A large gravitational anomaly has been discovered beneath the surface of the moon by researchers at Baylor University in Texas.
The anomaly was discovered when the scientists measured subtle changes in the strength of gravity around the moon, analysing data collected from NASA missions.
Researchers suspect it may be caused by a mass of metal which originated from deep space being buried beneath the moon’s surface and weighing it down.
“Imagine taking a pile of metal five times larger than the Big Island of Hawaii and burying it underground,” said the study’s lead author Dr Peter James.
“That’s roughly how much unexpected mass we detected,” Dr James said, explaining the research published in Geophysical Research Letters.
The metal, if that is what is causing the extra mass, is located on the moon’s South Pole-Aitken basin, an enormous crater at the bottom of the planet.
The crater – which is where the Chinese lunar explorer Chang’e 4 landed in January – is 1,600 miles (2,000km) wide, 8.1 miles (13km) deep and is believed to be the largest crater in our solar system.
But despite its enormity, the crater cannot been seen from Earth because it is located on the far side of the moon.
“When we combined [the gravitational data] 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,” Dr 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 dense mass – “whatever it is, wherever it came from” – is so heavy it is actually weighing the basin floor downward by more than half a mile, according to Dr James.
The team ran complicated computer simulations of large asteroid impacts which suggested that – under the right conditions – an asteroid which had an iron-nickel core could have dispersed into the moon’s upper mantle during an impact.
“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,” Dr James said.
But the team has considered other hypotheses too, including a concentration of dense metals which pooled when the molten moon began to solidify billions of years ago.
The South Pole-Aitken basin was created around four billion years ago according to Dr James.
It is the largest preserved crater in the solar system. Other larger impacts may have taken place, including on Earth, there is no lasting trace of them.
Dr James called the moon’s basin “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”.