You can only see one face of the Moon on Earth. Its cheek is turned away from our planet every year, and this far side is much more pockmarked with craters than the one facing us.

When we look up at our satellite, there are dark patches on the lunar maria, which are vast plains of volcanic basalt. The reason for this two-faced appearance has remained a mystery since the first mission to the Moon in the 1960s. A new simulation might have solved the Apollo-era puzzle.

Computer models support the idea that a massive lunar impact once resurfaced the Moon's near side in lava flows.

Astronomers have long believed that the near side was once covered in a sea of magma that smoothed the rocky landscape. The triggering for this volcanic activity is controversial.

There is a huge crater at the South Pole that could explain the differences.

The basin is a remnant of a large collision on the Moon. Simulations show that the SPA event occurred at the right time and place to initiate changes to the moon's mantle.

Experts think that the impact would have warmed the upper mantle to such an extent that it would have led to a concentration of elements like thorium.

The composition scientists have found in lunar rock samples from the near side is exactly what they have been looking for.

The planetary scientist Matt Jones from Brown University says that under any plausible conditions at the time that SPA formed, it ends up concentrating these heat-produced elements on the nearside.

We expect that mantle melting contributed to the lava flows we see on the surface.

The aftermath of the event would last hundreds of millions of years.

The most ancient volcanic plain erupted 200 million years after the impact events. Up to 700 million years after the impact, volcanic activity continued on the near side of the Moon.

The cheek of the Moon reacted to the hit more because of the location of the impact and the slight changes in gravity.

The upper mantle in the southern hemisphere heated up and began flowing toward the northern hemisphere.

The upper mantle on the far side was too cool to distribute the same material in a similar fashion.

The Moon's two faces were asymmetrical because of this difference.

The most significant open question in lunar science is how the PKT formed.

The Aitken impact is one of the most significant events in lunar history. I think our results are really exciting, because this work brings those two things together.

Science Advances published the study.