Astrobiology Field Research Editor Henry Bortman recently interviewed the ExoMars project scientist, Jorge Vago. In this, the first part of a two-part interview, Vago describes the goals of ExoMars and explains why, to achieve those goals, the rover will drill as much as two meters below the martian surface.

Astrobiology Magazine: Can you give us an overview of the European Space Agency's upcoming ExoMars mission?

Jorge Vago: The goal of ExoMars is to search for traces of past and present life. So we tried to come up with a mission that is well-positioned within the international Mars exploration scenario. The MER rovers have been moving geologists. NASA's MSL (Mars Science Laboratory) will target the issue of habitability; it will include a few instruments that will look for organics, and will have a very long range, but will only look at stuff on the surface.

At ESA we have had a long program of looking at exobiology over the past 15 or 20 years, and our goal has always been to go for life signatures. So what we have proposed is a mission that contains a rover that is like the MER rovers, but which has a very capable subsurface drill that is able to go down and collect samples from up to 2 meters (6.5 feet) in depth, and then analyze those samples.

The idea is that we want to pick up where Viking left off, in terms of looking for life signatures, and try to answer once and for all if indeed there are oxidants there, if the oxidants are destroying the organics, if we see a relationship - or actually, I should say an anti-correlation - between the presence of oxidants and organics as we move down in the subsurface.

And of course we also have instrumentation that will be able to answer other questions, more classical ones, related to mineralogy. To understand the possible biomarkers and interpret whether indeed they are biomarkers or not, we also need to know a bit about the geological context. So we have these other instruments with us that, even if we don't find any organics, will provide good science.

AM: Why is it important to go into the subsurface?

JV: There are three agents that we know of that potentially can destroy or degrade organic signatures. One is UV radiation, but this only penetrates a few microns into the surface. It basically affects anything that is lying unprotected on the surface. We don't expect to find any living cells or organic molecules lying out on the surface; the UV radiation will have probably destroyed them very quickly.

Then when you go into the subsurface, assuming that you will be able to see, an extra agent of concern are oxidants, if indeed they're there. Now, if you look at models for these oxidants, some say that they only diffuse about a few centimeters into the surface, and others say that go down to 5 meters (16 feet). But, of course, nobody knows until they go there. What we do know is the following: that the diffusion of oxidants depends on the porosity of the rocks; it also depends on the humidity.

One funny thing about these oxidants, or very aggressive radicals, is if the soil is very dry they can stay in the soil and do nothing. So you could have an organic molecule very close to an oxidant and they wouldn't do anything to each other.

Now, if you have a lot of water, then the water dilutes the oxidants, and then the effect on the organics is also not that aggressive. But if you have just a little bit of moisture, then the oxidants really wake up and become really aggressive, and will very quickly chew up whatever organics are there.

So we don't only need to look at the oxidants, we also need to understand what is the humidity or water interaction between the atmosphere and the shallow surface to get a more complete picture. That's why the instrument that we have in the payload looking at oxidants also has the capability of addressing issues related to humidity.

These are two nasty agents. And then we have another one, ionizing radiation, which is of major concern for the long-term preservation of organics, for traces of extinct life or fossil life. In this case we're talking about life that could have been there in the very early age of the planet, of the first, say, half a billion years - we're talking about 4 billion years ago. Ionizing radiation has a very slow but unstoppable degrading effect on organics. But the deeper you go, the less affected you are by it.

AM: What's the difference between ionizing radiation and UV?

JV: UV is light. Ionizing radiation has two sources: one is galactic radiation, very high-energy particles that are coming from elsewhere in the galaxy or even from other galaxies; the other source is solar proton events in the sun, associated with flares.

Now, the trick here is, how deep do you need to go? And that depends on the age of biomarkers that you expect to look at. Here again we go to models and experiments. What people have tried to do is to expose amino acids, which are an organic molecule of interest to us - to simulated doses of the ionizing radiation levels that you would expect to see on the martian surface.

They run the simulation for half a billion years, a billion years, 3 billion years - not real time, of course, but simulated time - and what they've found is that if you want to be able to access things that have been sitting there for about 3 billion years, given the detection levels of your instruments, you need to go down to the area of 1.5 to 2 meters in depth.

So, summarizing, everything points in the same direction. Whether it is present life or biomarkers of past life, all the clues seem to say go into the subsurface, look within geologies that can be associated with the past presence of water - this is also what NASA is doing - but go deep, go at least 1.5 meters below the surface, and that should maximize your chances of finding something. Of course, we don't have a crystal ball and we can't be sure that we're going to find what we're seeking, but we're doing everything possible to try to maximize our chances.

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