Meteoric nanodiamonds were formed from 'carbon onion' collisions
Australian researchers have developed a model to resolve the origin of meteoric nanodiamonds, a long-standing cosmological puzzle. Their work could also have an impact on an important process here on planet Earth: synthesising artificial diamond.
Until recently, investigating the early life of the universe has only been possible through spectroscopy. By observing ancient radiation from space, astronomers can effectively look back into history. This changed in the late 1980s when nanodiamonds (tiny particles of diamond of less than 2nm in size) obtained from meteorites were found to contain unusual noble gas isotopes that indicated their origins lay outside our solar system.
’These samples were really important because that was the first time we could say "This really came from outside our solar system,"’ says Rhonda Stroud, who studies meteoric nanodiamonds at the US Naval Research Laboratory in Washington.
However, since that discovery, nanodiamonds have been more confounding than revealing, with seemingly conflicting evidence regarding their age and origin frustrating attempts to develop a realistic model for nanodiamond formation that fits all the data. Now, Nigel Marks at Curtin University in Perth, Australia, and his colleagues have proposed a new model for nanodiamond formation, which they believe offers the simplest, most obvious solution.
The Marks model is based upon the collision of carbon ’onions’ - concentric layers of fullerene molecules that could occur naturally in space. ’Carbon onions are absolutely everywhere,’ says Marks, ’wherever you have hot carbon vapour, it cools and spontaneously forms these concentric onion structures. The Spitzer telescope has shown space is full of fullerenes and I’d be massively surprised if it wasn’t full of onions too. In fact, onions are easier to form.’ And as they form, the onions encapsulate other species, providing an ’elegant explanation for how the isotopes get caught inside’. When these onions then collide with each other, or other matter, at the right speed, the force of the impact brings about a phase transition to diamond.
Marks stumbled upon his discovery while conducting computer simulations to investigate structural anomalies in a thin carbon coating. ’We ran many, many simulations,’ says Marks ’and in a handful of cases we found that diamond was formed. We realised this big conundrum existed in astrophysics and when we looked at the conditions in our simulations, they were exactly those you find in space.’ Marks suggests that the commonplace conditions would permit nanodiamond formation prior to and during the formation of our solar system, resolving confusion regarding evidence for the age of nanodiamonds.
Rhonda Stroud says that Marks’ model is indeed convincing but may not be the only explanation. ’I suspect that there will be multiple origins, multiple populations of nanodiamonds and once we can measure them individually, we will be able to distinguish the diamonds of different origins’. Stroud also notes that unambiguously identifying the age and origin of specific nanodiamonds will require powerful analytical techniques that are only just becoming available.
’The process of shock transformation of carbon onions into nanodiamond is quite realistic,’ confirms Sasha Verchovsky at the Open University, UK, who also works on calculations of nanodiamond phenomena. ’It will be interesting to do this experimentally to produce nanodiamonds from carbon onions.’
For Marks, the experimental verification of his model and its implications for materials science are the most exciting aspect of his work. ’We now want to create an apparatus that contains only carbon onions and then control their collisions with surfaces,’ he says. ’That will be the killer piece of evidence ... we’ll be able to do things you can’t normally do with carbon ... and if it works, we’ll have a new way to make diamond.’
N Marks, M Lattemann and D McKenzie, Phys. Rev. Lett., 2012, 108, DOI:10.1103/PhysRevLett.108.075503