Spotty Mars rocks hint at ancient bacterial life

Unusual rocks discovered on Mars may hold the most compelling evidence yet of ancient life on the Red Planet. The mudstones - which have been found in a dried-up riverbed by NASA’s Perseverance Rover - are marked with unusual patterns scientists have dubbed “leopard spots” and “poppy seeds.” Researchers say the features are rich in minerals that could have been formed by microbes billions of years ago. It’s also possible they were created by natural geological processes, but NASA scientists believe the discovery could be the clearest sign so far that life once existed on Mars. Sanjeev Gupta is a planetary scientist from Imperial College London and one of the authors of the study…

Sanjeev - This finding, this result, was not in a place that we actually expected. We’d explored the delta and Jezero crater. There’s a big delta fed by a river valley. So this is the big river valley that cuts through the crater rim. In orbital images we could see these really light-toned rocks and they looked really intriguing and we decided that we would actually go and have a quick look at them. So we got there and we found out that they were really fine-grained and had these really fine laminations in them. And we also had some on the sides of the valley. We had some coarser-grained rocks, pebbles, etc. And so our inference is that these are the infill of a lake within an ancient river valley.

Chris - And when you then began to look at the rocks, when did it emerge that there was some interesting mineralogy there?

Sanjeev - We have a series of two key instruments attached to the rover arm. One is SHERLOC, which is a Raman spectrometer that can look for organics. And the other one was PIXL, X-ray fluorescence, which can do mineralogy. But importantly, what it does is relate that to the texture and the grains, etc. in the rocks. And so we set about looking at these rocks. Then we started seeing some really interesting chemistry.

Chris - Like what? What sort of chemicals stood out?

Sanjeev - Well, firstly, we saw these really little dark spots, which we called poppy seeds. They turned out to be rich in iron sulphides and phosphates. And then the most intriguing features we saw were these spots that we called leopard spots. And we think that those are iron-3-containing rocks or haematite-bearing rocks. And then when we looked at these spots, which are just a few millimetres in diameter, what we could see is that the centre of the spot was light-toned and then around it there was a rim of dark-toned minerals. And this is very, very intriguing. On Earth, you find these features and we call them reduction spots. That’s where you’ve got these redox reactions taking place.

Chris - Is that sort of an interface then? So you’re saying one group of chemicals is migrating in one direction, they meet something or a different environment coming in the other direction and at the battle zone between them, as it were, that’s where you get this line. And that’s what you’re seeing.

Sanjeev - Yeah, so that’s basically a reaction front, essentially. That’s fossilised in the rocks. And this was the first time we’ve seen something like this. But they’re really intriguing. And we see them on Earth. I don’t know whether you’ve ever been up to northwest Highlands, Scotland, where we have some of the oldest sedimentary rocks in Britain, the Torridonian group. And you can see these spots. So you’ve got this interesting redox chemistry taking place. But what drove these redox reactions? On Earth, there are two different mechanisms. You could do this either abiotically, but that often requires higher temperatures. Or you can do it biologically by microbes mediating that redox reaction. Also within these rocks, the SHERLOC instrument, the Raman spectrometer, has detected organics, carbon compounds. We’ve got organics. We’re in a habitable environment. And then we have these very interesting features that tell a story of past redox reactions.

Chris - So the microbes are eating some of the minerals. They’re producing basically the building blocks of other things that then other microbes can... And you get a community because the microbes are feeding microbes. And that suite of chemical reactions results in the deposition of these minerals with that intriguing pattern. So you’re seeing on Mars what we sometimes see on Earth, sometimes linked to life processes. And so it’s intriguing. We have organic material, these minerals, and a sort of configuration or conformation very similar to what we sometimes see here on Earth.

Sanjeev - Exactly it. And the reason we’ve called this out is because it’s something special. It’s not something we’ve seen before. And we’ve called it a potential biosignature. That has very, very specific meaning. So it’s not words being thrown around. It means that we have a feature in the rocks that could have either an abiotic, so non-biological, or a biological explanation. And we need to analyse it in more detail, study it in Earth laboratories, to be able to tease out between those two different hypotheses.

Chris - Why has it got to come back to Earth though, Sanjeev? Because I thought these rovers are now so damn good with all the instrumentation. Is there no chemical fingerprint or analysis we can do in situ that gets us a step closer, that moves our confidence further down that track to say, yeah, this is looking promising?

Sanjeev - Perseverance’s mission was always a sample return mission. So it doesn’t have some of the complex instruments that Curiosity has. Where these instruments would sit is occupied basically by our sample store.

Chris - We can’t drive Curiosity over there. It’s too far, is it?

Sanjeev - Way too far.

Chris - Are you not really frustrated now though? Because you’ve got this, it’s a bit like seeing a pot of gold at the end of a rainbow and you think, oh, it’s so tantalising, we’d love to get there. And you can’t.

Sanjeev - Oh, actually not. For me, certainly, it won’t be my science that analyses those rocks. That’s what I’m getting at.

Chris - I mean, it’s like you think, oh, I’ve opened the door, there’s something really cool to look at and you can’t look at it.

Sanjeev - Ah, no, it’s the long game, you see. It’s like a sweet shop up there, frankly. We studied these rocks about a year ago and we’ve been looking at the crater rim and these amazing rocks. So we’re well on our way to looking at other things. So it’s stored, it’s parked, it takes a long time to produce the paper and it’s there for future generations. So yeah, I’m happy to wait.