# Why are asteroids so much richer in precious metals than Earth's crust?

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

• en.wikipedia.org/wiki/Asteroid_mining Jun 10 at 4:15
• 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. Jun 10 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. Jun 10 at 22:31
• @ChristopherSchultz Gold-based alloys, then? And for electrical wiring, too? Jun 11 at 4:01
• 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? Jun 11 at 4:31

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.

• 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? Jun 9 at 20:20
• @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. Jun 9 at 20:46
• @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. Jun 9 at 22:32
• Here's a periodic table: en.wikipedia.org/wiki/Goldschmidt_classification Jun 9 at 23:09
• @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. Jun 10 at 18:39

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.