On the one hand, the start of a new year is a non-event from an astronomical perspective, a man-made convenience to mark time's passage. The Earth's spin about the sun continues as always: ceaseless, smooth, and unchanging. On the other hand, year's end does fall between two significant milestones in that orbit. The most well-known is of course the solstice on December 22, when the tilt of the Earth's rotational axis with respect to the sun is greatest, bringing winter to the north and summer to the south. Less commonly known is that two weeks later comes perihelion, the time of the Earth's closest approach to the sun. So remember, amidst all the snow and blustery cold, on January 3rd you were three percent closer to the sun than you will be in July. Doesn't that just make you feel all cozy and warm? We scientists celebrate these astronomical events by buying each other fancy new lab coats in holiday colors and exchanging carefully-wrapped gifts of test tubes and telescopes. Or maybe not!
But a real gift I actually did receive this year was a copy of Super Mario Galaxy, in which the intrepid plumber runs around outer space in gleeful defiance of most laws of physics. Along the way he gathers up tiny Star Bits, which are small spiky glowing balls that fall from the sky like meteors and can be shot at his enemies. Although I suspect this was unknown to Nintendo's game designers, it turns out that small bits of stars do fall from the sky from time to time, in a most unlikely fashion. You've heard it said that "diamonds are forever," and the truth is, maybe not forever, but some diamonds do appear to be far older than the sun itself. Tiny nanodiamonds inside meteorites appear to be true "star bits," born in the edges of dying stars long, long before our solar system ever formed.
Nano-needles in a Haystack
The first hint of this came back in the 1960s, from studies of isotopic abundances in meteorites. All bodies in the solar system are composed of different elements and isotopes in approximately the same ratios, reflecting the fact that they all formed out of the same original interstellar cloud. There are small differences from one object to the next, but generally the patterns of isotopic abundances are so predictable that the few tiny differences can be used to precisely date objects, such as the uranium-decay dating that establishes the overall age of the solar sytem. So it was a bit of a surprise when chemical studies of some meteorites indicated that they had completely wacky abundances for many elements, particularly for noble gases such as neon and xenon. The most likely cause of such anomalies is the presence of mineral grains which formed long ago and far away, in the atmospheres of dying stars or the furious aftermath of a supernova explosion. These grains are fossils of an ancient time, from before interstellar mixing produced the homogenous elemental abundances characteristic of the solar system; they truly are bits of other stars, each holding the evidence of the unique conditions of its origin.
But most of any given meteorite is composed of regular rock of solar-system origin, which makes actually finding any pre-solar grains a needle-in-the-haystack challenge. Following the initial discoveries, it took some two decades of work to finally identify the carriers of these strange abundances. By putting meteorite samples through a series of acids, solvents, and reagents, the bulk of the samples can be dissolved away to leave a fine residue of pre-solar mineral grains. (This process has been likened to burning down the haystack to find the needle. . . .) What scientists found was remarkable: the exotic isotopes were bound as impurities into incredibly small diamonds, typically only 2-10 nanometers across and containing a few million atoms each. These are really tiny diamonds: you'd need to gather over fifty trillion of them to equal a one-carat diamond ring. The unimaginably small size of these grains is made up for by their abundance, as they can indeed be found by the trillion in typical chunks of carbonaceous meteorites. Often they make up as much as a tenth of a percent of the total bulk of a meteorite, a huge amount for such a surprising mineral.
Subsequent studies have shown that most nanodiamonds are nearly pure carbon, with none of the captured exotic elements that can provide a clue to their origins, but perhaps one nanodiamond in a million contains a xenon atom or other such interloper. Using vast numbers of nanodiamonds, scientists can measure the abundance ratios of these atoms, confirming that the nanodiamonds are responsible for (for instance) the anomalous pattern of high abundances of heavy xenon isotopes found in meteorites. But where in the universe could these diamonds be coming from? Their discovery came as a complete surprise, at a time when there was no other evidence for diamond dust in space whatsoever.
On Earth, diamonds typically form deep underground at high temperatures and pressures. Even if such conditions exist on other planets, there's no way to get diamonds out of those planets and back into space (short of Death-Star-like destruction of planets, something which seems hard to invoke to explain the presence of so many nanodiamonds). But around the same time in the 1980s as nanodiamonds were first being discovered, others were learning how to grow diamonds in the laboratory one layer of atoms at a time, through chemical vapor deposition at high temperatures but very low pressure. It turns out that similar conditions are present in the atmospheres of some stars, or in the expanding nebula that is the debris from a supernova explosion. Voila, a theory is born! Stars burn hydrogen to helium, helium to carbon, carbon to other elements; stars die and explode, blasting those elements outward; as the hot debris expands, carbon atoms adhere to one another to form nanodiamonds; sometimes other elements get trapped in the growing diamond crystals, becoming a telltale clue that will be decoded by astronomers billions of years later.
Not to be outdone by their colleagues the planetary scientists, astronomers rushed to find observational evidence for nanodiamonds in space. Red and infra-red fluorescent glow hints at nanodiamonds around supernova remnants such as the famous Red Rectangle nebula. Similar spectra provide evidence for nanodiamonds in the outer atmosphere of asymptotic giant branch (AGB) stars, dying stars which are not quite massive enough to supernova. Meanwhile, infrared emission lines around young stars indicate that many (but not all) also have nanodiamonds mixed in to their circumstellar dust. So far, so good, right?
A Diamond Theory with a Flaw?
As usual in science, the first theory is never the final one. If nanodiamonds formed long before the solar system, and some persisted to become parts of asteroids and then later meteorites, then scientists reasoned that nanodiamonds should be even more abundant in the most pristine materials in the solar system: comet dust. As comets evaporate under the sun's glare, the blowing dust which makes up their dramatic tails gets spread about the solar system. A portion of it eventually falls to Earth, where it can be captured by special high-flying airplanes with collector boxes strapped to their wings.
Yet contrary to expectations, captured comet dust turns out to have little or no nanodiamond dust at all. This seems to be incompatible with the idea that nanodiamonds were common in the nebula from which the solar system formed, throwing a major wrench into the whole theory of pre-solar diamonds. Back to the drawing board! Instead, perhaps nanodiamonds might somehow form in the young solar system itself. Of course, that would not explain the odd elemental abundances, nor their apparent presence around supernova remnants such as the Red Rectangle. Another alternative might be that nanodiamonds are in fact pre-solar, but for some reason tend to gather preferentially in the inner solar system, far from the outer comet belt. That would fit with all known observations—but there's no good explanation for just why the diamonds would be concentrated like that.
Perhaps the true explanation lies somewhere in between. There are vast numbers of nanodiamonds, after all. Must they all share the same origin? Some researchers think not. Remember, only a miniscule fraction of nanodiamonds include the tracer elements that hint at supernova origins. The others are a mystery, and could plausibly originate in supernovae, around young stars, or somewhere else. Maybe most nanodiamonds were formed in the inner solar system, and contaminate asteroids, meteorites, and other near-solar debris. The tiny pre-solar fraction would be spread throughout the whole solar system, but alone might be too rare to be easily found in comet dust.
Recent research supports this mixed-origin model: by using improved techniques for extracting nanodiamonds from meteorite samples, and then sorting them by average size via differential centrifugation, researchers have identified several distinct populations of nanodiamonds. Just as extra heavy xenon atoms are a signature of element production in supernovae, an abundance of certain so-called s-process elements marks grains which were produced in AGB stars. It turns out that these s-process-rich-grains form an identifiable subset of grains, which are coarser and have slightly different chemical properties than "common" nanodiamonds. Using the most recent techniques, about one percent of all nanodiamonds seem to come from AGB stars, and maybe one tenth as many come from supernovae. This implies the vast majority, 98.9% or so, must have formed either around the infant sun or in the proto-solar nebula immediately beforehand, accounting for their lack of any exotic elements. But that lucky one percent truly does include diamonds older by far than the rest of the solar system.
All that doesn't glitter . . .
The diamonds-from-space puzzle has one final piece, perhaps the strangest yet. Diamonds come in many colors, depending on their impurities: pure white, bright yellow, pale blue, even black. Nanodiamonds can't really be said to even have a well-defined color, given that they're a hundred times smaller than the wavelength of visible light so they barely interact with light at all. But some much larger diamonds may also be from space: carbonado diamonds. These strange objects truly are the black sheep of the diamond family, almost literally: black, lumpy, full of frothy bubbles and residues of former radioactivity. Some weigh as much as 3600 carats, but they're so unlike typical diamonds that I would never have recognized one as a diamond without being told beforehand what I was looking at. Carbonado diamonds look more like half-molten lumps of glassy charcoal than anything you'd ever want to set in jewelry. They aren't even found in the same places as regular diamonds, which are usually mined from veins of igneous minerals such as kimberlite that rose up from deep underground in now-extinct volcanoes. Instead, carbonados are found in a wide layer of sedimentary rocks that stretches across Brazil and southern Africa (which used to be adjacent, remember, some megayears ago).
By now probably you can guess where this story is headed. "Where oh where can such bizarre diamonds come from?" cry out scientists in curiosity and wonder. "Why, outer space of course!" comes the answer. Last year, researchers illuminated some extremely pure samples of carbonado diamonds with exceptionally brilliant beams of infrared light produced by a particle accelerator. Measurements of the resulting absorption and emission spectra provide clues to the atomic structure of the carbonado diamonds. The wavelengths of infrared light which are absorbed closely match the spectrum of pre-solar nanodiamonds, suggesting a common origin. Other spectral lines indicate the presence of plenty of hydrogen bonded into carbonado diamonds, which has been interpreted to mean that they must have formed in a hydrogen-rich environment, such as interstellar space. Their presence in sedimentary rocks spread across South America and Africa suggests that the carbonado diamonds were all delievered to Earth by an asteroid impact hundreds of millions of years ago, when those two continents were joined.
Some scientists remain unconvinced, and argue that carbonado diamonds are gigantic compared to other space diamonds, literally ten to the 20th power times larger in volume than typical nanodiamonds. There have been suggestions for possible ways they might form in the Earth's crust instead. Yet the fact that not one carbonado diamond has ever been found mixed in with conventional diamonds, and that some carbonados contain tiny bits of exotic minerals such as osbornite which are only otherwise found in meteorites, seems to seal the case for their extraterrestrial origin. So are there more asteroids full of smoky clouded black diamonds waiting out there?
The twinkling of a star in the night has long been compared to a gemstone's gleam, "like a diamond in the sky." The real diamonds in the sky may not twinkle very much, being either far too small or far too ugly. But the fact they exist at all is a glorious surprise, a scientific Christmas present we're still in the process of unwrapping, one experiment at a time. These star bits that fall to Earth may eventually help us unravel the origins of the solar system and details of the deaths of stars—and probably along the way they'll raise even more questions for the curious. You never know what's going to turn up: humble, lumpy grey-black rocks can turn out to have surprising secrets. So go forth and explore! It's a new year in a wondrously strange cosmos. And diamonds are falling from the sky.