from
https://www.sciencealert.com/the-mystery-of-the-moon-s-two-faced-nature-could-finally-be-solved

The Far Side of The Moon Is Significantly More Cratered. We May Finally
Know Why
CARLY CASSELLA11 APRIL 2022
No matter where you're standing on Earth, you can only ever see one face
of the Moon. Its other cheek is perennially turned away from our planet,
and this far side is much more pockmarked with craters than the one
facing us.

The Moon's near side is covered in lunar maria, vast plains of volcanic
basalt that appear as dark patches when we look up at our satellite. The
reason for this two-faced appearance has remained a mystery – one that's
persisted since the first spacecraft orbited the Moon in the 1960s. But
a new simulation might now have solved the Apollo-era puzzle.

Piecing together the different features, computer models support the
idea that a massive lunar impact once resurfaced the Moon's near side in
lava flows. The differences are more than skin deep, since they're also
reflected by distinct geological compositions on each side of the Moon.

Astronomers have long suspected the near side was once covered in a sea
of magma that, when cooled, smoothed the rocky landscape, creating the
dark blemishes we see today. But the trigger for this volcanic activity
is contentious.

A massive crater at the Moon's south pole, known as the South
Pole–Aitken basin (SPA), could explain the differences.

This basin is a remnant of one of the largest and oldest collisions on
the Moon. Simulations show the SPA event, which happened about 4.3
billion years ago, occurred at just the right time and the right place
to initiate changes to just one side of the lunar mantle.

The immense heat produced by the impact would have warmed the upper
mantle on the near side to such an extent, experts think it would have
led to a concentration of potassium, rare earth elements, phosphorus,
and heat-producing elements like thorium.

To date, that's exactly the composition scientists have found in lunar
rock samples from the near side, especially in the Procellarum KREEP
Terrane (PKT), a large area known for this compositional anomaly.

"What we show is that under any plausible conditions at the time that
SPA formed, it ends up concentrating these heat-producing elements on
the nearside," explains planetary scientist Matt Jones from Brown
University.

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

The fallout of the SPA event would probably have lasted for hundreds of
millions of years.

In simulations, the most ancient nearside volcanic plain erupted 200
million years after the impact events. In fact, intense episodes of
volcanic activity continued on the near side of the Moon for up to 700
million years post-impact.

According to experts, the reason this cheek of the Moon reacted to the
hit more is both because of where the location of the impact centered
the transport of heat-producing materials, and also because of slight
changes in gravity.

In every scenario researchers examined, the upper mantle in the southern
hemisphere heated up and began flowing toward the northern hemisphere,
traveling via the near side.

Meanwhile, the upper mantle on the far side remained too cool to
distribute the same material in a similar fashion.

This difference could very well have generated the asymmetry observed in
the Moon's two faces.

"How the PKT formed is arguably the most significant open question in
lunar science," says Jones.

"And the South Pole–Aitken impact is one of the most significant events
in lunar history. This work brings those two things together, and I
think our results are really exciting."

The study was published in Science Advances.