Ever since the dawn of humankind, the moon has been a captivating presence.
We’ve marveled at it suspended in our sky, wondered what its purpose is, and wondered
how it came to be. In this, our space age of discovery, however, we’ve gotten to know the moon
better than ever before thanks to modern science and technology. But still, the apparent split
between the near side we continually see and the far (or “dark”) side of the moon is something
that has kept us guessing. So, this is Unveiled,
and today we’re exploring the extraordinary story of how an ancient asteroid split the moon in two.
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We know full well how integral asteroid impacts have been in the shaping of the history of Earth…
with the last major asteroid collision famously signaling the end of the dinosaurs and the rise
of the mammals. But, actually, in comparison to our own world, the moon has withstood a far
greater barrage of space rock. It’s why the lunar surface is so pockmarked with craters, scattered
like deep scars to show the cosmic turmoil that the moon has experienced. But, still, whenever
we look at the moon, we’re only ever seeing one side of the story. Because the moon is tidally
locked to Earth, we always see the same face of it… which means that there’s always another face,
looking out away from us, into the rest of space. It’s commonly known as the “dark” side of the
moon, although scientists prefer the term “far” side… because, really, it’s not especially dark
at all. And, contrary to popular belief, the far side is no longer completely hidden to us,
either, thanks to various space missions (fronted by America, Russia, and China, especially) that
have sent probes over to the “other side” to beam back images and data. One thing we know
for sure about the far side, then, is that for all the impact craters there seems to be on the
near side of the moon as we look at it… there are many, many more across the part we cannot see.
The impact craters aren’t fairly spread across the moon, with the far side hosting hundreds more of
them than the near side does. And, for decades, this has puzzled scientists and astronomers.
Why should that be? Why would what’s essentially a spherical object be more prone to asteroid
collisions in certain areas, but not others? If the findings of a new paper ring true, however,
then that puzzle may have finally been solved. Details of a multi-authored study led by members
of Brown University (and with contributions by NASA) were published in the journal “Science
Advances”, in April 2022. The study focuses on what it calls “A South Pole-Aitken impact
origin of the lunar compositional asymmetry”. In other words, the “lunar compositional asymmetry”
is a reference to the moon’s uneven crater distribution between the near and far sides…
while the “South Pole-Aitken impact origin” is what researchers think is to blame for that.
The South Pole-Aitken basin is one of the largest known craters not just on the moon,
but in the entire solar system. It’s some 1,600 miles across at its widest point,
and it was formed around 4.2 billion years ago, close to the beginning of the moon’s life. While
most of this colossal structure isn’t visible to us on Earth, however, because it stretches onto
the moon’s far side… one edge of the basin skirts the lunar south pole, the moon’s most southerly
point. This means that a small portion of the South Pole-Aitken basin is just about visible
for us, rising on the edge of the moon (as we look at it) before dipping out of sight.
But, why’s this region especially important for this particular video? Well, the April 2022 paper
suggests that the impact that caused the South Pole-Aitken basin may have had an even deeper
and more fundamental effect on the moon, beyond just the crater itself. Up until the impact,
it’s thought that the moon’s mantle - just below the lunar surface - was reasonably evenly spread.
Its geochemical makeup was balanced all over, and so the subsurface conditions within the
mantle anywhere on the moon were fairly uniform. But, after the impact event, that all changed.
It's thought the asteroid that caused the South Pole-Aitken basin also triggered a
disturbance or surge in the moon’s mantle, which resulted in a massive rise in volcanism
on the near side. This immense shift led to the creation of the lunar maria, which are the
dark patches we can see splodged all across the moon, all the time. In days gone by, astronomers
believed the maria to be literal seas… but now we know that they’re essentially lava pools,
long cooled off and darkened into the shadowy shapes we see today. But, if we were to view
the far side of the moon, we’d find many, many fewer examples of these lunar maria. There are
more craters back there, but there are also fewer lava leftovers. And that’s important because it’s
the lava pools - the maria - that have essentially filled in the craters on the nearside. And that’s
why there appears to be far fewer craters on one side of the moon, compared to the other.
So, to recap. The model put forward in the April 2022 paper says that when a massive asteroid
struck the moon some 4.2 billion years ago, creating the South Pole-Aitken basin, it also
triggered a surge in the moon’s mantle, which led to more volcanism on the near side specifically.
This volcanism then led to the formation of the lunar maria, which filled in many of the craters
that had been on the near side… and that’s why the near and far sides look so different nowadays.
Geologically speaking, they host wholly different conditions as a result of intrinsically different
histories, both of which were set in motion by something of a ‘sliding doors
moment’ in the solar system’s early timeline. In a statement released by Brown University
relating to the paper, study lead Matt Jones explains that he and his team ran a series of
digital simulations to calculate all the ways the South Pole-Aitken impact could’ve unfolded.
And, in all of them, the same skewing of the lunar mantle toward the near side of the moon was found.
“What we show,” said Jones, “is that under any plausible conditions at the time that South
Pole-Aitken formed, it ends up concentrating heat-producing elements on the nearside”.
He goes on to explain his team’s expectation that this “contributed to the mantle melting that
produced the lava flows we see on the surface”. It might seem bizarre that one event literally
billions of years ago could have such dramatic long-term consequences… but, really,
this type of profound realization isn’t unusual for astronomers. When we look into our planet,
our moon, or the solar system, we’re looking at a system that’s never stood still. It’s
always been on the move… and it shows no sign of slowing down any time soon.
For the billions of years of its life, there have been countless collisions, crashes,
moments of creation, and moments of destruction that have specifically shaped the future from
that point forwards… and the South Pole-Aitken impact event is one of the most significant
formative moments for the moon, in particular. Going forwards, though, it’s not as though
everything about this lunar region is now solved. In fact, the South Pole-Aitken basin remains one
of the most contested, controversial, and crucial areas of the moon, in general. In mid-2019,
news broke of the discovery of a large, mystery mass lurking beneath the crater - said to be five
times bigger than Hawaii’s Big Island. At present, we’re not exactly sure what it is… although
most theories tend toward it being some kind of metallic leftover from the asteroid that started
it all. Regardless, it’s a sure bet that this part of the moon will be one of the most studied,
as we move into a new era of space travel and technology in which (one day) it’s hoped that
we will return to the lunar surface, ourselves. When that day comes, perhaps we’ll be advanced
enough to stick around this time. To set up moon bases, and stage missions that last for weeks or
months, and not just a few days. The Apollo moon landings of the 1960s and ‘70s remain some of the
most iconic moments in modern human history… but all signs are that we’ll be adding to that history
soon. And, if we do ever set foot back there, then perhaps a journey to the far side will be at the
top of the agenda. Because at least, now, when we get there, we’ll have a better understanding
of just why it looks so strange. And that’s how an ancient asteroid split the moon in two.
What do you think? Is there anything we missed? Let us know in the comments, check out these other
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