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Moon Formed In Interplanetary Hit-and-Run Incident, Say Astronomers

A new theory of lunar formation suggests Earth was the victim of a hit-and-run incident–and that the culprit may still be at large

The origin of the Moon is one of the more important problems for planetary geologists and in recent years, they’ve made giant strides in understanding how it happened. That’s largely because of a much improved understanding of the Moon’s composition an interior structure.

It turns out that our interplanetary companion has a similar composition to Earth, including an iron core. The consensus is that this rules out the possibility that the Moon formed elsewhere and was later captured by the Earth’s gravity. Instead, it must have formed from the debris created by giant collision between the Earth and a Mars-sized body. 

Astronomers believe this collision must have occurred at a slow relative velocity and a shallow angle to ensure that the debris entered orbit around Earth and stayed there, eventually forming the Moon. 

Such a slow impact collision would also have preserved the angular momentum of the system too. This places an additional constraint on the bodies before impact, since they cannot have had an angular momentum much higher than the Earth-Moon system today.

Computational astronomers have simulated this kind of slow, grazing impact in detail and shown that an Earth-Moon system could indeed have formed given the theory’s constraints on mass and angular moment.

But there is a problem with this model. The silicate surfaces of both the Moon and the Earth have a similar isotopic signature indicating that they must have formed from the same stuff. 

But in a slow, grazing impact, most of the debris that ends up in orbit and forms into the Moon comes from the Mars-sized impactor, which is unlikely to have had the required isotopic signature. That’s a major problem.

Today, Andreas Reufer at the University of Bern in Switzerland and a few pals say they’ve come up with an alternative hypothesis that fixes this problem. They say the Earth must have been grazed by a larger object travelling at much higher velocity. 

This extra speed caused much of the impact debris to escape, hence the hit-and-run moniker. However, the debris that became trapped in orbit would have been a mixture of Earth and impactor material with an isotopic content that matches the observed signatures here and on the Moon.

Of course, the debris that escaped would have carried away angular momentum as well as mass. This makes such a scenario challenging to model because it is hard to find a suitable set of starting conditions–mass, angular moment, impact angle etc–that produce a realistic Earth-Moon system. In fact, astronomers have discounted this scenario in the past for precisely this reason. 

But with improved simulation techniques, Reufer and co have revisited the scenario. They’ve used a model consisting of about 500,000 particles in which the Moon ends up being made of some 10,000 particles. That’s more or less state of the art. 

They say it can produce Earth-Moon-like systems for a reasonable set of starting conditions, while at the same time reproducing the observed isotopic signatures. 

That’s interesting work, not least because it has important implications for the conditions on Earth at the time of the impact, about 4.5 billion years ago. In particular, Reufer and pals say the more energetic impact would have heated the Earth’s mantle to temperatures of about 10,000 Kelvin and heated the debris that formed the Moon to higher temperatures too.  

This clearly has implications for the early history of both the Earth and the Moon and may help lead to additional evidence that could back up this hypothesis.

But it also leaves an interesting unanswered question–what happened to the impact debris that escaped from Earth orbit? These guys ought to be able to calculate the properties of this stuff, perhaps even its isotopic signature. That would allow astronomers to hunt for it, perhaps by examining meteor falls from the relatively recent past.

Finding evidence of the culprit would be an important clue, which means there’s some fascinating detective work ahead!

Ref: A Hit-And-Run Giant Impact Scenario

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