Could another planet host mirror life?

Finally, if mirror life exists, it would show that life can develop with a different molecular handedness than what we see on Earth. Even if we never find it, studying the possibility helps us understand how life works. So, could it exist on another planet? And could it be problematic? Here’s Lewis Dartnell, an astrobiologist at the University of Westminster…

Lewis - When we're searching for life on other planets, we're expecting to find organic life, certainly on places like Mars or Europa. Carbon chemistry based life. And let's suppose we do find life on the surface of Mars or maybe molecular fossils, one thing that would be a really good sign that that life is different from terrestrial life would be if those biomolecules on Mars had the opposite handedness. So rather than life on Earth, which uses on the whole left-handed amino acids and right-handed sugars, if we found the opposite situation on Mars, that would be a very, very clear signal that that Martian life is genuinely alien, that it came from a completely separate origin, and we're not finding contamination from Earth.

Chris - So aside from its interest and its academic and possibly industrial applications, the mere fact that you might find mirror image life would be a really, really good endorsement of its veracity, if you like.

Lewis - Yeah, absolutely. And certainly with astrobiology, that chirality would be a very, very useful indicator that the biosignatures, the signs of life we found are from life which is very different from us, very biochemically distinct and we're not just finding contamination from the lab tech when we built the rover or the Mars probe. Or possibly that there seems to be the distinct possibility that life could have been transferred between the inner planets during the earliest chapter of solar system history.

So during the late heavy bombardment, when all the rubble left over from building the planets is still flying around through space and chipping off bits of Mars and Earth as meteorites, it's almost like the inner planets were sneezing in each other's faces constantly. So there seems a distinct possibility that life could have been transferred from Earth to Mars, or maybe vice versa, from Mars to Earth in the very early Solar System.

Chris - The rovers that we've got trundling around on Mars, some of them are quite advanced analytical laboratories in their own right, aren't they? But can they detect chirality? Are they equipped to do that? And have we found anything in that respect yet?

Lewis - We're essentially sending miniaturised laboratories to Mars with wheels on the bottom, so they're mobile and can find the most interesting spots, and solar panels on the back or other power sources to drive them around. And this chirality, that signature, is exactly the sort of thing that we're trying to detect on Mars. And although we have found organics on Mars, we've not detected yet any unambiguously biologically produced biomolecules.

So that would be the next step, the hopeful next step for astrobiology.

Chris - When we make one particular handedness of a molecule, there is this process of what they call racemisation or spontaneous isomerisation, where in fact the things can rearrange themselves to end up the other handedness. And that happens with time, just by chance, doesn't it?

So is there a possibility, if life went extinct on another body in the Solar System a long time ago and it was one handed, could it not have mixed everything up since then? And so we end up seeing a balance and we just assume, well, there was no life because there was no chirality, no bias in handedness.

Lewis - Let's assume we don't find life which is extant on Mars. So we find molecular fossils, the breakdown products of what once were cells, or at least we believe once were cells. And over time with just thermal degradation, just chemical breakdown, and the action of things like cosmic rays, radiation from outer space bombarding the Martian surface (which is my own field of expertise) these would all act to break down the large, complex, very obviously biological molecules, things like DNA or proteins, break them down to simpler molecules, the monomers that they're built from, which we'd expect to find anyway. And you're right about that racemisation process as well. If you start with a distinct bias in either direction in the anatomers, over long periods of time, that starts getting pulled back towards an equilibrium to equal ratios of both.

So it's almost like time is erasing, is rubbing out those bias signatures and making it harder and harder to have unambiguous evidence that there was once life on Mars.

Chris - Are we being adequately cautious? And have we always been adequately cautious when we've sent things farther afield to land on far away worlds to make sure that we don't pollute them? Because it seems to me that there's a possibility we could quite easily upset the apple cart.

Lewis - We try very hard with exploration of other planets to sterilise our probes as far as is reasonably possible. And this falls under a set of international laws called planetary protection.

We don't want to contaminate Mars with the very thing we're trying to find there. But the next step of exploration would be a mission called Mars Sample Return. So rather than sending a miniaturised laboratory to Mars to do all of our experiments in situ on the surface, we'll send a mission, send a probe, to collect some promising samples, maybe dug up from the ground, maybe rock samples, and then launch those back to Earth. So we can then use all the laboratories around the world to scrutinise these samples of Mars and try to find bias signatures. And in that case, we'd be facing the opposite problem. We want to not accidentally contaminate Earth with any life that might have come from Mars. So people are already designing and building containment facilities to make sure these pristine samples of Mars are kept completely isolated from the outside world when they return to Earth.

Chris - Wouldn't it be safer to do that sort of thing in space? We have some kind of orbital laboratory like the ISS and do the analysis off planet.

Lewis - This is quite a common trope of science fiction. It's just incredibly expensive to launch an entire capable laboratory into low Earth orbit. And I suspect it might be the worst of both worlds.

If you're having to try to miniaturise your laboratory equipment, your instrumentation, to make it flight capable, to make it capable to fly to Mars, you're not really gaining much by having to use similarly miniaturised equipment just in low Earth orbit, rather than using all the very, very capable instrumentation laboratories we have here down on Earth.

Chris - Those samples you're referring to, they're already being collected on Mars, aren't they? Perseverance, the rover that's there, is picking them up. But what are the prospects of actually getting them back? Because that would really clinch it, wouldn't it, if we can get those samples and give them proper rigorous analysis? Is it going to be in the near term or are we looking at decades to get those samples?

Lewis - With space exploration, it always comes down to getting the budgets in place, getting the missions designed, and then actually built and launched. And although Mars Sample Return has been on the horizon for a while yet, it's not yet been completely fully funded and given the final green light to go ahead. And who knows what's going to happen in the current American administration and government with funding for NASA and for science missions like this.