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SYFY WIRE Bad Astronomy

Did a whopping huge impact at its south pole cause our Moon to be two-faced?

Immense south polar impact may have rewritten the Moon's surface.

By Phil Plait
A topographical map of the Moon’s south polar region

For all of human history until 1959, the far side of the Moon was invisible to our eyes. The Moon rotates once every time it orbits the Earth once, and this synchrony keeps one side of the Moon pointed toward us and the other forever pointed away.

But then the Soviet Union sent the Luna 3 spacecraft into orbit around the Moon, and for the first time we got a glimpse of the landscape that until then was synonymous with “mysterious and hidden.” It was shocking: It was completely unlike the near side! The side we can see has two major components: Rough, cratered highlands, and smoother, much darker low areas, the latter created from lava flowing across the surface. These look like bodies of water from Earth, so they are called maria (singular; mare), meaning seas.

But the far side has only a single, small mare, and the rest is completely covered in craters. Years later, as technology improved, other differences were found. The near side has much more of the elements thorium and titanium than the far side, as well as what’s called KREEP terrane: That stands for potassium (the symbol for which is K), rare-Earth elements, and phosphorus. Eventually it was discovered that the crust on the far side is much thicker than on the near side as well.

Near (left) and far (right) sides of the Moon

Over the years some pretty clever ideas have been proposed to explain this. One is that after a Mars-sized protoplanet whacked the Earth hard enough to blast enough material into orbit to coalesce and form the Moon, it actually formed two moons; a big primary one and a smaller one. The second one eventually impacted the Moon, forming the thicker crust on the far side. Another is that when the Moon formed it was so close to Earth that the still-hot-from-the-giant-impact planet heated it, causing material to flow around to the Moon’s far side and condense, thickening the crust. In both cases this is then linked to more volcanism on the near side, which changed the elemental abundances.

A new idea has just been published, though, that is fairly different: The planetary scientists implicate a massive impact on the Moon itself that was so huge it changed the way the Moon’s hot mantle flowed, creating the difference in hemispheres’ surface mineral composition [link to paper].

In the southern part of the lunar far side is an immense basin, called the South Pole-Aitken (or SPA) basin, the result of an impact so huge it staggers the imagination. It’s approximately 2,500 kilometers across — over half the width of the United States! The impact that caused it must have been simply apocalyptic. It’s one of the largest impact basins in the solar system.

In the new work, the scientists wondered if this enormous event could somehow be related to the near/far side chemical difference. The timing was about right; the impact occurred roughly 4.3 billion years ago, around the time the maria started repaving the lunar near side. The material brought up in the maria volcanism came from the Moon’s mantle, the hot fluid rock beneath the crust, so the scientists focused their attention there.

South Pole-Aitken basin impact on the Moon

Using physical models of heat flow through the Moon after the impact, they simulated different scenarios after the giant SPA impact. What they found is pretty interesting: Under pretty much any realistic conditions, the impact generated a vast plume of heat that moved through the Moon’s interior, changing the way the mantle flowed. 

A huge pulse of heat would have moved to the opposite side of the Moon — technically, the antipode — and sequestered a lot of KREEP there. Potassium has an isotope that’s radioactive, and other radioactive elements such as thorium and uranium are associated with KREEP material as well. This would have heated the underside of the crust there, which led to a lot — a lot — of volcanism. This spot on the Moon is in the sprawling Oceanus Procellarum, a very large mare in the lunar northwest that’s the most obvious lunar feature you can see by eye on the full Moon. This region is known for having a lot more thorium and titanium (which is also associated with KREEP), so that fits. 

Not much later, about 3.9 billion years ago, another huge impact blasted out Mare Imbrium, one of the last of the giant impacts on the Moon. That too let loose a lot of KREEPy stuff from under the surface, further modifying the surface. 

A topographical map of the Moon’s south polar region

This is still hypothetical, but the science fits, and I have to say it doesn’t involve any special circumstances; we know the South Pole-Aitken basin exists, and was big enough to affect the entire Moon. That’s an added benefit on top of the idea of the physics working out as well; scientists like hypotheses that use what we already know and don’t ask for any one-off occurrences. The hypothesis is agnostic about the difference in crustal thickness, though it’s possible some other physics was involved that could lead to that as well.

A lot of this evidence, like KREEP minerals, was discovered during the Apollo era exploration of the Moon, and of course this entire idea was started by the Soviet orbiter missions. We still don’t really understand the Moon’s origin and evolution over the past 4.4 or so billion years, but we’re about to embark on a new age of lunar exploration. What else are we about to learn?