Although diamonds on Earth are rare, extraterrestrial diamonds (diamonds formed outside of Earth) are very common. Diamonds so tiny 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.
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.
In meteorites, nanodiamonds make up about 3 percent of the carbon and 400 parts per million of the mass.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.
If most nanodiamonds did form in the Solar System, that raises the question of how this is possible. 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. 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 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.
The most carbon-rich meteorites, with abundances up to 7 parts per thousand by weight, 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 either the shock of the impact with Earth or with 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.
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]
#diamond #neptune #uranus
