How an Ancient Asteroid Split The Moon in Two | Unveiled

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Why are there more craters on the far side of the moon?? Join us... to find out!

In this video, Unveiled takes a closer look at the new theory on why the two sides of the moon are so radically different from one another. The near and far (or dark) side of the moon share some similarities but the far side has considerably more craters... and now, scientists believe that they've finally worked out why!

How an Ancient Asteroid Split the Moon in Two

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.

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.