Three hundred kilometres below the surface of the moon lurks something massive — and scientists aren't completely sure what it is.

According to a study published in the journal Geophysical Research Letters, the mass sits below the moon's South Pole-Aitken basin (SPA), a huge, oval-shaped impact crater on the far side of the moon that is 2,000 kilometres wide and several kilometres deep. (By comparison, the moon's circumference is roughly 11,000 kilometres.) The SPA is also the oldest basin on the moon, formed four billion years ago when something slammed into the celestial body.

Using data collected by NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission and the Lunar Reconnaissance Orbiter, scientists found that there is extra mass in that basin — and a lot of it.

"We estimate that the minimum mass is something in the order of 2x10¹⁸ kilograms," said Paul Byrne, the paper's co-author and an assistant professor of planetary geology at North Carolina State University. "Which is like two quadrillion tons."

Though researchers believe they know its mass, they don't know how large it actually is. And as for how it got there, they're pointing the finger at whatever created the basin in the first place.

"The single best explanation, I think, at least right now, is that it's the remnant core or chunk of whatever slammed into the moon to make SPA," said Byrne.

Scientists suggest a rock struck the moon roughly four billion years ago, when the solar system was in its infancy. But instead of sinking to the core, the rock remained closer to the moon's mantle.

In fact, Byrne cites a recently published paper in the journal Nature that suggests the Chinese lunar lander, Chang'e, launched earlier this year, found evidence of the moon's mantle at the surface, which would have been ejected by the collision.

This illustration details the structure of the moon. (Wikimedia Commons/Kelvinsong)

But there could be another explanation: A mass concentration of dense oxides that were left over after the moon became solid. As the magma ocean cooled, heavier material settled below.

Byrne said that although it's a possibility, it doesn't quite fit: The magma ocean would have been global, so it would be puzzling for the oxides to have settled in only one particular place.

The only way to know for sure, though, is to send a lander or people to further study the basin.

Understanding Earth

During the early formation of the solar system, Earth would have been as similarly bombarded as the moon. But because of plate tectonics and weather, there's no evidence of it. That's why it's important to study bodies like the moon, Mercury and Mars to better understand what was going on four billion years ago.

And while scientists have studied craters on Mercury and Mars, the best place to turn to when attempting to unravel the mystery of the early solar system is to look to the closest thing to Earth.

"The moon is like a time capsule for events that happened on Earth, because it's the closest celestial body to Earth. And because they're in such close proximity, whatever happened to the moon must have happened to Earth as well, in one way or another," said Sara Mazrouei, a planetary scientist with Western University's Centre for Planetary Science and Exploration (CPSX) who was not involved in the study.

"By understanding the moon, we get a much, much better understanding of Earth's past as well."

Mazrouei is familiar with the SPA; she worked on a project at the Lunar and Planetary Institute in Houston, Texas, a few years ago, picking the best landing sites for possible future missions.

"I know all the cool spots to go to," she said with a laugh.

A mosaic of the moon from the Lunar Reconnaissance Orbiter, centred in the middle of the South Pole-Aitken basin. (NASA/GSFC/Arizona State University)

According to Mazrouei, this new research makes the SPA even more alluring. "It just makes a much, much stronger case — not that we need a stronger case — of why we need to go to the SPA," she said.

That might be a real possibility: Just two months out from the 50th anniversary of the Apollo 11 moon landing, NASA announced this past May that it is planning to return to the moon in 2024.

While this new research is an important step, Byrne said he'd like to see landers, or people, go to the region to study it, as it's important to understanding our local planetary neighbourhood.

"One of the biggest values of this is not just understanding this fascinating feature on the moon — although that's in and of itself really interesting — but it really does help us understand more broadly what it means for large impacts and the role they have in shaping the planets in the solar system."