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Potassium isotopic evidence for a high-energy giant impact origin of the Moon Kun Wang & Stein Jacobsen 2016 Nature doi 10.1038/nature19341
The Earth–Moon system has unique chemical & isotopic signatures, compared with other planetary bodies. The Moon is substantially depleted in volatile elements (e.g. potassium) compared with the Earth & the bulk solar composition. It has long been thought to be the result of a catastrophic Moon-forming giant impact event. Volatile-element-depleted bodies (e.g. the Moon) were expected to be enriched in heavy K isotopes during the loss of volatiles, but such enrichment was never found.
Here we report new high-precision K isotope data for the Earth, the Moon & chondritic meteorites: the lunar rocks are significantly (>2σ) enriched in the heavy isotopes of K, compared to the Earth & chondrites (by around 0.4 parts/1000). The enrichment of the heavy K isotope in lunar rocks (vs the Earth & chondrites) can be best explained as the result of the incomplete condensation of a bulk silicate Earth vapour, at an ambient pressure
>10 bar.
We used these coupled constraints (the chemical loss & isotopic fractionation of K) to compare 2 recent dynamic models to explain the identical non-mass-dependent isotope composition of the Earth & the Moon. Our K isotope result is inconsistent with the low-energy disk equilibration model, it supports the high-energy, high-angular-momentum giant impact model for the origin of the Moon.
High-precision K isotope data can also be used as a "palaeo-barometer" to reveal the physical conditions during the Moon-forming event.
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Chemical Analysis Turns Moon Origin Theories on Head David DeMar 29.7.16 <http://www.newhistorian.com/chemical-analysis-turns-moon-origin-theories-h ead/7313/#respond> <http://www.nature.com/nature/journal/vaop/ncurrent/full/nature19341.html>
... Based on ultra-sensitive methods for detecting concentrations of potassium isotopes, Stein Jacobsen & Kun Wang determined the amount of K-41 present in both Earth & Moon rock samples: 0.4 parts/1000. The only way this K isotope could have condensed in both moon rock & samples from the Earth's mantle was an incredibly energetic impact, much bigger than even the largest estimations, with an object that would have vaporized both the impactor & nearly all of the Earth at that time.
The findings support a 2015 theory that suggested the impact that created the Moon was an extremely violent one. The vaporized Earth's mantle would have mixed with the remnants of the impactor, to form an atmospheric cloud 500 x larger than the Earth as it is today. The Moon is composed of a condensed version of this incredibly dense vapor, which was mixed so thoroughly that Wang called it a "supercritical fluid": a substance that has qualities of both liquids and gases, able to both dissolve materials & flow through solid ones.
Wang: It was this high-Tp process that separated K isotopes to cause K-41 to accumulate in the manner it has in both the Moon & terrestrial geology. With K-41 being a heavier isotope, it would have fallen out of this supercritical vapor first.
According to their calculations, the entire process occurred at an atmospheric pressure that was c 10 x the typical pressure experienced on Earth at sea-level.
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