Key Takeaways:

  • The South Pole-Aitken Basin on the far side of the Moon is the largest crater in the Solar System.
  • Scientists found a massive structure (possibly metallic) buried 300 kilometers deep under the basin.
  • The mass is believed to be leftover from the asteroid that created the crater 4 billion years ago.
  • The discovery was made possible by NASA’s GRAIL mission which studies the Moon’s gravity.
  • The South Pole-Aitken Basin is a valuable area for studying the Moon’s history, composition, and impact events.
This false-color graphic shows the topography of the far side of the Moon. The warmer colors indicate high topography and the bluer colors indicate low topography. The dashed circle shows the location of the mass anomaly under the basin. NASA/Goddard Space Flight Center/University of Arizona

Our own Moon has one of the largest preserved craters in the Solar System, the South Pole-Aitken Basin, located on the far side of the satellite. The area is at the center of many investigations, with various orbiters and even the Chinese lander Chang’e-4 studying the region. Under its surface, scientists have now found something strange.

A structure that stretches more than 300 kilometers (186 miles) in depth and weighs 2.18 billion billion kilograms has been found by planetary scientists. The team speculates that the mass may have come from the asteroid that created the crater, as stated in a report published in the journal Geophysical Research Letters.

“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,” lead author Peter B. James, from Baylor University, said in a statement.

NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission, which tracks tiny shifts in the Moon’s gravitational field, made the discovery possible. We can examine the internal composition of our natural satellite using these observations. It turns out that the mass they measured is sufficient to cause the basin floor to drop by over half a mile, or nearly a kilometer. Considering the crater’s approximate diameter of 2,500 kilometers (1,550 miles), that is quite the pull.

“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 explained. “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 anomaly was explained by computer simulations that the team ran. It’s possible that instead of sinking into the core, the asteroid stayed embedded in the mantle when it struck about 4 billion years ago. A different theory focuses on the Moon’s solidification and hypothesizes that the concentration of dense oxides developed as the magma ocean cooled and settled.

Interest in the South Pole-Aitken Basin from various space agencies is due to how special it is. This area can be used for studying the Moon’s history as well as its interior composition. It’s also the best lab to study the aftermath of a catastrophic impact on a rocky planet’s surface.

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