Astrobiology Magazine: It's my understanding that the instruments on ExoMars will be looking for biomolecules typical of terrestrial life. You don't plan to look for indicators or patterns that could be caused by biology with a different chemical basis than that of terrestrial life. Is it true that ExoMars will focus on looking for Earth-like life, and if so, why?

Jorge Vago: Well, there are two answers to that. One is that, in reality, what we're looking for is organic molecules, carbon-based. And that is of course based on what we know about life on Earth. The other thing we do know for sure is that there has been a very large exchange of meteoritic material between the two planets. So we can be pretty sure that Mars was in some way seeded with organic matter that came from Earth in the early period of the planet. Would this give rise to life on Mars as we know it or not? - well, I don't know.

On the other hand, we know that carbon is a very able atom when it comes to producing complex molecules, under the thermodynamic conditions that we know to prevail on Earth and on other terrestrial planets, including Mars. We have no evidence that point to more exotic chemistry being active on Mars. And when you have to design a mission, you have to be practical. So we go for the things we know. If we don't find any organics - we should at least be able to find the organics that we know are delivered to Mars daily by cometary and meteoritic dust - if we don't find that, even in the subsurface, then at that point we will have moved the bar for trying to find life on Mars much, much higher. The next mission that will try this will have to probably go much, much deeper.

AM: Finding fossil life is also tricky. There's a lot of debate about the oldest fossils on Earth, in which there are supposedly traces of life. There are questions about what you can really see, how easy it is to distinguish ancient fossils from the effects of purely chemical processes, what are reliable biomarkers. If it's that hard to find ancient life on Earth, how are you going to do it on Mars?

JV: Well, actually, it might be easier to find reliable biomarkers for very ancient life on Mars than on Earth. Because on Earth you have had all these plate-tectonics and reformation processes and volcanism that have made it extremely hard for us to find old rocks, older than 3.5 billion years, where you can say, Yeah, I'm looking at a rock that is more or less in the same state it was in when it was formed 3.5 billion years ago. This is simply not possible on our planet. Now Mars seems to have gone into a dormant stage very early in its history, so it is likely - and this will depend, of course, on our landing site - that we can access a place that is very, very old, where the rocks will be more or less in the same state that they were in when they were deposited there, 3.5 or even 4 billion years ago. What are we going to find there? Nobody knows. But chances are that whatever organics might be encased in those rocks will be in a much better state than you would hope to find in Earth rocks of a similar age.

AM: You're developing the capability to look both for extant life and for fossil life. How do you find a landing site that provides you with both opportunities?

JV: That would be very, very hard. My guess is that we will target some formation that we can definitely say is related to the past presence of water on old Mars. A site that may be of more interest for present life, is one where the Odyssey gamma-ray spectrometer indicates there may be water ice within one meter from the surface. Off course, water ice doesn't mean there's life there. It's really, really cold, but it's interesting. If anybody were to find some sort of heat in the next couple of years associated with the presence of water ice, then that would be interesting for a possible site for present life. I would say that from the point of view of the objectives of the mission, if we want to maximize the chances of finding something, the priority will be on targeting fossil life, or past life. And if we can get to a site that also has the potential for present life, that would be a bonus.

AM: Do you think Mars is, or ever was, a living world? What do you expect ExoMars to find?

JV: What I would like to answer with this mission is the question of where are the organics that we know should be there from the meteoritic delivery, and I think we have the instrumentation to do that. Hopefully these oxidants do not extend so deep into the subsurface that we are unable to find anything.

Now, regarding the possible emergence of life on Mars, I think if it happened, it happened very early, more or less at the same time when it did on Earth, and it will be very, very hard to prove with a single mission that there was life on Mars. But maybe we can at least plant a seed with this mission, and find a few strong indications that there could have been life there. To establish for sure that there was life on Mars may require more than one mission. But we can get started, and that would be a good thing.

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