[schiffern]

>PS: It may be a mainstay of sci-fi, but there is little actual evidence that asteroids are going to have particularly high levels of any of the really rare stuff. (other than H3)

Untrue. Iridium is the rarest element in the Earth's crust. The majority of the known deposites come from asteroids.

It's not just Iridium, either. Compare these two charts:

https://en.wikipedia.org/wiki/File:Elemental_abundances.svg and https://en.wikipedia.org/wiki/File:SolarSystemAbundances.png

Why are they so different? Basically all the heavy and iron-loving stuff sank.

https://en.wikipedia.org/wiki/Iron_catastrophe https://en.wikipedia.org/wiki/Planetary_differentiation

[kens]

I was puzzled by this, since obviously there are elements that are more rare than iridium, so I checked the original data source. They eliminate most radioactive elements which kind of makes sense. I was a bit surprised to learn that polonium and radium are less common in the crust than iridium. They also eliminate noble gases from the list, of which krypton, xenon, and radon turn out to be rarer in the crust than iridium. Also a couple sources have rhenium rarer than iridium.

(I'm not trying to be pedantic and start an argument, but just fill in some information in case anyone else wondered about this data.)

See http://en.wikipedia.org/wiki/Abundances_of_the_elements_(dat... and http://en.wikipedia.org/wiki/Abundance_of_elements_in_Earths...