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ref : https://science.ubc.ca/news/asteroid-impact-erosion-helped-make-earth-habitable

According to the theory of impact erosion, the early crust was presumably much hotter w/ radioactive isotopes (U & K, primarily) and thus less congenial for the later development of life. Asteroid bombardment - I suppose during the late heavy bombardment - then blew off a sufficient chunk of this material to lower net radioactivity in the crust.

Was this simply because beforehand for some reason there was an upward gradient of radioactivity in the primeval crust ? I'm at a loss to conceive a selective mechanism for this phenomenon. Any paleo-geologists in the house ?

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  • $\begingroup$ "I'm getting the unsurprising feedback that this theory has no basis." -> That is not at all what my answer was suggesting. This study has some strong arguments. It is one of the main ways to explain a chemical disequilibrium between the early Earth's building materials and today's Earth geochemistry as we measure it. It is debated, there are other ways to explain it: for example, a hidden reservoir somewhere in the mantle. $\endgroup$ May 23, 2023 at 12:15
  • $\begingroup$ Now you can pick your favorite hypothesis: either the missing elements have been lost to space, removed by impacts; or they are hidden somewhere deep in the mantle, and somehow we never had the chance to sample them through volcanism. So, is it debated? Yes. Does some people believe in another hypothesis? Sure. But is it garbage science with no basis? No, not at all. $\endgroup$ May 23, 2023 at 12:18
  • $\begingroup$ @Jean-Marie Prival I don't hear any support for the notion that U & K are preferentially, blasted off into the interplanetary space. That is the essence of the theory, as I understand it. What possible mechanism exists for it ? What you are describing is an entirely different geochemical process of sequestration in the mantle. A very different proposition. $\endgroup$
    – theRiley
    May 24, 2023 at 2:23
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    $\begingroup$ Furious Arcturus answer is wrong in many aspect. His comparison with the atmosphere is misleading: of course atmospheric gases stay bounded to Earth by gravity, they have no energy to reach escape velocity. However, impacts provide plenty of kinetic energy, allowing the impacted material to reach escape velocity and to leave Earth's gravitational field. We found martian meteorites on Earth, how do you think they got here? They escaped Mars gravity after an impact. $\endgroup$ May 24, 2023 at 8:48
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    $\begingroup$ Here is another, more recent paper about the same hypothesis: doi.org/10.1126/science.abq7351 and the related perspective (both pay-walled unfortunately): doi.org/10.1126/science.add3199 Again, this is serious research, published by serious scientists in serious peer-reviewed journals, supported by serious arguments. You don't have to believe it, but many geologists agree with it. $\endgroup$ May 24, 2023 at 8:53

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All the original research paper is saying is that there might be less radioactive elements in the silicate Earth (crust + mantle) than previously thought. And if there had been more of those elements, there would have been more internal heat, thus a different tectonic regime, a different volcanism, a different geodynamo, a different atmosphere... And maybe, maybe, these different conditions would have prevented the emergence of life.

Also, don't forget that even is uranium is dense, it doesn't "sink below the crust". The mantle is solid rock, uranium is trapped in the crystal lattice of minerals, but it is an incompatible element, meaning it will preferentially go into the melt when the mantle partially melt. These melts then rise by buoyancy and actually make the bulk of the continental crust. So it makes perfect sense that the crust is uranium-enriched and the mantle uranium-depleted. Then, if you remove some of this crust after its formation, you will indeed remove some radioactive elements compared to the original composition (I'm not saying it happened, but the model is perfectly logical).

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  • $\begingroup$ This makes sense. The crust enrichment relative to the mantle makes an asteroid-induced ejection as a means of selective reduction feasible.. $\endgroup$
    – theRiley
    May 25, 2023 at 1:48

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