When we look at the two sides of the Moon, they look so different as to appear to be different worlds. The near side is a mix of light regolith and vast seas of dark mare that people often see as the “Man in the Moon”. The far side – the side we don’t get to see as the Moon goes around and never shows us its back – that far side is just craters next to craters, all in the lighter colored material.

How the Moon ended up this way is a matter of debate. The Moon’s high-density core is off-center, which is also weird. To try and piece all this weirdness together, we have to look at the history of our Earth-Moon system. 

Once upon a time, in the early days, a Mars-sized object hit what was trying to become Earth. In the collision, both worlds were destroyed, and a bunch of their lighter crust materials splashed away while the two higher density cores merged together to create our planet Earth. As the lighter material orbited the Earth, it somehow eventually ended up in a single Moon, but it may have done this in two steps, with the bulk of the Moon coming together and then getting hit by a smaller pocket of leftover material that we now see as the Moon’s far side. This gives us a layer of material to make the Moon’s core appear off-center, and it may explain the difference in appearance, but the details don’t all just come together, and when scientists see inelegant data, they often go looking for a better solution.

And that better solution may have just been published.

In a new paper in Science Advances that is led by Matt Jones, researchers describe simulations of the impact that created the Moon’s south pole’s Aiken Basin. This is the second-largest crater in the solar system, and when the Moon was struck, the energy of the impact was transformed into heat, and that heat, the models say, traveled through the Moon. With the heat, there was melting and the movement of material through the lunar interior. Another way of saying that is the asteroid impact created magma – the stuff of volcanoes – and it flowed through the Moon carrying the specific chemistry of material we see in the lunar lava fields.

As Jones says: How the [chemistry of the lava] formed is arguably the most significant open question in lunar science. 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.

I have to say I think this is exciting, too, and I love how impacts are both digging down deep and building up the surface of the Moon in different ways.

More Information

Brown University press release

“A South Pole–Aitken impact origin of the lunar compositional asymmetry,” Matt J. Jones et al., 2022 April 8, Science Advances