What Asteroid Bennu Samples Are Revealing About Solar System

Researchers continue to analyze the sample brought back to Earth from the asteroid Bennu — and among the things they’ve found is that Bennu’s parent asteroid was made up of materials from all over the solar system.

Those findings are among those published in a trio of new papers based on the results of 2023’s OSIRIS-REx mission.

Tom Zega is a professor of Planetary Sciences at the Lunar and Planetary Laboratory at the University of Arizona. He’s been among the scientists studying the return sample.

Zega joined The Show to talk about some of what he and his colleagues have found.

MARK BRODIE: What stands out to you about what the team has found out so far about the composition of Bennu?

TOM ZEGA: Yeah, so the kind of work that we do in studying meteorites and samples returned from asteroids and people at other institutions that do this, we sort of have been at the mercy of what falls from the sky, right? So we can't get into our Battlestar Galactica or Starship Enterprise and fly out somewhere in the solar system and easily collect stuff. Not until we had sample return missions like this one, like OSIRIS-REx.

And one of the unique things about having a sample return mission like this is that it brings back samples from the surface of an asteroid that are not weathered or altered in any way by Earth's atmosphere. And among the major findings from our study is that the sample is composed of 80% or more of water-bearing clay material.

BRODIE: What is the significance of that?

ZEGA: Well, we know, of course, that life, at least life on Earth, requires water, and we know that two-thirds of Earth's surface is composed of water. So one big question is where did all this water come from? How, how did the early Earth get its water? And there are different hypotheses in the scientific literature about how Earth got its water. Among them, is things like comets could have delivered water to the early Earth because we think comets have lots of water locked up inside them in the form of ice or in other water bearing minerals like clay minerals.

It turns out, looking at a carbonaceous asteroid like Bennu, a lot of it, most of it is composed of this water-bearing material and organic compounds. And so it's very possible that material like what we're finding in an asteroid venue could have very, very likely seeded the early Earth with organic compounds and possibly even, even water.

BRODIE: And that would, I would imagine, indicate that there is, if not currently, then at one point the possibility of water elsewhere somewhere in the solar system, right?

ZEGA: Absolutely. So when we look at the, you know, when we look at the minerals that are comprising the asteroid samples, not just Benu but also Ryugu, which was material returned by the Japanese aerospace Exploration Agency, or JAXA, those two contain lots of, of, of water-bearing clay material. And we think that what was happening some 4.5 billion years ago is that these asteroids accreted a combination of silicate dust, some metal with some carbon, organic material and icy material from the outer solar system.

It could have been water ice. It could have been CO2 ice. It could have been ammonia bearing ice. It could have been a combination of those things. So once that ice accreted together with all this dusty material from the early solar system, all we needed, or all it needed was a little bit of heat to melt that ice, make a liquid. That liquid now is in contact with solids, and now you have everything you need to start doing all kinds of chemistry.

BRODIE: So what questions then does that raise for you? Like, what do you want to find out next based on that particular finding from Bennu?

ZEGA: Well, among the next phases of the mission would be, we know what it's mostly composed of. We generally know the kinds of chemical reactions that led to the products that we see today. One of the outstanding questions is when exactly did that happen? Can we actually put a time stamp on it?

And so there's a way to measure the timing or dating of that, and that's by looking at radioactive isotopes that may have been part of the sample for, early in the solar system history. And so by measuring some of the radiogenic isotopes with sophisticated instruments and mass spectrometers, we can get a sense of when all this chemistry happened and actually put a very precise age date on some of this chemistry.

Another one of the key questions, of course, is when we look at the organic compounds in the sample, and there are quite a few organic compounds. We've identified 15 of the 20 amino acids that life uses to build proteins, and we've identified all five nucleo bases that are the genetic components of DNA in the return sample.

So we'd like to know is that the complete diversity of those organic compounds, or do we see more organic compounds in the sample? And so the next phases of sample analysis will almost certainly involve continued measurements of the organic component.

BRODIE: So I know that one of the other things that you guys found with this sample is that the parent comet where Bennu, the assumption is, broke off from, had material from sort of all over the place, and I'm wondering how typical or atypical is something like that.

ZEGA: Yeah, that's an excellent question. So, again, those of us that have been studying meteorites for, for many decades, there's always been this general idea or hypothesis about how meteorites form and where they come from. So to give you, to give you a feel for it, meteorites come from parent asteroids. And asteroids largely reside in a belt between the orbits of Mars and Jupiter, OK? And so we tend to think of this belt as being static, it's there, not much is really going on.

But in the early solar system, we don't think it was so static. Things were moving around, there were lots of impacts, things were dynamic, right? And as it turns out, when we look at the return sample, we don't find some features in it that we would have expected based on what we had seen from meteorites or what we've looked at in meteorites for many decades now. And so it's sort of upending our idea of how the solar system maybe was put together.

And so samples from asteroids like Bennu and Ryugu look very similar to another group of meteorites that we have in our terrestrial collections called CI chondrites, and they seem to suggest that much of the material that comprised these asteroids came from the outer solar system rather than the inner solar system. And there may be a lot more of them in the asteroid belt than we previously thought based on what we have in our terrestrial collections.

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