Possible in situ formation of meteoritic nanodiamonds in the early Solar System

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Abstract
Grains of dust that pre-date the Sun provide insights into their formation around other stars and into the early evolution of the Solar System1,2,3,4. Nanodiamonds recovered from meteorites, which originate in asteroids, have been thought to be the most abundant type of presolar grain3,4. If that is true, then nanodiamonds should be at least as abundant in comets, because they are thought to have formed further out in the early Solar System than the asteroid parent bodies, and because they should be more pristine5,6,7. Here we report that nanodiamonds are absent or very depleted in fragile, carbon-rich interplanetary dust particles, some of which enter the atmosphere at speeds within the range of cometary meteors8,9. One interpretation of the results is that some (perhaps most) nanodiamonds formed within the inner Solar System and are not presolar at all, consistent with the recent detection of nanodiamonds within the accretion discs of other young stars10. An alternative explanation is that all meteoritic nanodiamonds are indeed presolar, but that their abundance decreases with heliocentric distance, in which case our understanding of large-scale transport and circulation within the early Solar System is incomplete11.