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Moon formation: Was it a 'hit and run' accident?


Computer simulations suggest a large, fast-moving body impacted the Earth to create the Moon.

Scientists have proposed a fresh idea in the long-running debate about how the Moon was formed.

What is certain is that some sort of impact from another body freed material from the young Earth and the resulting debris coalesced into today's Moon.

But the exact details of the impactor's size and speed have remained debatable.

In a report online to be published in Icarus, researchers suggest that the crash happened with a much larger, faster body than previously thought.

Such theories need to line up with what we know about the Moon, about the violent processes that set off the creation of moons, and what computer simulations show about the more sedate gravitational "gathering-up" that finishes the job.

In recent years, scientists' best guess for how the Moon formed has been that a relatively slowly moving, Mars-sized body called Theia crashed into the very young Earth.

That would have heated both of them up and released a vast cloud of molten material, much of which cooled and clumped together to give rise to the Moon.

That would suggest that the Moon is made up of material from both the early Earth and from Theia, which should be somewhat different from one another.

Impact factor

What complicates that story is a number of observations of "isotopic compositions" - the ratios of naturally-occurring variants of some atoms - taken from the Earth and from lunar samples.


The mix of similar and different "isotope ratios" confounds the issue.

While the Moon has an iron core like Earth, it does not have the same fraction of iron - and computer models supporting the Theia impact idea show just the same thing.

However, the ratio of the Earth's and the Moon's oxygen isotopes is nearly identical, and not all scientists agree on how that may have come about.

Confounding the issue further, scientists reporting in Nature Geoscience in March said that a fresh analysis of lunar samples taken by the Apollo missions showed that the Moon and the Earth shared an uncannily similar isotope ratio of the metal titanium.

That, they said, gave weight to the idea that the Moon was somehow cleaved from the Earth itself.

Now, Andreas Reufer, of the Center for Space and Habitability in Bern, Switzerland, and colleagues have run computer simulations that suggest another possibility: that a far larger and faster-moving body made an even more glancing blow with the young Earth.

They said this body would have lost only a small amount of material and most of it would have continued on after the "hit-and-run".

That results in a much hotter disc of debris from the collision, but matches up with what would be needed to make a Moon-sized body.

The authors suggest that since most of what became the Moon would have been liberated by the impact from the Earth, similarities between the isotope fractions should be more pronounced.

More analyses of different elements within lunar samples - and a great deal more computer simulations that result in a Moon like our own - will be needed to settle the debate.

No writer of the article was mentioned here, so I'll give you the official link in this case:

photostill

It has long been thought that the moon could have formed by a collision process. It's one of only a couple of theories on how the earth could have gotten such a big moon. Compared to other planets, the earth's moon is rather large.

Computer simulations have been run over and over, trying to nail down the speed and angle it would take to eject such a mass of material. It comes out that there is only one real way this could happen and that's by a glancing hit, where the original body continues on. It wasn't a straight in hit, like a two cars crashing bumper to bumper but more like a billard ball where someone is trying with a glancing blow to angle off the other ball.

This leaves the question of what happened to Theia after that. So far no answers. Could have dove into the sun, could have left the Solar System all together, who knows.

I find in all this the interesting proposition that there is a missing planet. Bode's law sets out that there should be a spacing between planets and that you should expect to find a planet in that area, going out from the sun. Other than Neptune and Pluto, the highest error of being off is Mars at 5.26%. If that law is valid and much of it does hold up, there is a space between Mars and Jupiter that should hold a planet. Only one is not there. Instead there are the asteroids. This is where the idea of the asteroids being a broken up planet come from.

There is also one other possibility on what happened to Theia. Our solar system is an oddball. The reason for that is that it is not a binary star system. The majority of star systems are binary with some rare ones even having a trinary system. Especially under trinary systems but also less common in binary star systems is the lack of stability in planets that have to adjust their orbits for long term stability or either be ejected from the system or swallowed by the parent star or another massive planet.

Recent studies suggest that ejection from the system is not an uncommon event. This leads to wandering hobo planets without a star. They normally can't be seen because there is no light to show them. Since they are out in interstellar space wandering around they tend to also be cold so infrared will not show them up. In all the numbers of times this may have happened, odds say that there will be collisions within distant star systems. Most will probably pass undetected and unnoticed. Every once in a while one must pass through a system close enough to planets to disrupt the gravitational orbit. When that happens, it's anyone's guess what the results will be. It is not outside the realm of possible this could have been a spark to start the events.

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