Will Mars rovers find signs of life?

Will - How to find a fossil on Mars. It's a fantastic time to be interested in the planet Mars. We have multiple surface Rover missions going on at the moment. We've got NASA's Curiosity and it's more recent Perseverance rover working their way across specifically targeted parts of the planet's surface today. And Mars has a gift for the scientist, people like myself, in that it has a very pristinely, beautifully preserved ancient geomorphic records, a surface record of its former landscapes, whereas on earth today, such landscapes, they don't last too long because we have active plate tectonics. Over geological times scales our landscapes are destroyed. We also cover up these beautiful landforms with buildings and they're covered by vegetation. It's quite hard to actually get a handle on the ancient geomorphic surfaces that comprise Earth. But Mars is different. Mars' ancient geomorphic record goes back billions of years and it tells us a history of a far warmer, wetter past.

Chris - Why have we got plates moving around and resurfacing the earth with volcanoes and earthquakes but Mars doesn't?

Will - Yeah it's a big question that lots of people are still wrestling with an answer for. We think very early in Mars' history, it probably did have some manner of plate tectonics, but pretty early on the process that's internal to the planet that actually caused continents to move around, above plates on Earth today. These processes shut down on the planet Mars and essentially left it vulnerable. It's got very low erosion rates, throughout earth history. So it basically left these final snapshots of these processes amenable to observation for billions of years.

Chris - In other words, for a fossil hunter, it's heaven if earth was like that, you'd find this amazing record of everything going back over all time.

Will - Absolutely, And this is why the rover's missions are critical to our success. Because if we could actually get geologists to the planet Mars, and actually go and touch and sniff and lick rocks, I think we'd be there by now. Right now we have to make very strategic decisions on where to go and look for our fossils on the planet. So Mars' geomorphic record, we know we've got ancient rivers, we've got lake systems, even Deltas where rivers might have met the sea. These are very habitable environments, so it's very plausible that 3 billion years ago Mars was inhabited. But that's one thing.

Chris - Have we seen anything interesting yet or anything that's got scientists excited?

Will - We found environments in which life could plausibly exist, or could have existed. We're talking 3 billion years ago, so even by a geologist's standard, this is a long time ago.

Chris - But what happened to that fossil that looked like it had some funny bacterium in it that made history? NASA said, look, have we found life on Mars? They said, are these fossilised microbes or something. Did that amount to nothing in the end?

Will - It did amount to nothing and this is going to be the curse of the planetary geologist if you like. It's very hard. We are not expecting to find, sorry for the sci-fi fans, aliens with faces. The period of history we're talking, 3 billion years ago, it's 1 to 2 billion years older than when multicellular life existed on earth. We are looking for microbial life, but when it comes to microbial life, it's very, very simplistic. So it can leave certain signatures of its potential former presence, a variety of textures which basically developed as a result of the microbial surface's interactions between the substrates and the atmosphere. But there's a lot of overlap between simple processes that form abiotically, but makes similar looking textures.

Chris - We're looking for those and are we looking for the chemical fingerprint as well then that life, even though it might not be there now, was there?

Chris - Absolutely. And this is again why possibly when we actually do get somewhere, we're not going to be finding this alien with a face. We are looking for muds. Muds are fantastic at preserving signs of life, but there's certain clay types that can actually only form through soil forming processes linked to biology. So it's those clays we're after.

Chris - And what about not Mars, but much farther away, Zander, you were saying that we can now see what's in the atmosphere of distant planets. Does this mean potentially we could go after Mars type environments or are the telescopes not up to that yet?

Xander - Well, they are getting better and better and, as I mentioned, with the advent of the James Webb Space Telescope, we are finally being able to detect molecules in the atmospheres of these distant planets, which we would never have been able to do before. In recent months, James Webb has found the first signatures of carbon dioxide, sulfur dioxide, quite considerably strong components of many of these atmospheres.

Chris - Do we know what we are looking for, though? If we were to see a distant world, would we know what the signature in the atmosphere to look for that would tell us, hey, there's probably life there?

Xander - Well, there are certain chemicals that people, we call these biosignatures, which could potentially be evidence of life there. Some of these are quite controversial. For example, I think the main one that we are really looking for is oxygen, because it's quite difficult to have oxygen in an atmosphere for a very long time without there being loads and loads of trees and algae constantly pumping it into the atmosphere as we have here on earth. But as I say, this is quite controversial because there are some people who think that there are certain processes that exist, which could generate oxygen under abiotic processes without life. So I think even when we find some kind of signature that there might be life somewhere, it will be argued over quite a lot.

Chris - The other thing that people often bring up is water. And they say this is a critical ingredient for life. Why?

Xander - I'm not a biologist, but I think it's just because water is such a crucial solvent and so it just is completely crucial for any form of life. It's a bit of a case of it's necessary, but not really sufficient, because you need water in order to have life, but just because you find some water there doesn't mean there is necessarily life there. In the first press releases of the James Web space telescope, it found water on an atmosphere of a sort of Jupiter size exoplanet. But this was an incredibly harsh environment, ridiculously high temperatures, thousands of degrees. So there would certainly be no life there, even though there is water

Chris - Some sauna, isn't it?