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Did the majority of Earth's precious metals sink below the crust during Earth's formation?

zucculent
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    https://en.wikipedia.org/wiki/Asteroid_mining – Nilay Ghosh Jun 10 '22 at 04:15
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    There's an observation selection bias inherent in your question. Those metals are "precious" because they're rare in earth's crust. If gold were abundant, we'd be making mundane things like paperclips and beverage cans out of it, and never even consider mining asteroids for it. – dan04 Jun 10 '22 at 17:08
  • @dan04 I agree with you, but your examples are bad because gold would not be appropriate for those applications. I think a better example would be golden showers. – Christopher Schultz Jun 10 '22 at 22:31
  • @ChristopherSchultz Gold-based alloys, then? And for electrical wiring, too? – Rosie F Jun 11 '22 at 04:01
  • It would be good to give the motivation of the question. The accepted answer assumes you are interested in asteroid mining. Another reason for the same question would be to understand the significance of the layer between the Cretaceous and Tertiary periods being enriched in iridium and whether that is good evidence for an asteroid impact. Those deserve very different answers. Fundamentally, the answer is yes. Is that sufficient? – Ross Millikan Jun 11 '22 at 04:31
  • @RosieF Gold is really quite useless for virtually any practical application at all until your civilization's technology level reaches electronics. Gold is an amazing conductor, not as good as silver but unlike silver it doesn't tarnish. It doesn't make for good electrical wiring because of its weight, but for electronics where tiny trace wires are needed, it's ideal. Before that, gold is practically useless for literally anything other than looking pretty. – Mason Wheeler Jun 11 '22 at 16:43
  • @MasonWheeler copper is a better conductor than gold, 5.8E7 instead of 4.8E7 S/m. But gold is used in a very thin layer to protect the surface of copper conductors against oxidation. Tiny traces are possible with copper or aluminum. Connectors are plated with gold to get a lower contact resistance by protecting the copper below against oxidation. HF parts are plated with gold due to the skin effect, the current concentrates close to the surface, therefore the surface should be free from copper oxide. – Uwe Jun 12 '22 at 11:36
  • "Precious" is relative. When/if we actually start doing any asteroid mining or living in space, water, methane and similar mundane substances could well prove more valuable than many precious metals as we'll need fuel and food. A ton of gold versus a ton of water might be a very easy choice - grab the water, leave the useless gold. :-) – StephenG - Help Ukraine Jun 12 '22 at 11:36
  • @Uwe So yeah, what I said above. Gold is an amazing conductor, not as good as silver or copper but unlike silver or copper it doesn't tarnish. – Mason Wheeler Jun 12 '22 at 13:10

2 Answers2

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This is in part marketing hype by wanna-be asteroid mining companies. That said, some asteroids are suspected to be richer in precious metals than is the Earth's crust. For example, the Earth's crust is significantly depleted in gold compared to the solar system as a whole. I wrote about the reasons why this is the case at physics.stackexchange.com.

Gold and related precious metals are siderophiles, which means "iron-loving". When the Earth differentiated, the iron and nickel that sank to the center of the Earth took other siderophiles with them. In a sense, the precious metals are more siderophilic than is iron itself. Gold et al. easily dissolve in molten iron. Precious metals are so chemically inert that they do not readily combine form compounds with other elements.

There is a lot more gold and other precious metals in the Earth's core than there is in all of the asteroids combined.

David Hammen
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    So it sounds iron-nickel asteroids are the ones suspected to have more gold in them? What about other elements that, while not necessarily precious, are still rare and expensive, like tantalum and iridium? – zucculent Jun 09 '22 at 20:20
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    @zucculent Correct (regarding iron-nickel asteroids.) The cost involved in sending mining and smelting equipment to an asteroid and bringing the mined metals back to Earth overwhelms the value of those precious metals. (That ignores that doing so is TRL-3 at best.) Asteroid mining, if it ever does become commercial, will initially focus on boring substances such as water and methane, and using those materials in space. The cost of sending a kilogram of water into orbit exceeds the value of a kilogram of gold mined on the Earth. – David Hammen Jun 09 '22 at 20:46
  • I'm not really interested in the concept of bringing all the mined minerals back to Earth. I agree that it's far from being practical, if it ever will be practical. I'm just interested in knowing the concentration of rare and expensive metals in those asteroids compared to Earth's crust. – zucculent Jun 09 '22 at 21:05
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    @zucculent Some asteroids were large enough and hot enough to have differentiated. The isotope $^{26}\text{Al}$ (aluminum-26), with a half life of 717,000 years, is widely hypothesized to have provided significant additional heating during the formation of the solar system. Massive collisions later cracked some of those differentiated asteroids into parts. Many, many parts. – David Hammen Jun 09 '22 at 22:32
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    Here's a periodic table: https://en.wikipedia.org/wiki/Goldschmidt_classification – PM 2Ring Jun 09 '22 at 23:09
  • So are the siderophilic elements the only ones we'd find in much higher concentrations in certain asteroids, since the Earth's crust has plenty of rock and (I imagine) chalcogen ores? – zucculent Jun 09 '22 at 23:24
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    @DavidHammen: You are way off! A kilo of gold costs about $60,000; putting a kilo of water into orbit on a Falcon 9 can be done for about $4,000. – TonyK Jun 10 '22 at 18:39
  • @TonyK That's the cost to send something to LEO. The water will need to be stored in a container, which has mass. Something a bit stronger and hence more massive than a milk jug will be needed. Besides, LEO is not where the interesting sci-fi level space construction will occur. It will be a construction depot in a pseudo-orbit about one of the Earth-Moon Lagrange points. Sending water in a container from the Earth to such a depot will cost a whole lot more than $4000/kg, even after taking into account how disruptive SpaceX has been. – David Hammen Jun 12 '22 at 08:07
  • Comments are not for extended discussion; this conversation has been moved to chat. – called2voyage Jun 13 '22 at 14:57
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I have no knowledge of the quantified specifics, but would like to point out two effects that may be relevant:

  • We already exploited the easiest precious metal deposits on the top of Earth's crust to the extent that we could find and access them. This goes back millenia and intensified in recent centuries. Modern Earth is not natural.
  • Asteroids can be cherry-picked. You want to mine the freak asteroid, not the normal asteroid.
Emilio M Bumachar
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