Nanodiamonds are Forever? Maybe Not
By 1993, scientists discovered that in some nanodiamonds the ratio of different isotopes of the inert gas xenon resembled those detected in supernova explosions. Over time, scientists came to believe that nanodiamond story began with a formation in star explosions that distributed the tiny grains through the universe. One destination was the solar nebula that coalesced into our solar system about 4.5 billion years ago.
Among other places, the tiny pre-solar (older than the solar system) grains wound up in asteroids, which eventually broke up to form the meteorites in which the first nanodiamonds were discovered. Meteorites originate chiefly in the asteroid belt, which is relatively protected from the ravages of sunlight, heat and chemical reactions that probably destroyed nanodiamonds elsewhere in the solar system.
If that understanding was correct, nanodiamonds should be even more common in regions like the Kuiper belt, home of many comets, which is even farther from the Sun. And thus the surprise when, last summer, a new study found no nanodiamonds in cometary debris.
The study turned the conventional wisdom inside out, says author John Bradley, director of the Institute for Geophysics and Planetary Physics at Lawrence Livermore National Laboratory. "If nanodiamonds are truly pre-solar, they should get progressively more abundant as you go further out, and they should be more abundant in comets than in asteroids. But we did not find any in comets, which suggests that abundance falls off with distance from the sun, rather than increases."
Nanodiamonds are detected using a series of acid baths in a process that has been compared to "burning down a haystack to find the needle." Once the diamonds are isolated, a spectrographic analysis is made of their isotopic compositions.
As expected, the meteorites that came from asteroids carried nanodiamonds at roughly one part per thousand by mass. A class of smaller dust particles that were not clustered together, however, showed no trace of diamond. Because these smaller particles appeared more "pristine" - less processed by light and chemistry - the researchers concluded that they had come from comets, not meteorites.
The absence of diamonds was surprising, says Bradley, since supernova explosions should have "salted" nanodiamonds through the solar nebula: "This is turning the picture around completely."
In retrospect, however, the finding does shed light on a certain inconsistency in the link between nanodiamonds and supernovae. "It was anomalous xenon isotopes that tagged them as pre-solar, but the bulk isotopic composition was the same as the solar system," says Bradley. In other words, although the rare xenon atoms trapped in the diamonds were clearly non-solar, the far more numerous carbon atoms had run-of-the-mill solar-system isotopic ratios.
The resolution of this conflict may lie in the fact that most nanodiamonds do not contain even a single xenon atom, says Alan Boss, in the Department of Terrestrial Magnetism at the Carnegie Institution of Washington. "It could be that only one in a million nanodiamonds carries the xenon, and maybe those diamonds are still from supernovae, but the rest of the diamonds come from other processes."
But how could nanodiamonds form in the nascent solar system? One possibility, Boss says, is shock caused by continual collisions in the asteroid belt. A stronger possibility, however, is chemical vapor deposition (CVD), a process used to make diamond film in the laboratory. "The microstructural evidence from nanodiamonds indicates that they probably formed by chemical vapor deposition," says Bradley.
For two reasons, CVD, in which gases undergo chemical reactions before condensing, was until recently a doubtful source of nanodiamonds. First, CVD works better in non-oxidizing conditions, and the solar nebula apparently had a substantial amount of oxygen. Second, CVD is more efficient when the carbon starts to crystallize on a substrate of atoms such as silicon, but substrate atoms are not found in nanodiamonds. Now, it appears that diamonds can form without substrates, under some oxidizing conditions.
As the conventional wisdom about nanodiamonds is revised, Bradley stresses that the question of origins does not require an either/or answer. "My personal view is that nanodiamonds probably form everywhere throughout the galaxy, under all sorts of conditions."
For the search for life beyond Earth, the implications of a revised nanodiamond theory could be momentous. If, due to solar heat and asteroid impacts, early Earth was, as Boss puts it, "a molten body with a steam atmosphere," then where did the oceans and organic compounds come from?
If the source was, as some suspect, a rain of comet fragments, then understanding the circulation of material through the early solar system is critical to understanding the origin of life. If the source or sources of nanodiamonds can be pinned down, the little crystals could provide a rare source of data on material flow through the primordial solar system, Boss says.
Understanding the physics of the nebula is a tough job, he stresses. "This detective story is difficult to unravel. We are 4.5 billion years after the crime, and even though we are at the scene of the crime, we need every piece of evidence to make a cohesive story."
For now, the nano-detectives may turn to laboratories rather than the usual tools of spacecraft and telescopes. If most nanodiamonds formed via chemical vapor deposition, researchers need to know more about artificial diamonds. Processes that work today in the lab, after all, may also have worked in the solar nebula 4.5 billion years ago.