We have a couple of very exciting dates on the way here in 2020 when it comes to the collection and return of near-Earth asteroid samples: On October 20, NASA’s OSIRIS-REx will drop to the surface of Bennu and collect at least 60 grams (but up to 2 kilograms) of material to be sent back to Earth for arrival in 2023. And on December 6, Japan’s Hayabusa2 will deliver to Earth samples it has already collected from Ryugu. Hayabusa2 is well on its way back to Earth as we speak.
We were very fortunate here on Earth to have a rare and spectacular meteorite find in Costa Rica just last year, and that has helped rekindle a lot of interest in the molecular origins of life on Earth, and how we may have gotten some help in that regard from outer space. That makes the upcoming asteroid events even more exciting to look forward to.
Did the molecules that kick-started life on Earth come from space? Where did we get amino acids to make proteins? Sugars and nucleotides to make DNA and RNA? Long-chain hydrocarbons to make fats and membranes?
Life on Earth today uses a crazy array of thousands of organic (carbon-containing) molecules, from simple things like acetic acid to very complicated ones like vitamin B-12:
But even our most primitive ancestors would have needed a pretty good selection of organic molecules like this — some building blocks, at least — to get anything resembling life started.
Where could such a library of molecules have come from?
Stanley Miller’s apparatus
I must have been a teenager when I read about Stanley Miller’s famous 1953 experiment, the one where he generated lots of organic molecules similar to the ones used by living organisms here on Earth simply by exposing water, methane, ammonia, and hydrogen to some heat and sparks. I felt such a sense of wonder and relief at the same time, that at least some of the special molecules you and I need to survive could have been generated by simple processes on the early Earth, that no miracles were needed to explain where they came from. Special intervention from a deity would have been the boring result. To me, the fact that these things could have arisen from natural forces in the Universe was far more wondrous.
In 1969, the Moon landing dominated the news, and rightfully so, but another very important astronomical event took place: On September 28, a very large meteorite (over 200 pounds total) fell to Earth in Murchison, Victoria, in Australia. It wasn’t like other meteorites, most of which contain just stone or metal; here was a large body full of carbon-containing compounds that must have formed even before Earth did:
A fragment of the Sept. 28, 1969 Murchison meteorite
Only about 20 of this type of meteorite have been documented in the last 200 years, and most have been smaller and not recovered immediately, leaving them subject to unknowable modifications as they sit in the rain and heat and absorb terrestrial material. But Murchison was collected right away.
The Murchison meteorite turned out to contain almost all of the amino acids used in proteins on Earth, and nearly 100 different amino acids in total. For those keeping score at home, it contained much more than that as well:
ppm = parts per million
This 2006 list doesn’t mention sugars, but just last November they, too, were found in the Murchison meteorite. One very important example was ribose, the sugar that makes up the backbone of RNA.
On April 23, 2019, another Murchison-like meteorite fell over Costa Rica, and it, too, was recovered right away. It is named for the village it fell near, Aguas Zarcas:
A fragment of the April 23, 2019 Aguas Zarcas meteorite
Aguas Zarcas is said to be the best meteorite of its kind ever to reach us, not necessarily because it’s inherently better than Murchison, but because our methods of preservation and analysis are much better than they were 50 years ago.
“If I had to start a new museum collection for meteorites, and I could only select two, I would choose Murchison and Aguas Zarcas,” says Philipp Heck, who curates the meteorite collection at Chicago’s Field Museum. “If I could choose only one, I would choose Aguas Zarcas.”
The doghouse of Rocky the German Shepherd, whose roof was penetrated by a piece of the Aguas Zarcas meteorite
So far the content of Aguas Zarcas is shaping up to be similar overall to that of Murchison:
CM2 refers to the classification of the meteorites (carbonaceous chondrite, class M)
There are already claims that intact proteins (that is, several amino acids connected together) are being found in Aguas Zarcas, as a direct result of improved analytical methods, but those claims haven’t been peer-reviewed yet. Quite a substantial find if it holds up, though. Imagine, protein from outer space!
But for now let’s keep the focus on amino acids because the facts on this are well-established. Amino acids are an especially interesting class because they have chirality, or “handedness”. Here’s one of the simplest amino acids, alanine, to illustrate:
The two different “handed” versions of the amino acid alanine
All proteins on Earth, in all known organisms, contain only one version, the “L”. It was an arbitrary choice; they could just as well have ended up all “D”. But it had to fall one way or the other. Meteorite amino acids, when they do show a bias, lean toward the “L” somewhat. So if meteorites pelted us for awhile during a period like the Late Heavy Bombardment, providing us with lots of extraterrestrial material, maybe that helps explain why “L” ultimately got picked here on Earth.
Why do meteorite amino acids lean toward the “L”, though? It’s been shown that if you expose a mixture of “D” and “L” amino acids to circularly polarized ultraviolet light, the kind that can be emitted by nebulae surrounding areas of rapid star birth, you can induce excesses of “L” over “D” by degrading the “D” a bit faster than the “L”. That’s one way the bias could have arisen in these meteorites before they glommed on to their parent asteroids.
Meteorites always have at least some terrestrial contamination, even if they were collected right away. Fortunately, you can tell the terrestrial compounds from the extraterrestrial by measuring their ratios of 13C/12C and 2H/1H (that is, carbon and hydrogen atoms with an extra neutron versus the usual kind). These ratios are much higher in asteroids than on Earth, so for the most part you can sort out what’s what. But you always have that asterisk; you just don’t know the extent that terrestrial conditions influenced the extraterrestrial compounds.
Astrobiologists are still enthusiastically analyzing Aguas Zarcas, and lots more information is still to come. It’s certainly worth noting, too, that Murchison and Aguas Zarcas contain flecks of 7-billion-year-old material that long predates the formation of the Solar System. So there are many fascinating reasons, even without the biology part, to keep studying them.
But we’re about to take all of this to a new level with pristine samples from asteroids. That begins right here in 2020.
Just a few days ago, NASA’s OSIRIS-REx completed its final rehearsal for the sampling of near-Earth asteroid Bennu, which will take place on October 20. Things went very well:
Bennu has mostly big boulders and only a few small clearish sites (at most about half the size of a basketball court) from which to grab good samples. The primary target sampling area is called the Nightingale site:
Nightingale, the primary sampling site for OSIRIS-REx on Bennu. The spacecraft’s dimensions are laid over the image to illustrate the little room for error.
The site is small, but it is on the northern part of the asteroid, meaning it stays relatively cool and thus well-preserved. Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, adds:
“The primary objective of OSIRIS-REx is to bring back organic material and water-bearing material from the early Solar System, so, being in those high latitudes we think gives us the best chance to preserve that kind of material.
Just thinking that we’re going to get this sample back to Earth sends chills down my spine.”
The REALLY big date on the calendar for 2020, though, is December 6. That’s when Japan’s Hayabusa2 drops off samples from Ryugu down to Earth.
Hayabusa2 had a lengthy opportunity to characterize Ryugu while it was there and took many detailed images. Ryugu has lots of intact inclusions that make it seem like not only a carbonaceous chondrite similar to Murchison and Aguas Zarcas, but one that has gone through little aqueous alteration (reaction with water) while within its parent body, and obviously no entry through Earth’s atmosphere or contact with the ground, so an extremely good look at the materials available in the early Solar System:
Color photo of material on the surface of Ryugu. The inset shows inclusions highlighted in red.
Hayabusa2 did its successful sample collection back on July 11, 2019:
Hayabusa2 itself will keep going on to another asteroid target, it was just announced, but that’s OK. It doesn’t need to return to Earth to drop off its payload. It will release its samples on December 6 down to the Woomera Range Complex, a very large military base in South Australia:
It’s fair to say they’ll be the most valuable minerals we have ever encountered. What will we find inside? Lots of “L” amino acids? Intact proteins? DNA components? Something totally unexpected?
2020 hasn’t been kind, but there’s hope on the horizon. We’re going to change things profoundly for the better this November, and then we’ll all be in a much better place when we receive a truly special gift, right from the sky.
