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. 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. They were dated at 4.5 billion-year-old crystals and were formed at pressures greater than 20 gigapascals. They contained inclusions of iron- and sulfur-bearing minerals, the first inclusions to be found in extraterrestrial diamonds. : 264 However, much larger diamonds were found in fragments of a meteorite called Almahata Sitta, found in the Nubian desert of Sudan. ![]() Diamonds are common in highly shocked ureilites, and most are thought to have been formed by either the shock of the impact with Earth or with other bodies in space. : 241 These have no known parent body and their origin is controversial. The most carbon-rich meteorites, with abundances up to 7 parts per thousand by weight, are ureilites. 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. The crossover in stability is between 1 and 5 nm. In very small minerals, surface energy is important and diamonds are more stable than graphite because the diamond structure is more compact. However, nanodiamonds are close to molecular size: one with a diameter of 2.8 nm, the median size, contains about 1800 carbon atoms. On the surface of Earth, 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. If most nanodiamonds did form in the Solar System, that raises the question of how this is possible. ![]() 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. 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. Only a very small fraction of them contain noble gases of presolar origin and until recently it was not possible to study them individually. It is unclear how many nanodiamonds in meteorites are really from outside the Solar System. ![]() Collectively they are known as presolar grains or stardust and their properties constrain models of nucleosynthesis in giant stars and supernovae. Grains of silicon carbide and graphite also have anomalous isotopic patterns. In meteorites, nanodiamonds make up about 3 percent of the carbon and 400 parts per million of the mass. 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. Analyses of additional primitive meteorites also found nanodiamonds. ![]() Trapped in them were noble gases whose isotopic signature indicated they came from outside the Solar System. In 1987, a team of scientists examined some primitive meteorites and found grains of diamond about 2.5 nanometers in diameter ( nanodiamonds). Artist's conception of a multitude of tiny diamonds next to a hot star.
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