area. A huge excess of mass in the southern interior of the South Pole-Aitken basin corresponds with the central depression, outlined here with a dashed gray circle. Black lines mark the best fit ellipses for the South Pole-Aitken basin’s inner ring, outer ring, and exterior scarp. Credit: James et. al., 2019.

The South Pole-Aitken Basin Crater is Different Than Other Craters on The Moon.

The South Pole-Aitken basin is different from many other impact craters on the Moon. Other craters on the Moon have a bulls-eye pattern of gravity anomalies, but the South Pole-Aitken basin doesn’t. James told Universe Today that the South Pole-Aitken basin – has a broad region of low gravity associated with a topographic depression that is produced by the dense mantle anomaly weighing down the lunar surface.

Additionally, according to James, the South Pole-Aitken Basin is one of the best natural laboratories for examining catastrophic asteroid impact events, an ancient process that shaped all of the rocky planets and moons we see today.

Unlocking the secrets of the South Pole-Aitken basin will need more work. Ruling out either oxide from magma solidification or an asteroid impact as the cause of the anomaly will require enhanced simulations. 

Finally, as James told Universe Today, – The most reliable way to rule out one scenario or the other would be to make newer and better simulations.

Thanks for reading this article. If you want to know more secrets about our Moon, then head over to this interesting article: 11 Things You Didn’t Know About the Moon.

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