Sara Russell: Revelations from the Bennu asteroid

In this edition of Titans of Science, Chris Smith chats with a planetary scientist charged with analysing rock samples brought in from outer space, Sara Russell…

Chris - Once you were installed at the Natural History Museum, what was the furrow that, you weren't a peasant, but what was the furrow you were ploughing with your meteorites from there?

Sara - Yeah, so I'm still really interested in how the protoplanetary disk formed, but also I became more interested in the slightly later history of what happened once an asteroid was there. So by looking at the mineralogy of the asteroid itself, and especially the ones that formed with ice, you can look at how much water it originally had and how the waters interacted with the rocks. And I became interested more in learning about basically how our Earth got to have water on it. And we think that the Earth got all the water from its oceans by being impacted in its early history with asteroids that are probably asteroids that contain clays. And so I wanted to look more about how that formed and where it came from.

Chris - So, brief history of time then, we have this massive cloud of gas and dust and something kind of buffets it to start under gravity coalescing, it forms a primitive star, which then gets big enough that it fires up. And meanwhile, around its sort of middle, it forms this disk of material that the planets are born out of. But they're all like the Earth, the inner ones, they're all dry, and they get their water from these asteroids. So why, why do the asteroids have water? And the planets didn't then?

Sara - Yes, so that is a great question. And we've really only quite recently in the last few years started to have a big model of how this happened. And it's a crazy idea. But I think it could be true. It's probably true that when the giant planets formed, they probably didn't form where we see them now, they may have moved around. So they may have moved into the innermost part of the solar system and then moved outwards again.

Chris - This is Jupiter, Saturn, Neptune, those sort of gas giant planets, not little rocky planets like us.

Sara - No, just thinking about the giant planets that make up most of the mass of the planets of our solar system, but yet these are these outer planets. So while they moved around, this would have been really disruptive to the small bodies like asteroids and more icy bodies that were around at the time. This would have meant that these smaller bodies kind of were moving around in and out of the solar system, many of them being ejected from the solar system as well. But one of the results of this would have been the ones that formed in the outermost part of the solar system where there was a lot of ice. So they accreted with a lot of ice, may have been ejected into the inner solar system and then been available to hit the Earth, not just the Earth, but also Mars and Venus and Mercury as well.

Chris - Were the inner bodies like the Earth and Venus when they were forming? Was it just too hot then that they blew off all their water? And that's why they were dry and rocky. And they got their water later when things were cooler from these outer solar system bodies coming in.

Sara - Yes, that's our current hypothesis. There are some scientists who think that they may have retained some of their primordial indigenous water. But I think they probably just became too hot as they accreted and they wouldn't have had any water at that stage.

Chris - This was presumably this work, the Prelude that ended up with you working on the OSIRIS-REx mission, which that's been the real pinnacle so far of what you've achieved, isn't it? Being to actually lay your hands on pristine samples. You don't have to go to Antarctica. You've effectively reached out into deep space and got a sample of an asteroid to look at.

Sara - Yeah, it's been an absolute privilege of my career to work on this mission, OSIRIS-REx. So this is a NASA mission that visited asteroid Bennu and collected over 100 grams of material from its surface to bring it back to Earth, which makes it the biggest NASA sample return mission since the Apollo era. So it's a really big deal for the NASA teams. But it was exciting for us, those of us who study meteorites, because as I said before, one of the things that's really annoying about meteorites is that we don't know where they came from. When I was trained as a geologist here in Cambridge, one of the things that was beaten into us was that we needed to do fieldwork. We need to go out into the field, look at the regional geology, and then pick a rock to bring back to the lab for further study. With meteorites, we can't usually do that. But with OSIRIS-REx, we can. That was our fieldwork. We went out to this asteroid. We spent two years mapping it. So we really understood its surface in a huge amount of detail before selecting the area that we would sample and then bring back to Earth. We have this incredible context of knowing exactly where it came from. That helps us not only understand Bennu, but it also helps us understand all the other meteorites in our collection as well, because we can look at how it's the same or different to these other meteorites as well.

Chris - Why did you pick on Bennu? Why was that the target of the mission? Was it just because it was there, or were there some other features that meant you thought that's a good one to go for?

Sara - Yes. It was in a good place to be able to reach quite easily. It's a near-Earth asteroid, which means that it has an orbit similar to that of the Earth. That made it more accessible. But also, it was a target because it looks dark. It looked almost black. That made us think that it might be rich in carbon, which we were particularly interested in because we wanted to use this mission to try to address some of the big questions about how the Earth may have become seeded with organic material.

Chris - Has it answered those questions? Because that's the really big one, isn't it? We're all thinking, well, the planet's been here for four and a half billion years, but we've got evidence from your geological and Earth sciences colleagues that life got started here pretty quickly. The question is how? Have you got some insights now off the back of what you're seeing out there in the asteroid belt and beyond that might give us insights into what could have happened here?

Sara - Yes. One of the things that really surprised us about the return sample from Bennu is that it's incredibly rich in carbon. That carbon is mostly in the form of organic carbon. We knew that we had organic carbon in meteorites, but in meteorites, it's really hard to say how much of it is indigenous to the meteorite and how much of it might be contamination because as soon as a rock comes to Earth, it immediately gets crawled over by worms and bugs and people will pick it up. It immediately gets contaminated, but we knew that we could keep Bennu in a completely pristine state. We knew that all of this organic carbon it contained was pristine. It showed us this incredible kind of zoo of organic molecules. We found lots of amino acids there. We found nuclear bases. We found really these very biologically important organic molecules, although we don't think the ones in Bennu form biologically, but these would have rained down on the early Earth and surely must have helped kickstart life on Earth. As well as the organic material, we also found loads of salts that would also be really bioessential. Phosphates, for example. Bennu contains loads of phosphates, which obviously is used here on Earth as a fertiliser and is really an important constituent of all living things. As well as that, we found lots of ammonia as well as sulphur. So we found really kind of biologically important material, as well as the water. So Bennu contains a huge amount of water that's trapped within the clays that make up most of the rock.

Chris - How did the asteroids come by this then? The Earth didn't have it. You've argued that it was hot and a horrible environment here to start with, which would have boiled off a lot of this stuff. But where did the original chemicals come from or how did they come into existence that are aboard asteroids like Bennu?

Sara - We think that Bennu's parent body formed right in the outermost part of the solar system. So there, it didn't get heated by the young sun. It would have stayed very, very cold. So it was really rich in these volatile elements that are actually very abundant in the universe. So going back to the work on nucleosynthesis, the most common elements in the universe are hydrogen, oxygen, carbon and nitrogen. So these are really important elements in Bennu as well. So it accreted from this very kind of primordial material and it wasn't boiled off like it was in the innermost part of the solar system. But then we think Bennu, in its early history, probably had pods of water, salty water, and these would have been fantastic environments to make more sophisticated organic molecules like the amino acids, for example, may have formed inside this parent body itself. And so all of these wonderful ingredients for life were being cooked up in Bennu's parent body, not just Bennu, but also loads of other small bodies around the solar system as well. So these sorts of environments were likely very common in the outermost part of the solar system. And all it needed was some mechanism to eject them to the inner planets.

Chris - This is really intriguing then. So we end up with the Earth being a potential home, but it's not ideal and it hasn't got things really that it can use to kickstart life yet. But in a much more propitious environment, way out in space, it's much colder. You've got all the raw materials there and they're in a body which can cook them up like a kind of pressure cooker out in space. And then they're conveniently dislodged and chucked at the Earth by other bigger planets jockeying for position. And that brings these molecules that have been cooked out in deep space to Earth where they can potentially trigger life processes.

Sara - It sounds almost fantastical when you say it, but it kind of makes sense that the outer solar system would have been this environment full of all of these really important molecules. The planet migration, we think, may happen on exoplanets as well. So that means that potentially these processes are not just limited to our solar system. They may be universal processes that are found, we may discover on exoplanets as well.

Chris - Did asteroids like Bennu have a heat source then? Because some of this chemistry can't have happened to make these exotic molecules at the really low temperatures that are out there. It must have needed some heat. So where would that come from?

Sara - There were some radioactive isotopes around in the early solar system. Probably the most significant example is aluminium-26. This is a radioactive isotope with a half-life of just three quarters of a million years. This was around in the early solar system and it could have provided enough heat for asteroids like Bennu to heat up enough, at least to make liquid water.