1. School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA
2. Department of Astronomy, University of Washington, Seattle, Washington 98195, USA
3. Laboratory for Extraterrestrial Physics, Code 691, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
4. Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, UK
5. Present address: Institute for Geophysics & Planetary Physics, Lawrence Livermore National Laboratory, Mail Stop L-413, 7000 East Avenue, Livermore, California 94550, USA.

Correspondence to: J. P. Bradley^1,2 Correspondence and requests for materials should be addressed to J.P.B. (e-mail: Email: jbradley@igpp.ucllnl.org).

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 System^{1, 2, 3, 4}. Nanodiamonds recovered from meteorites, which originate in asteroids, have been thought to be the most abundant type of presolar grain^{3, 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 pristine^{5, 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 meteors^{8, 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.