Asteroid Organic Chemistry

Well, since we were talking about primitive amino acids and abiotic chemistry of biomolecules yesterday, let’s have a look at a series of papers that just came out in Science. These are the reports from the teams analyzing some very precious material: samples from the asteroid 162173 Ryugu, brought back to Earth in a sealed container by the Hayabusa2 mission.

That probe was launched in 2014, the successor to the original Hayabusa spacecraft, which was also designed to return asteroid material (but had a number of problems along the way). The first mission ultimately retrieved a small sample from asteroid 25143 Itokawa (the first such return ever accomplished) but Hyabusa2 performed very well indeed. It released 3 small landers (returning the first images from an asteroid’s surface), and lightly touched down itself to collect samples from the untouched surface (as disturbed by a 5g projectile of pure tantalum) as well as from below that layer, after firing a 2.5kg copper impactor to excavate a ten-meter-wide crater.

These samples were stored in separate compartments in a re-entry capsule. Hayabusa2 changed its orbit to come back close to Earth, and the capsule was released to aerobrake into Earth’s atmosphere in December 2020, parachuting then into the Woomera Test Range in southern Australia. Hayabusa2 itself still has xenon propellent left for its ion engine, and is heading for flyby encounters with two more types of asteroid in 2026 and 2031. As a space exploration fan since my childhood watching the Apollo missions, I absolutely love writing sentences like this.

The sample capsule had about 5 grams of material from Ryugu, just what it was designed for, and that is a ton of stuff by modern analytical chemistry standards. That’s some of it in the photo! I think it’s safe to say that every single useful technique and instrument you can think of has been used on the stuff by now! Ryugu itself is a dark carbonaceous chondrite asteroid – chunks of such stuff have landed on Earth many times (notably the Murchison and Tagish Lake falls), and we’re really in need of some “ground truth” about how many types of such asteroids exist, what their composition is, how they differ, and how the various meteorite falls match up with them. We’ve done as much spectroscopy as possible from here on Earth, but there’s no substitute for a physical sample. So this is a massive step forward for the field, pristine material collected in situ. What’s in it?

Well, the overall composition and fine structure match up most closely with the CI type of carbonaceous chondrite, an extremely rare class with only a few finds known on the planet (the most recent from a 1965 fall in Canada). The prototype is the Ivuna meteorite, which fell in 1938 in Tanzania. About 700g of that one were recovered, and pieces of it are now scattered around the world in museums, research institutes, and private collections. The CI meteorites are thought to be extremely old, largely unaltered since the very early days of the solar system. Their elemental composition is the closest to that of the Sun than any other meteorite class, for example. But every meteorite that drops out of the sky here has had some rough treatment during its flaming descent and violent landing in our wet, biology-filled landscape, and some of them have been sitting around for a while before being discovered. The Ryugu samples differ from the Ivuna ones in ways that suggest that the earlier meteorite samples were indeed altered by their re-entry and exposure to Earth weather.

Now, organic chemistry is my field, so I’m going to skip over a lot of the geology and inorganic findings. What those tell us, though (through isotope ratios and other means) is that Ryugu is only a small part of what used to be a much larger body in the early solar system that seems to have formed in the first two million years of the solar nebula coming together. About three million years after that (still very early days!) the internal temperature of that planetismal got warm enough for liquid water to exist, and it stayed that way (roughly around 20 to 40 C) for several million years, almost certainly through the heat liberated by the decay of radioactive elements. This warm wet period altered the mineral character in unmistakeable ways. Subsequent cooling literally froze things in place, and after about a billion years, something catastrophic happened, almost certainly a large impact that broke the parent body into fragments. Some of these gravitationally reassembled into Ryugu (and doubtless many other bodies), and about five million years ago it wandered into its current orbit, where it’ll stay until it makes enough close passes to larger bodies to move it around again.

The Ryugu material is about 3% carbon by weight, and that carbon is distributed into thousands and thousands of different organic compounds. The chemical diversity is far higher than you get from biologic samples, where things fall into specific classes of known biomolecules – carbonaceous chondrite material is just complete chemical soup in every direction. The mass spec signatures of the soluble material indicate constant methylation, hydroxylation, sulfur addition and other such reactions over and over, making a bit of everything. It’s worth noting that many of these compounds are older than the solar system itself: there are grainy regions with anomalies in the H/D and nitrogen isotope ratios that are thought to be from fractionation in cold interstellar clouds. But many other compounds were formed by subsequent reactions during those millions of years of gentle cooking in an aqueous environment.

There are aromatic hydrocarbons (substituted benzenes and polycyclics such as fluoranthrene, pyrene, and more), and heterocyclic aromatics (a huge number of substituted pyridines, pyrroles, pyrimidines, and imidazoles). There are aliphatic amines (the smaller volatile ones surely occurring as salts – they would have evaporated away by now otherwise after a few billion years of hard vacuum). And indeed there are plenty of carboxylic acids, so that might be your answer right there. To that hypothesis, small volatile organics like methanol, ethanol, acetone, acetonitrile and diethyl ether were not detected; they either reacted with other compounds long ago or are long evaporated. There’s a good amount of less-soluble macromolecular organic stuff as well. That is likely a mix of polycyclic aromatics crosslinked by and substituted with aliphatic chains in various ways, with plenty of functional group diversity scattered around it (OH groups, carboylic acids, ketones, etc.)

And there are amino acids. As mentioned yesterday, you always get those under such abiotic conditions – the Ryugu material has ten to fifteen different ones, including those found in Earthly biology  such as glycine, alanine, and valine, as well as some of those nonbiological ones like alpha (and beta)-aminobutyric acid, norvaline, isovaline, and so on. Those are the very ones addressed in yesterday’s post on how we might have ended up with the proteins we have. All of the chiral amino acids are in racemic form.

So this is yet more detailed evidence of the way that small organic molecules (including those in classes important for “life as we know it”) are ubiquitous. You find them in asteroids, in comets, on the surfaces of other planets and moons and floating around in their atmospheres, and sitting around in interstellar clouds. These things undergo mixing, heating (everything from palm-of-a-human-hand to volcanoes), cooling, dissolution in warm water, in liquid ammonia, in lakes of ethane and molten sulfur and who knows what else, high pressures, solar irradiation, gamma rays, and ion bombardment. Every organic compound you can imagine being formed under those conditions has probably been formed, because all of this has been going on for billions of years.

If your image of outer space is of something stark, clean, and black-and-white, revise those thoughts. We find ourselves, as I once put it, in a galaxy full of gunk.