Scientists can't explain mysterious large mass discovered on Moon

Rodiano Bonacci
Giugno 11, 2019

James and his team looked at data from NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission and the changes in gravity they discovered surprised them.

The anomaly - "whatever it is, wherever it came from", James added - is weighing down the basin floor by more than half a mile. Essentially, something caused a giant hole on the Moon billions of years ago, and astronomers have just discovered that there's something big - really big - buried underneath the surface.

"[The basin is] 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", James said. Despite its size, it can not be seen from Earth because it is on the far side of the Moon. "That's roughly how much unexpected mass we detected". At about 2,000 km wide the South Pole-Aitken basin is the largest crater known to scientists, with the newly-discovered mass underneath it being large enough to affect the moon's gravity.

In the new study, the researchers ran impact simulations to show that this underground debris could, theoretically, be the remnants of a heavy iron-nickel asteroid that got stuck part way to the lunar core.

But when the team compared these findings with the lunar topography data collected by NASA's Lunar Reconnaissance Orbiter, the results showed something else: a mass of about 2.18 quintillion kilograms (that's a number with 18 zeroes), extending more than 300 kilometres (184 miles) below the surface.

Researchers say it could be a huge lump of metal from the asteroid that formed the South Pole-Aitken basin.

The South Pole-Aitken basin was created around four billion years ago according to Dr James.

According to the published study, "Plausible sources for this anomaly include metal from the core of a differentiated impactor or oxides from the last stage of magma ocean crystallization", which hypothesizes the moon's surface was once a molten liquid ocean of magma.

Whatever formed the basin almost 4 billion years ago remains a mystery, but the blow was so strong that it likely punched all the way through the moon's crust and tossed part of the lunar mantle - a deeper geologic layer - onto the surface.

"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", lead author of the paper, Peter B. James, Ph.D., said in a statement. The Lunar Reconnaissance Orbiter has spent almost 10 years at work and has made billions of measurements of the precise height of the moon's surface.

The colliding body is sometimes called Theia, after the mythical Greek Titan who was the mother of Selene, the goddess of the moon.

Several different theories have emerged over the years to explain the similar fingerprints of Earth and the moon.

A third possibility is that the moon formed from Earthen materials, rather than from Theia, although this would have been a very unusual type of impact.

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