The Fingerprint of the Giant ImpactWhen the Apollo program returned samples from the Moon, the rocks

The Fingerprint of the Giant ImpactWhen the Apollo program returned samples from the Moon, the rocks clearly differed from rocks on Earth. Different minerals, different abundances of minerals, clearly different histories and chemistry, but there were also surprising links between the two bodies.Oxygen is the most abundant atom in the Earth, making up nearly half of the planet’s mass. Oxygen has 3 isotopes with masses 16, 17, and 18. Modern instruments can measure the ratios of these 3 isotopes in rocks and one key observation is that almost every object in the solar system has a different mixture. Mars is different from the Earth, the Earth is different from meteorites, meteorites from one asteroid differ every other asteroid.But there was one exception; to within error, the Earth and the Moon matched.This match was a big deal at the time and still is today. Since oxygen is so abundant it’s impossible to get a match by random chance: it’s like mixing 8 different types of pop at a soda fountain and coming out with the same exact mixture 2 times in a row. Seems impossible right?Out of this data was born the idea that the Moon was formed by a giant impact into Earth. Late in Earth’s history ~50 million years after the planet started forming, an object at least the size of Mars, if not bigger, slammed into the Earth, spraying debris into orbit around the planet. That debris came together under gravity to form the Moon.That model could explain the Moon’s chemistry; identical to Earth in its oxygen isotopes but slightly different in its chemistry since it formed in a different way. The model fits many other parameters, like where the Moon formed, the ages of the Moon rocks, and why the Earth spins at the rate it does, so it is a very good model.But there was a problem. To understand how a giant impact could occur, scientists used advanced computer simulations to model how the energy and mass moved around during the impact (described here: https://www.facebook.com/TheEarthStory/posts/556021494458899). While doing this, they noticed something odd; the Moon kept being formed mostly out of material from the impactor.There was a lot of exchange of mass between the two bodies since most of the mass of the moon was literally vaporized by the impact, but some parts of the impactor, known as Theia, should have been big contributors to the Moon. If this is the case, then how in the world could the Moon’s oxygen isotopes match the Earth so perfectly?It turns out, modern analyses may have solved this problem. Over time, we’ve been able to measure oxygen isotopes more precisely as better instruments are developed, and better measurements can detect smaller differences. A few years ago, a first study published that there was a potential difference between the oxygen isotope ratios of the Earth and the Moon, and just this year another study was able to not only confirm the difference between the Earth and the Moon, but also isolated specific rock types where the differences were larger. It turns out that the rocks of the Moon’s mantle are distinctly different from the Earth, while the rocks of the Moon’s crust are much closer to the Earth - suggesting that the rocks near the surface did exchange oxygen with gas from the Earth, but the deep part of the lunar mantle was unaffected.This is about as exciting of a measurement of an incredibly tiny difference as we can get. This measurement fits with exactly what the people who developed the Giant Impact model for the Moon’s formation have been predicting and have been unable to explain for years. There should be tiny differences between the Moon and Earth if the Moon was splashed off of the Earth in a giant impact, and scientists finally just found one!Scientists created a model for how the Moon formed based on the Apollo samples, early measurements gave us a conundrum, but later measurements turned out to be precise enough to explain a missing part of the story. How neat is that!-JBBImage credit:https://astrobiology.nasa.gov/news/support-for-a-catastrophic-formation-of-the-moon/Original papers:https://www.nature.com/articles/s41561-020-0550-0http://www.sciencemag.org/content/344/6188/1146 -- source link
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