Tiny diamonds found in exotic stony meteorites called “ureilites” were recently proposed to have formed much like diamonds on Earth: from graphite deep inside a planetary body where pressures are high enough to change the crystal structure from graphite to diamond over long periods of time. To grow 100 micron diamonds, it was thought that pressures larger than 20 GPa (Giga Pascals) were needed. This led some to suggest that the early solar system body from which ureilites originated must have been the size of Mars or Mercury, perhaps proof of an earlier generation of planets since lost.
Now, in a paper published in the journal PNAS (Proceedings of the National Academy of Sciences of the United States of America), a group of researchers led by Fabrizio Nestola (University of Padova, Italy) and Cyrena Goodrich (Universities Space Research Association at the Lunar and Planetary Institute), report that a shock wave from a big but fairly common planetary collision in the early solar system can do the same. They studied the pristine meteorites that were collected shortly after the impact of small asteroid 2008 TC3 in the Nubian Desert of Sudan by teams led by co-authors Muawia Shaddad (University of Khartoum) and Peter Jenniskens (SETI Institute).
The team found that metal impurities can catalyze the change from graphite (blue in the picture above) to diamond (shown in red), just as is done in the fabrication of industrial diamonds, and that the graphite in ureilites is spiked with such impurities. They also calculated that shock waves from impacts between planetary bodies would keep peak pressures high for as much as four to five seconds, rather than microseconds. Shocks less or about 15 GPa would be enough to change some graphite into diamond. Damage from such shock intensity is also seen in other components of the meteorites.
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Nestola F., Goodrich C. A., Morana M., Barbaro A., Jakubek R. S., Christ O, Brenker F. E., Domeneghetti C. M., Dalconi M. C,, Alvaro M., Fioretti A. M., Litasov K. D., Fries M. D., Leoni M., Casati N. P. M., Jenniskens P., and Shaddad M. H. (2020) Impact shock origin of diamonds in ureilite meteorites. Proceedings of the National Academy of Sciences (Sept. 28, 2020).
