Astronomers Find a “Massive, Dense Structure” Beneath the Largest Crater on the Moon

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

Astronomers have found a strange, humongous, deep-mass structure beneath the largest crater in our solar system; the Moon’s South Pole Aitken basin.

There, researchers discover an anomaly with a massive unexpected mass. Although it’s not aliens, astronomers say that the mysterious mass may very well contain the metallic remnants from the asteroid that slammed into the Moon, forming the crater.

“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.


This photo was taken by Apollo 8 during the 1968 mission on their first approach to the Moon. Credits: NASA.

An oval crater

The crater located on the far side of the Moon is an oval-shaped region around 2,000 kilometers wide.

Measurements throughout the years have revealed it is several miles deep.

The new, anomalous mass beneath the crater is detailed in the study “Deep Structure of the Lunar South Pole-Aitken Basin,” published in the journal Geophysical Research Letters.

To spot the anomaly, astronomers analyzed data from NASA’s Grail Mission, which allowed them to accurately measure gravity strength changes around the Moon.

“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.”


The location of the underground structure circled in the South Pole-Aitken Basin. Image Credit: NASA/Goddard Space Flight Center/the University of Arizona.

Mysterious structure

The researchers revealed that the dense mass of the structure –“whatever it is, wherever it came from” is causing the entire basin floor to weigh in by more than half a mile.

The researchers’ team revealed through computer simulations of asteroid collisions into the Moon that, under the right conditions, iron-nickel cores from asteroids could have been dispersed into the Moon’s upper mantle during collisions.

“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 explained.

In addition to coming from asteroid impacts, astronomers say that the mysterious structure’s origin could be related to the concentration of highly dense oxides left there by the last phases of lunar magma ocean solidification.

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