Space News Deep Dive: Bennu, the Organic Asteroid

So I know I just wrote about an asteroid last week, but to be fair it wasn’t the asteroid I thought I would be writing about. The news about the latest data release on the asteroid Bennu kind of got lost in the shuffle of the news about 2024 YR4, but the Bennu story is also awesome and deserves its own attention.

So please pardon the asteroid double-up here at Spacing Out as we take the opportunity to dive into what the latest studies about Bennu reveal and what it might mean for this rock and for the history of the solar system in general.

Remembering OSIRIS

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Just as a quick reminder, Bennu is a rubble pile asteroid that was visited by the OSIRIS-REx spacecraft from 2018-2021, with the spacecraft scooping up a sample from the asteroid in 2020 (and almost getting swallowed by Bennu in the process). 

That sample container, safely encased in a re-entry capsule, was successfully chucked at Earth by OSIRIS-REx when it sailed past the planet in 2023 (it’s off on a new mission to the asteroid Apophis under the new name OSIRIS-APEX). When scientists opened the capsule they found not only the sample container, but an abundance of extra Bennu dirt all over the top of the container thanks to the fact that the asteroid tried to eat the spacecraft and wound up spewing dirt all over it.

After carefully scooping up all that precious extra sample, scientists took several months longer than anticipated to get the actual sample container opened because two of the fasteners on the lid were stuck and the scientists had to invent a whole new tool and get it sterilized and into the clean room to cut them.

Space Time Capsules

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Why were we so hyped to get our hands on dirt from Bennu in particular? To quote, well, myself actually, from a blog post about the then-imminent return of OSIRIS-REx written in August 2023: “One of the reasons asteroids make great sites for sample return missions is that they’re pristine fossils from the solar system’s early days. They haven’t changed much in the last 4.5 billion years, so information about our system’s infancy which has been erased from other worlds can still be found in asteroids. They’re like time capsules. Giant, dirty, hard-to-reach time capsules.”

Bennu is a type of asteroid chock-full of carbon-rich materials from the earliest days of the solar system. If we want to figure out things like how the heck life managed to manifest itself on Earth (which we do), then you really have to know what kinds of stuff the solar system was working with at the time. Bennu can help tell us that. 

While the uncooperative sample container was an unexpected obstacle that severely delayed the first major studies of the Bennu bits, preliminary examinations began almost immediately on that surplus dirt from the outside of the container. That alone was enough to tell us the trip was probably worth it. Even a very preliminary look indicated the sample was full of organic materials.

Organics in Space

It’s probably worth clearing something up at this point. “Organic” here does not refer to food grown without GMOs. It does not refer to something ecologically produced. It doesn’t refer to something alive. It doesn’t even refer to biology. It means carbon.

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Carbon is the major building block for life as we know it here on Earth. We are, as they say, carbon-based lifeforms. So is everything alive on Earth. No carbon, no life (does that mean that’s the only way for life to be everywhere in the universe? Hmm, no, perhaps not, but it’s the only type we know how to look for and the only type we know definitely works). 

The exact definition of an organic material can vary—some folks say there has to be certain kinds of carbon bonds present, others say there just needs to be carbon. NASA in the stricter camp, defining “organic molecules” in a 2018 article about findings by the Curiosity rover on Mars (another place we find organics) as “Organic molecules contain carbon and hydrogen, and also may include oxygen, nitrogen and other elements.”

So organic materials are not biology, but they’re necessary for biology to happen. That makes learning how they originally formed and where they came from in our solar system a source of major interest. Which is what makes the recent news about the Bennu sample so intriguing.

A Bennu Paradise?

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Bennu, it turns out, is teeming with good stuff life needs in order to, you know, be a thing. All the major building blocks are there. Carbon, yes, but we knew we’d find carbon on this rock. What we didn’t know is that we’d find it in complex forms like amino acids and nucleobases.

Amino acids, according to the Encyclopedia Brittanica (what, I’m a physics type and generally wave anything biology-related off as “squishy science”, I had to look it up) are “any of a group of organic molecules that consist of a basic amino group, an acidic carboxyl group, and an organic R group that is unique to each amino acid.” I hope that clears things up. Point is, they’re not simple things. Life on Earth uses twenty amino acids as the basis of all the proteins our myriad forms use to go through their varied existences.

Fourteen of those twenty essential amino acids are on Bennu, as well as nineteen others Earth life doesn’t even bother with. 

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And then there are the nucleobases! All life on Earth uses DNA and RNA as the vessel on which our genetic information is carried. Nucleobases are one of the building blocks DNA and RNA molecules are made of. Life on Earth uses five of them: guanine, cytosine, adenine, thymine, and uracil. No nucleobases, no DNA. 

All five of those nucleobases are on Bennu.

Of course life isn’t all about carbon. There are other things it needs, such as liquid water. Bennu, to be clear, does not have any. BUT it does have a number of salt and clay minerals that would have formed in a liquid environment.

Asteroids as we see them today are often the fragments of older, larger rocks that have been smashed up over the eons. That includes Bennu, which we suspect once broke off of a larger body that probably no longer exists as a single entity. And the evidence points to that object having veins and pools of liquid water running under its surface, the remnants of which are the salts and clays still found on Bennu today. 

And don’t even get me started on all the ammonia and nitrogen-rich materials found in the sample! Bennu is a smorgasbord of important ingredients for life.

Questions…

We went to Bennu to learn more about how the conditions in the early solar system could have led to life arising on Earth. Of course, whenever we seek out answers, we always wind up with more questions.

As an example, there’s a question about “handedness” in the amino acids on Earth vs those on Bennu that had me (again, not a biology person!) boggled—amino acids can be right or left handed?? I thought about diving into it here but there are other questions I’d rather ponder.

For instance, it appears that Bennu (or its parent body) had pretty much all the ingredients needed for life. But we see no evidence of actual biology—no signs of any form of life. Why not? What would have prevented even a very basic form of life from arising on either Bennu or its ancient parent? Can life even form on an asteroid, even if the right ingredients are there? 

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Did Earth originally form with this same delicious smorgasbord of ingredients? Is that how life was able to arise on this planet? Or was it all delivered to the baby Earth via rocks like Bennu? Some ammonia molecules we see on Bennu suggest that it formed far from the Sun—perhaps for a planet to have life it needs to form close in and then have the vital ingredients delivered from farther out, with Bennu-like asteroids acting as the GrubHub drivers of the solar system.

And if all of these life-critical goodies are found on a random asteroid, then it suggests that the ingredients for life were common in the early solar system. If that’s so, then Earth wouldn’t have been the only place getting a rain of amino acids and nucleobases from the heavens. But then why is Earth the only place with an obvious biosphere? What makes Earth special?

That’s the big one that a lot of our questions keep coming back to. Statistically, analytically, logically, probably, Earth as a planet should not be distinctive. It’s one planet around one average star in one decently-sized galaxy among a few trillion in the known universe. It shouldn’t be special.

And yet here we are, complex life forms looking up through our nitrogen-oxygen atmosphere at rocks like Bennu and asking questions. Maybe someday we’ll find the answers.