<p>All the <a href="https://doi.org/10.1038/ngeo2488" rel="nofollow noreferrer">original research paper</a> 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.</p>
<p>Also, don't forget that even is uranium is dense, it doesn't &quot;sink below the crust&quot;. 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).</p>

CC BY-SA 4.0
2023-05-25T01:48:29.853Z
This makes sense.  The crust enrichment relative to the mantle makes an asteroid-induced ejection as a means of selective reduction feasible..
theRiley
CC BY-SA 4.0
2023-05-22T08:54:05.873Z
2023-05-22T08:54:05.873Z
Jean-Marie Prival
<p>ref : <a href="https://science.ubc.ca/news/asteroid-impact-erosion-helped-make-earth-habitable" rel="nofollow noreferrer">https://science.ubc.ca/news/asteroid-impact-erosion-helped-make-earth-habitable</a></p>
<p>According to the theory of impact erosion, the early crust was presumably much hotter w/ radioactive isotopes (U &amp; 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.</p>
<p>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 ?</p>

CC BY-SA 4.0
2023-05-23T12:15:43.947Z
"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.
Jean-Marie Prival
CC BY-SA 4.0
2023-05-23T12:18:03.480Z
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.
Jean-Marie Prival
CC BY-SA 4.0
2023-05-24T02:23:02.757Z
@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.
theRiley
CC BY-SA 4.0
2023-05-24T05:00:53.623Z
The crust is generated by partial melting of the mantle. When the mantle partially melts, uranium goes into the melt, i.e. ends up in the crust. So the crust is enriched in U while the mantle is depleted. Now if you remove some of this crust (by impacts), you end up with a planet with less uranium than originally. Less uranium means less internal heat, so maybe a different tectonic regime, etc.
Jean-Marie Prival
CC BY-SA 4.0
2023-05-24T07:31:24.047Z
Ok, I get the idea of the crust being enriched relative to the mantle undergoing net reduction of U given some means of getting rid of this crust off-planet.  But we still have the difficulty of shipping large amounts of rock out into interplanetary space, given Earth's large gravitational field, as noted by Furious Arcturus.  The earth is not littered with orbiting primeval debris, other than the large moon - so where did it all go, and what pushed it out sufficiently without tearing the planet to pieces ?  We should have seen the signal in the crustal content between the moon & earth.
theRiley
CC BY-SA 4.0
2023-05-24T08:48:01.877Z
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.
Jean-Marie Prival
CC BY-SA 4.0
2023-05-24T08:53:25.157Z
Here is another, more recent paper about the same hypothesis: https://doi.org/10.1126/science.abq7351 and the related perspective (both pay-walled unfortunately): https://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.
Jean-Marie Prival
CC BY-SA 4.0
2023-05-24T22:48:12.813Z
@theRiley -- You chose the wrong answer.
David Hammen
CC BY-SA 4.0
2023-05-25T01:52:55.530Z
Have adusted the selected answer.  The original explanation was clear enough, I was a little fuzzy on ejection.  It still leaves me with the question why we don't have a large amount of debris floating around in near-earth orbit, if such collissions were sufficient to make an appreciable dent in crustal composition.
theRiley
<geophysics><earth-history>
Impact Erosion Selectivity In Offloading Radioactive Material
theRiley
# Impact Erosion Selectivity In Offloading Radioactive Material

(Asked by: username_0 on 2023-05-21)

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 ?

**Tags:** geophysics, earth-history

> **Comments:**
> **username_1**: "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.  
> **username_1**: 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.  
> **username_0**: @username_1 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.  
> **username_1**: The crust is generated by partial melting of the mantle. When the mantle partially melts, uranium goes into the melt, i.e. ends up in the crust. So the crust is enriched in U while the mantle is depleted. Now if you remove some of this crust (by impacts), you end up with a planet with less uranium than originally. Less uranium means less internal heat, so maybe a different tectonic regime, etc.  
> **username_0**: Ok, I get the idea of the crust being enriched relative to the mantle undergoing net reduction of U given some means of getting rid of this crust off-planet.  But we still have the difficulty of shipping large amounts of rock out into interplanetary space, given Earth's large gravitational field, as noted by Furious Arcturus.  The earth is not littered with orbiting primeval debris, other than the large moon - so where did it all go, and what pushed it out sufficiently without tearing the planet to pieces ?  We should have seen the signal in the crustal content between the moon & earth.  
> **username_1**: 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.  
> **username_1**: Here is another, more recent paper about the same hypothesis: https://doi.org/10.1126/science.abq7351 and the related perspective (both pay-walled unfortunately): https://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.  
> **username_2**: @username_0 -- You chose the wrong answer.  
> **username_0**: Have adusted the selected answer.  The original explanation was clear enough, I was a little fuzzy on ejection.  It still leaves me with the question why we don't have a large amount of debris floating around in near-earth orbit, if such collissions were sufficient to make an appreciable dent in crustal composition.  

---

## 1 answers

### Answer by username_1 on 2023-05-22. Score: 2 | Accepted answer

All the [original research paper](https://doi.org/10.1038/ngeo2488) 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).

> **Comments:**
> **username_0**: This makes sense.  The crust enrichment relative to the mantle makes an asteroid-induced ejection as a means of selective reduction feasible..  

---

