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Extraterrestrial diamonds

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Although diamonds on Earth are rare, extraterrestrial diamonds (diamonds formed outside of Earth) are very common. Diamonds so small that they contain only about 2000 carbon atoms are abundant in meteorites, and some of them formed in stars before the Solar System existed.[1] High pressure experiments suggest large amounts of diamonds are formed from methane on the ice giant planets Uranus and Neptune, while some planets in other planetary systems may be almost pure diamond.[2] Diamonds are also found in stars and may have been the first mineral ever to have formed.

Meteorites

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Artist's conception of a multitude of tiny diamonds next to a hot star.

In 1987, a team of scientists examined some primitive meteorites and found grains of diamond about 2.5 nanometers in diameter (nanodiamonds). Trapped in them were noble gases whose isotopic signature indicated they came from outside the Solar System. Analyses of additional primitive meteorites also found nanodiamonds. The record of their origins was preserved despite a long and violent history that started when they were ejected from a star into the interstellar medium, went through the formation of the Solar System, were incorporated into a planetary body that was later broken up into meteorites, and finally crashed on the Earth's surface.[3]

In meteorites, nanodiamonds make up about 3 percent of the carbon and 0.04% of the total mass.[4][3] Grains of silicon carbide and graphite also have anomalous isotopic patterns. Collectively they are known as presolar grains or stardust and their properties constrain models of nucleosynthesis in giant stars and supernovae.[5]

It is unclear how many nanodiamonds in meteorites are really from outside the Solar System. Only a very small fraction of them contain noble gases of presolar origin, and until recently it was not possible to study them individually. On average, the ratio of carbon-12 to carbon-13 matches that of the Earth's atmosphere, while that of nitrogen-14 to nitrogen-15 matches the Sun. Techniques such as atom probe tomography will make it possible to examine individual grains, but due to the limited number of atoms, the isotopic resolution is limited.[5]

If most nanodiamonds did form in the Solar System, that raises the question of how this is possible. On the Earth's surface, graphite is the stable carbon mineral, while larger diamonds can only be formed in the kind of temperatures and pressures that are found deep in the mantle. However, nanodiamonds are close to molecular size: one with a diameter of 2.8 nm, the median size, contains about 1800 carbon atoms.[5] In very small minerals, surface energy is important and diamonds are more stable than graphite because the diamond structure is more compact. The crossover in stability is at between 1 and 5 nm. At even smaller sizes, a variety of other forms of carbon such as fullerenes can be found, as well as diamond cores wrapped in fullerenes.[3]

The most carbon-rich meteorites, with abundances up to 0.7% by mass, are ureilites.[6]: 241  These have no known parent body and their origin is controversial.[7] Diamonds are common in highly shocked ureilites, and most are thought to have been formed by the shock of the impact with either Earth or other bodies in space.[6][8]: 264  However, much larger diamonds were found in fragments of a meteorite called Almahata Sitta, found in the Nubian Desert of Sudan. They contained inclusions of iron- and sulfur-bearing minerals, the first inclusions to be found in extraterrestrial diamonds.[9] They were dated at 4.5 billion-year-old crystals and were formed at pressures greater than 20 gigapascals. The authors of a 2018 study concluded that they must have come from a protoplanet, no longer intact, with a size between that of the moon and Mars.[10][11]

Infrared emissions from space, observed by the Infrared Space Observatory and the Spitzer Space Telescope, has made it clear that carbon-containing molecules are ubiquitous in space. These include polycyclic aromatic hydrocarbons (PAHs), fullerenes and diamondoids (hydrocarbons that have the same crystal structure as diamond).[3] If dust in space has a similar concentration, a gram of it would carry up to 10 quadrillion of them,[4] but so far there is little evidence for their presence in the interstellar medium; they are difficult to tell apart from diamondoids.[3]

Planets

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Solar System

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Uranus, imaged by Voyager 2 in 1986.

In 1981, Martin Ross wrote a paper titled "The ice layer in Uranus and Neptune—diamonds in the sky?" in which he proposed that huge quantities of diamonds might be found in the interior of these planets. At Lawrence Livermore, he had analyzed data from shock-wave compression of methane (CH4) and found that the extreme pressure separated the carbon atom from the hydrogen, freeing it to form diamond.[12][13]

Theoretical modeling by Sandro Scandolo and others predicted that diamonds would form at pressures over 300 gigapascals (GPa), but even at lower pressures methane would be disrupted and form chains of hydrocarbons. High pressure experiments at the University of California Berkeley using a diamond anvil cell found both structures at only 50 GPa and a temperature of 2500 kelvins, equivalent to depths of 7000 kilometers below Neptune's cloud tops. Another experiment at the Geophysical Laboratory saw methane becoming unstable at only 7 GPa and 2000 kelvins. After forming, denser diamonds would sink. This "diamond rain" would convert potential energy into heat and help drive the convection that generates Neptune's magnetic field.[14][12][15]

There are some uncertainties in how well the experimental results apply to Uranus and Neptune. Water and hydrogen mixed with the methane may alter the chemical reactions.[14] A physicist at the Fritz Haber Institute in Berlin showed that the carbon on these planets is not concentrated enough to form diamonds from scratch. A proposal that diamonds may also form in Jupiter and Saturn, where the concentration of carbon is far lower, was considered unlikely because the diamonds would quickly dissolve.[16]

Experiments looking for conversion of methane to diamonds found weak signals and did not reach the temperatures and pressures expected in Uranus and Neptune. However, a recent experiment used shock heating by lasers to reach temperatures and pressures expected at a depth of 10,000 kilometers below the surface of Uranus. When they did this to polystyrene, nearly every carbon atom in the material was incorporated into diamond crystals within a nanosecond.[17][18]

Extrasolar

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Main article: Carbon planet
On Earth, the natural form of silicon carbide is a rare mineral, moissanite.[19]

In the Solar System the rocky planets Mercury, Venus, Earth and Mars are 70% to 90% silicates by mass. By contrast, stars with a high ratio of carbon to oxygen may be orbited by planets that are mostly carbides, with the most common material being silicon carbide. This has a higher thermal conductivity and a lower thermal expansivity than silicates. This would result in more rapid conductive cooling near the surface, but lower down the convection could be at least as vigorous as that in silicate planets.[20]

One such planet is PSR J1719-1438 b, companion to a millisecond pulsar. It has a density at least twice that of lead, and may be composed mainly of ultra-dense diamond. It is believed to be the remnant of a white dwarf after the pulsar stripped away more than 99 percent of its mass.[2][21][22]

Another planet, 55 Cancri e, has been called a "super-Earth" because, like Earth, it is a rocky planet orbiting a sun-like star, but it has twice the radius and eight times the mass. The researchers who discovered it in 2012 concluded that it was carbon-rich, making an abundance of diamond likely.[23] However, later analyses using multiple measures for the star's chemical composition indicated that the star has 25 percent more oxygen than carbon. This makes it less likely that the planet itself is a carbon planet.[24]

Stars

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It has been proposed that diamonds exist in carbon-rich stars, particularly white dwarfs; Carbonado, a polycrystalline mix of diamond, graphite, and amorphous carbon, which is one of the hardest natural forms of carbon, is also present,[25] and could come from supernovae and white dwarfs.[26] The white dwarf BPM 37093, located 50 light-years (4.7×1014 km) away in the constellation Centaurus, has a diameter of 2,500 miles (4,000 km), and may have a diamond core, which would make it one of the largest diamonds in the universe. For this reason it was given the nickname Lucy.[27][28]

In 2008, Robert Hazen and colleagues at the Carnegie Institution in Washington, D.C. published a paper, "Mineral evolution", in which they explored the history of mineral formation and found that the diversity of minerals has changed over time as the conditions have changed. Before the Solar System formed, only a small number of minerals were present, including diamonds and olivine.[29][30] The first minerals may have been small diamonds formed in stars because stars are rich in carbon and diamonds form at a higher temperature than any other known mineral.[31]

See also

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References

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  1. ^ Daulton, T. L. (2006). "Extraterrestrial Nanodiamonds in the Cosmos". (Chapter II) in "Ultrananocrystalline Diamond: Synthesis, Properties, and Applications" editors O. Shenderova and D. Gruen. pp. 23–78.
  2. ^ a b Max Planck Institute for Radio Astronomy (25 August 2011). "A planet made of diamond". Astronomy magazine. Retrieved 25 September 2017.
  3. ^ a b c d e Tielens, A. G. G. M. (12 July 2013). "The molecular universe". Reviews of Modern Physics. 85 (3): 1021–1081. Bibcode:2013RvMP...85.1021T. doi:10.1103/RevModPhys.85.1021.
  4. ^ a b Vu, Linda (26 February 2008). "Spitzer's Eyes Perfect for Spotting Diamonds in the Sky". JPL News. Jet Propulsion Laboratory. Archived from the original on 9 October 2016. Retrieved 23 September 2017.
  5. ^ a b c Davis, A. M. (21 November 2011). "Stardust in meteorites". Proceedings of the National Academy of Sciences. 108 (48): 19142–19146. Bibcode:2011PNAS..10819142D. doi:10.1073/pnas.1013483108. PMC 3228455. PMID 22106261.
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  7. ^ "Ureilites". Northern Arizona Meteorite Laboratory. Northern Arizona University. Retrieved 23 April 2018.
  8. ^ Hutchison, Robert (2006). Meteorites : a petrologic, chemical, and isotopic synthesis. Cambridge: Cambridge University Press. ISBN 9780521035392.
  9. ^ Gibbens, Sarah (17 April 2018). "Diamonds From Outer Space Formed Inside a Long-Lost Planet". National Geographic. Archived from the original on 18 April 2018. Retrieved 23 April 2018.
  10. ^ Salazar, Doris Elin (18 April 2018). "Diamonds in Meteorite May Come from a Lost Planet". Scientific American. Retrieved 23 April 2018.
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  12. ^ a b Scandolo, Sandro; Jeanloz, Raymond (November–December 2003). "The Centers of Planets: In laboratories and computers, shocked and squeezed matter turns metallic, coughs up diamonds and reveals Earth's white-hot center". American Scientist. 91 (6): 516–525. Bibcode:2003AmSci..91..516S. doi:10.1511/2003.38.905. JSTOR 27858301. S2CID 120975663.
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  14. ^ a b Kerr, R. A. (1 October 1999). "Neptune May Crush Methane Into Diamonds". Science. 286 (5437): 25. doi:10.1126/science.286.5437.25a. PMID 10532884. S2CID 42814647.
  15. ^ Kaplan, Sarah (25 August 2017). "It rains solid diamonds on Uranus and Neptune". The Washington Post. Retrieved 16 October 2017.
  16. ^ McKee, Maggie (9 October 2013). "Diamond drizzle forecast for Saturn and Jupiter". Nature News. doi:10.1038/nature.2013.13925. S2CID 124933499.
  17. ^ Cartier, Kimberly (15 September 2017). "Diamonds Really Do Rain on Neptune, Experiments Conclude". Eos. doi:10.1029/2017EO082223.
  18. ^ Kraus, D.; et al. (September 2017). "Formation of diamonds in laser-compressed hydrocarbons at planetary interior conditions". Nature Astronomy. 1 (9): 606–611. Bibcode:2017NatAs...1..606K. doi:10.1038/s41550-017-0219-9. S2CID 46945778.
  19. ^ Di Pierro S.; Gnos E.; Grobety B.H.; Armbruster T.; Bernasconi S.M. & Ulmer P. (2003). "Rock-forming moissanite (natural α-silicon carbide)". American Mineralogist. 88 (11–12): 1817–21. Bibcode:2003AmMin..88.1817D. doi:10.2138/am-2003-11-1223. S2CID 128600868.
  20. ^ Nisr, C.; Meng, Y.; MacDowell, A. A.; Yan, J.; Prakapenka, V.; Shim, S.-H. (January 2017). "Thermal expansion of SiC at high pressure-temperature and implications for thermal convection in the deep interiors of carbide exoplanets". Journal of Geophysical Research: Planets. 122 (1): 124–133. Bibcode:2017JGRE..122..124N. doi:10.1002/2016JE005158. OSTI 1344574.
  21. ^ Perkins, Sid (25 August 2011). "Diamond Planet Orbits a Pulsar". ScienceShots. American Association for the Advancement of Science. Retrieved 25 September 2017.
  22. ^ Lemonick, Michael (26 August 2011). "Scientists Discover a Diamond as Big as a Planet". Time. Retrieved 2 September 2017.
  23. ^ Duffy, T. S.; Madhusudhan, N.; Lee, K.K.M. (2015). "2.07 Mineralogy of super-Earth planets". In Gerald, Schubert (ed.). Treatise on Geophysics. Elsevier. pp. 149–178. ISBN 9780444538031.
  24. ^ Gannon, Megan (14 October 2013). "'Diamond' Super-Earth Planet May Not Be So Glam". Space.com. Retrieved 25 September 2017.
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  26. ^ Shumilova, T.G.; Tkachev, S.N.; Isaenko, S.I.; Shevchuk, S.S.; Rappenglück, M.A.; Kazakov, V.A. (April 2016). "A "diamond-like star" in the lab. Diamond-like glass". Carbon. 100: 703–709. Bibcode:2016Carbo.100..703S. doi:10.1016/j.carbon.2016.01.068.
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  30. ^ Hazen, R. M.; Papineau, D.; Bleeker, W.; Downs, R. T.; Ferry, J. M.; McCoy, T. J.; Sverjensky, D. A.; Yang, H. (1 November 2008). "Mineral evolution". American Mineralogist. 93 (11–12): 1693–1720. Bibcode:2008AmMin..93.1693H. doi:10.2138/am.2008.2955. S2CID 27460479.
  31. ^ Wei-Haas, Maya (13 January 2016). "Life and Rocks May Have Co-Evolved on Earth". Smithsonian. Retrieved 26 September 2017.

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