While we can only see one “face” on the Moon from our vantage point here on Earth, the far side is actually quite different. Observations have shown that the “back” of the Moon has a thicker crust and an extra layer of material compared to the side that faces Earth. Now, simulations have revealed one possible, explosive explanation – an impact with a huge object early on in the Moon’s life.

NASA’s GRAIL orbiters spent a little over a year whizzing around the Moon, mapping out the strength of its gravity and studying its interior composition. This data showed the stark difference in structure between the near and far sides of our natural satellite.

A long-standing hypothesis for why the Moon is so asymmetrical is that at some point in its past, it collided with a large object, such as an asteroid or even a dwarf planet. If the smash up was big enough and fast enough, this could have sent fragments of the Moon’s crust flying off one side and settling on the other.

The differences in topography, crust thickness and thorium distribution between the near and far sides of...

Using GRAIL data as a starting point, the team used computer simulations of impacting objects of different sizes and traveling at different speeds, to see which scenario – if any – best explained the observations.

After running 360 simulations, the team pinpointed the scenarios that came closest to the real-life structure of the Moon. The best fit was that an object about 480 mi (780 km) wide smashed into the near side of the Moon at 14,000 mph (22,500 km/h). Or, a slightly-smaller object – measuring 450 mi (720 km) wide – could have had a similar effect if it was moving a little faster, say 15,000 mph (24,500 km/h).

In either case, the simulations showed that the debris thrown to the far side of the Moon would add an extra 3 to 6 mi (5 to 10 km) of material on top of the crust. And according to GRAIL, that’s how much is over there.

The process of how an impact by a large object would have spread material to the...

The thickness of the crust isn’t the only evidence for this smash-up theory. The Moon has far higher amounts of certain potassium, phosphorus and tungsten isotopes than Earth, which shouldn’t be the case if the two objects have the same origin, as is believed. These extra elements could have come from the impactor itself, the team suggests.

As with any science dealing with the distant past, astrophysics and computer simulations, this matter is far from settled. But it’s a fascinating theory nonetheless.

The research was published in the Journal of Geophysical Research: Planets.

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