The story of the Bennu asteroid

Now, on the subject of how life’s chemistry could have come from outer space, the nearby asteroid Bennu was recently visited by the OSIRIS-REx mission, which managed to collect and retrieve a sample from Bennu’s surface, some of which landed on the desk of Sara Russell from the Natural History Museum in London where she’s been analysing it. And it’s revealed intriguing insights into how asteroids might have cooked up all kinds of life-linked exotic molecules in deep space and then potentially delivered them to Earth…

Sara - The reason that we picked Bennu is we thought it was a stony meteorite. And these are really interesting because they formed right at the beginning of the Solar System four and a half billion years ago, and they can tell us what the environment was like when planets were first forming. And Bennu was particularly interesting because it's very dark, which implies that it might have a lot of carbon in it. So we were hoping that it would be full of organic material and water. And it turned out that it was. And that's really exciting because then it can help tell us perhaps about how the Earth became habitable, how the Earth got its water and how carbon came to the Earth.

Chris - And how did you actually study it then? What was the mission that went there and retrieved the samples that you've now got?

Sara - The mission was called OSIRIS-REx.It was a NASA mission that launched in 2016, and it sent a spacecraft to go around the asteroid Bennu, which is about 500 metres across. And it spent about two years mapping the surface. So, we've got some fantastic data about what the whole of the asteroid looks like, and learned about what it was made of. One of the things that we learned was that it's actually a rubble pile. So, it's not a solid body - but it's just a pile of boulders sort of just loosely stuck together under its own gravity.

The climax of the mission, this arm came down to the surface of Bennu, and we were originally thinking that it would be quite a solid surface, but actually it was like a ball pit and the arm went straight through. Luckily it had some thrusters to kind of push it back again, and it collected about 121 grams of material, which then brought back to Earth.

Chris - Of course, there's one other way that you could get your hands on a sample of Bennu, which is if it has a slightly closer encounter with Earth than we would have liked. Is that on the cards or is it way off on an orbit that means it's very unlikely to hit us?

Sara - No, it is a near-Earth object and it does have a small chance of hitting the Earth.
So it has a chance of 1 in 2700 of hitting the Earth in September 2182. So, we don't need to worry about it for another 150 years, but it would be quite a nasty collision if it happens.

Chris - Something we've learned about it, as you say, it's a rubble pile. So, does that have implications for how we might deal with it and its ilk? There's presumably a number of them that are like this. And if we just shot something at it, is there not moreover a chance that it'll just fall apart into lots and lots of bits so we turn one problem into a million?

Sara - Well, yes, there is that problem. So, the NASA DART mission actually tried to see if the trajectory of an asteroid could be changed by having an impact into its surface. And there's an ESA, European Space Agency, follow-up mission called HERA that's on its way to see what exactly happened. But it seems like that was successful. We can move an asteroid so we can deflect it out of the way. But you're right, if it's a rubble pile, then that does add an extra bit of complexity. And I think the key would be to do it as early on as possible so we can shift its trajectory enough that all the bits will miss the Earth.

Chris - And what have you learned so far with what you've got? What's been the sort of take home from the samples that have arrived back on Earth from Bennu?

Sara - Well, we've learned so much about Bennu already. And honestly, there's more to come. So my colleagues and I are still working really hard on Bennu. But the project that I've been most closely working on at the moment is looking at salts in Bennu. So, we found that there are salts (like table salt, sodium chloride, also phosphates and carbonate, sodium carbonate) in Bennu. And we think that these formed in a brine. So, there were pods of actual water, salty water, underneath the surface of Bennu's parent body, which slowly evaporated away and left behind these salts.

This is really interesting because we think we see brines across the whole Solar System. So there's a moon of Saturn called Enceladus, for example, that spews brines out. So we can see them on other asteroids like Ceres. So, this might be a really widespread process that happens across our Solar System that we're having a chance to actually look at in our labs today. And we think that these brines may also help catalyse organic reactions. So, we might actually make important new organic reactions inside these brines. So, our organics colleagues have been looking at this in Bennu and they found nuclear bases and they found amino acids and all of these molecules that are really familiar to us as being the building blocks of life.

Chris - I was waiting for you to say ‘watch this space.’ We made a programme last week about chiral chemistry and the handedness of molecules, and one aspect of that looked at astrobiology and the possibility that life out there uses mirror image chemistry. Are you looking at chirality in these samples and are you seeing any interesting trends there?

Sara - Yes. So again, I'm not on the organics team, I'm only on the mineralogy team, but my organics colleagues have been looking at this and the thing that they found, which actually blew their minds because they weren't expecting it at all, is that Bennu seems to be racemic, which means that it has equal amounts of left-handed and right-handed versions of each of the amino acids and other organics.

And that's different to what we've observed before in meteorites that do seem to have the same bias that life has. And what that slightly sadly means is that maybe our meteorites are contaminated by being on Earth and not being curated as beautifully as the Bennu asteroid is being curated. Ideally, we'd love to get more asteroidal material to try to test this.

But yeah, it's really interesting to see that maybe the organics in Bennu have both left and right-handed versions. So maybe life on Earth, when it started, it picked a team, it decided to go one way. And from then on, all the life on Earth became that way.