Did life begin with the Bennu asteroid?

The life of brine...
31 January 2025

Interview with 

Sara Russell, NHM

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Bennu

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Five years ago, a NASA spacecraft successfully collected samples from the asteroid Bennu, and they were brought back to Earth in September of 2023. Analysis from these samples is now starting to trickle through, and one startling finding published in Nature has concluded that Bennu contains samples of evaporated brine - which is not dissimilar to the composition of salt flats in the United States and Bolivia. This implies that Bennu’s parent asteroid could have hosted pockets of liquid water as well as plenty of other minerals and molecules that act as the building blocks of life. To talk us through the paper’s findings, we put in a call to the Natural History Museum’s Sara Russell. She spoke to us from NASA’s press conference on Bennu…

Sara - At the Natural History Museum, we have a great collection of meteorites. So we have some expertise in looking at asteroids, which is what most meteorites are fragments of. So we got an allocation of this material and we've been analysing it for the last year or so.

Chris - What does it actually look like when it turned up before you began analysing it? What does asteroid dust look like? Sara - Well, this asteroid is very dark. What we got looked like about a teaspoon full of black sugar, if you can imagine that. So it's that sort of grain size, but completely black.

Chris - Is it black because it's been hit by sunlight or is it naturally like that? Has anyone scratched below the surface so you know roughly what you're analysing?

Sara - Yeah, absolutely. We've basically done everything to it, including cutting it in half, the bits in half. And yes, it's black all the way through mostly, except there are some patches of white, which is part of the interesting part of what we're talking about today.

Chris - So apart from looking at it visually and chopping up the grains, how have you actually analysed it then?

Sara - So we've done a whole suite of analyses on it. So for each individual grain, we CT scanned it. So just as when you go to hospital, you might get a CT scan, which shows what your insides look like. We did the same thing on these grains to see what they look like on the inside. And then we put them in an electron microscope to look at them in much greater detail. And we also looked at their chemistry, what elements they were made of.

Chris - Wow. So come on, put us out of our misery. What is the bottom line?

Sara - The rock is mostly made of a clay mineral, which traps a lot of water. So it's very water rich. And we were kind of expecting that because that's what it looked like from space. But what we weren't expecting was that it also contains little crystals of salt. And that was completely unexpected to us. And we think that these probably formed in little underground pods in the asteroid of this briny, salty fluid that then started to evaporate away and leave the sequence of salts. And we see similar kind of sequences on Earth, on lakes that have dried up, can leave these layers of salty material on them. So we've been comparing them to terrestrial salts. But also we think that this might happen quite a lot over the solar system. So we see similar salts in moons of Saturn like Enceladus, also on Ceres, which is the biggest asteroid in the asteroid belt. And so we wonder if these salts are actually telling us something pretty universal about what happens to rocks in space.

Chris - How would it have been in a salty environment then, this body that's now an asteroid?

Sara - So we think that Bennu's parent body probably formed in the outermost reaches of the solar system. There it's very, very cold and icy. So it would have accreted ice and rock. And then the ice started to melt. And then this water interacted with the rock and produced both the clays. But then there was obviously some leftover that made these pods that became salty because they had had the ions from the rock added into them.

Chris - When you say it melted, would that be when it came closer into the solar system? What would have melted it?

Sara - Yeah, no, it was probably melted very, very early in its history. Because when it first formed, it would have been very slightly radioactive. And so the radioactive heat could have melted the water.

Chris - Ah, right. Okay. So how did it end up in its present position then, if it formed way out yonder in the solar system?

Sara - Chris, that's one of the big questions that we're trying to understand. How things moved across the solar system from the outermost parts to the inner part. So it might have been in the early solar system, Saturn and Jupiter were not in the stable place they are today. They might have been moving inwards and outwards in their distance to the sun. And that might have disrupted these bodies and forced them to hop into the innermost part of the solar system.

Chris - Apart from the saltiness, are there any other interesting chemicals in there that make us kind of go, hmm, that is interesting?

Sara - We also have an organics team. And so they've been looking at all the carbon-based molecules. And they found there's this huge zoo of different, complicated organic molecules. So these aren't things that were formed by life, but they are molecules that may potentially be the ingredients for life. They found things like amino acids and nuclear bases, which are molecules that make up part of the DNA of living things. So what we think is, firstly, these briny fluids might have actually helped make these organic compounds. So they might have been a really great place for these organic stuff to get cooked up and be built.

But then also, if asteroids like Bennu were around in the early solar system, which they certainly were, and they impacted the Earth, they would have seeded the Earth with both the organic molecules, plus also water, plus also essential nutrients from the salts like phosphorus and sulphur that life needs to flourish.

Chris - Do you think then, or would a model then be that in the early solar system, we've got lots of raw materials floating around, some of it accretes into bodies like Bennu, which just also happen to be radioactive, so they've got their own inbuilt cooker. That provides the conditions together with some of that water to cook up, as you put it, some exciting chemistry, which then through gravity is dumped onto nascent planets like the early Earth.

Sara - Yes, you've exactly got it. I don't think this would have been an unusual asteroid. So if an asteroid formed early in the solar system, it would have been slightly radioactive. It's very likely to have had these very common elements of carbon and hydrogen and so on that make up these organic materials and water and salts as well. So I see this as probably being a widespread process across the solar system. It is inevitable that asteroids like this would have raided down on the early Earth and would have made it a lot more conducive to life.

Chris - They would have rained down on a lot of other bodies as well, presumably. So therefore, they were giving everywhere an equal opportunity.

Sara - Yes, absolutely. If this model is right, then absolutely. It should be quite a widespread process. So why not have life everywhere in the solar system that has enough heat to support it? And of course, also in early Mars as well. So Mars is a bit cold now, but it would have been much more habitable in the early solar system. So there's no reason why life couldn't have started there as well.

 

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