Scientists studying the far side of the moon have discovered a massive underground structure buried beneath a crater on the moon. Stretching 2,000 kilometers, the Moon’s South Pole-Aitken basin is the largest preserved crater in the solar system but cannot be seen directly from Earth. Nevertheless, analysis suggests that beneath the lunar surface lies a deposit of heavy metal so large it’s distorting the moon’s gravitational field.
A new study—Deep Structure of the Lunar South Pole-Aitken Basin—published in the journal Geophysical Research Letters, describes the discovery. Researchers used data from orbital spacecraft servicing the National Aeronautics and Space Administration (NASA) Gravity Recovery and Interior Laboratory (GRAIL) mission, as well as topography data from the Lunar Reconnaissance Orbiter. Their data showed that approximately 186 miles underneath the crater, an enormous heavy metal structure weighing 2.4 quadrillion US tons is further distending the basin by more than half a mile.
The study’s lead author Peter B. James offered a description of how much material is present:
“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.”
Despite the sensational headlines that can result when discussing an underground metal structure on the moon, scientists say that the origin of the deposit is likely the leftover iron-nickel core of the asteroid which struck the lunar surface 4 billion years ago and left the enormous south pole crater. Computer simulations show that the metallic remnants of some asteroids can be subsumed into a planetary body’s mantle.
“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.
Another possibility is a concentration of dense oxides resulting from late-stage lunar magma ocean solidification.
Scientists believe this deposit—and the crater it lurks beneath—could offer a rare opportunity to research asteroid collisions. James called it “one of the best natural laboratories for studying catastrophic impact events, an ancient process that shaped all of the rocky planets and moons we see today.”
Other scientists say the discovery offers evidence that the inside of the moon is more solid than previously thought, otherwise the metal deposit would be gravitationally pulled further into the core.