Scientists studied the chemistry of moon rocks and Earth rocks, and, in a new paper published Monday in the journal Nature, they say they now have hard evidence that a enormous, high-energy collision between the Earth and another large space rock could be the origin story for our lunar companion. The study is a radical concept that could totally change our understanding of the planetary system. In one model, a low-energy impact leaves the proto-Earth and moon shrouded in a silicate atmosphere; in the other, a much more violent impact vaporizes the impactor and most of the proto-Earth, expanding to form an enormous superfluid disk out of which the moon eventually crystallizes.
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Speculation about the origin of our lunar companion has always been the focus of scientific inquiry.
That’s when this early planet collided with another, perhaps the size of Mars. Debris from the impact later coalesced into the moon.
Scientists have long debated the circumstances which led to the formation of the moon, with two main theories attempting to explain this event. Instead, a more spectacular, violent story began to emerge.
In the 1970s, two sets of astrophysicists independently came to the conclusion that the Moon was created by a glancing collision between a Mars-sized object and the still-forming Earth. “Physically there’s no problem”.
“Our results provide the first hard evidence that the impact really did (largely) vaporize Earth”, said Kun Wang, a geochemist at Washington University in St. Louis, in a release.
These discoveries threw the giant-impact hypothesis into crisis because previous computer simulations of the collision predicted that 60 to 80 percent of the material that coalesced into the moon came from Theia rather than Earth. But that’s not the case with the Earth and the moon: chemically, they are practically identical.
The new information comes after researchers studied Apollo-era moon rocks with new technology.
Around 4.5 billion years ago, an object slammed into Earth vaporising most of the planet into a scorching cloud from which the moon was born. It was only in 2015 when a model suggested that the collision was actually violent.
The only way to explain this higher abundance of potassium-41 in the Moon’s composition is to accept that there was a more violent impact, says Wang.
But Dr. Wang’s new measurements don’t match that scenario either. The samples in question contained isotopes of potassium, and the differences in isotopic compositions between lunar rocks and Earth rocks were measured and compared.
The isotope signatures were the same, except for more of a heavy potassium isotope in the moon samples, which would have required extremely hot temperatures to separate out. Potassium-41 is heavier than potassium-39.
Their finding that the lunar rocks are enriched in the heavier potassium isotope does not favor the silicate atmosphere model, which predicts lunar rocks will contain less of the heavier isotope than terrestrial rocks, the opposite of what the scientists found. This theoretical model is so new that it has only been described in conferences and has yet to be published in a scientific paper.
In this model, the impactor and Earth’s mantle vaporized and mixed together, forming a dense mantle atmosphere.
“We can measure much smaller differences between Earth and the Moon, so we found a lot of things we didn’t find in the 1970s”. As opposed to the lighter isotope, the heavier isotope would preferentially drop out of the vapor and condense.
At first, scientists thought more data would help solve this conundrum but as more data pilled up it only confirmed that the Earth’s and moon’s isotopic signatures are not distinguishable.
Wang himself doesn’t seem too fazed by the criticism, saying every new hypothesis takes time to settle in and become accepted as the evidence mounts around it.
“There are many new models – everyone is trying to come up with one – but two have been very influential”, Wang said in a statement.
Two such models have been previously suggested to account for this. “I think it may be true that the model can explain this data”, she says, “but I don’t think it’s clear that the other models couldn’t explain this data”. “It’s a key new constraint” for the models.
“Now we need to rethink the ideas that we had about the giant impact”, Wang told Space.com.
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This cast doubt on the giant impact hypothesis as it stood at the time.
