Astrobiology Magazine (AM): You've outlined three things that any place on Mars would need to be considered habitable: liquid water, an energy source, and the chemical building blocks of life.

Carol Stoker (CS) : That strategy for assessing the habitability of a site was laid out by MEPAG: the Mars Exploration and Payload Analysis Group. They've developed a program for the exploration of Mars over the next decade, and they appointed a committee to devise a strategy for the astrobiology field laboratory. That's the mission after Mars Science Laboratory which is supposed to try to detect life. The committee recommended how to pick a landing site for that mission, and also what are the precursor things that need to be done before you can fly a mission to look for life.

To some extent, this was informed by the 1970s Viking missions to look for life, which we sort of did in the blind. At that time, our understanding of Mars was very primitive compared to what it is now. We had a Lowellian view where we thought Mars was a warm, wet, habitable planet. So we had optimistic assumptions that we would find life wherever we landed.

For the astrobiology field lab, the working group at MEPAG recommended a strategy to evaluate three probabilities: the probability that an environment has or had liquid water in the past, the probability of there being an energy source to support life, and the probability of there being a chemical environment that's conducive to supporting life. The product of those three probabilities creates a habitability index.

We'll use the observations from Phoenix to evaluate those three parameters, and see what kind of a score we get. The selection of the landing site for Phoenix and the choice to explore the northern plains was based on theoretical modeling that suggests we should get a high habitability score there.

AM: Have they picked a site yet for Phoenix, or do you mean that the northern high latitudes generally look promising in terms of habitability?

CS: You would expect anywhere in the northern plains to be pretty good. Selecting the landing site is largely based on finding places where it will be safe to land, but another key factor has been selecting a site where there's ground ice. Mars Odyssey discovered there's near-surface ground ice virtually everywhere in the northern plains north of 60 degrees, and to a large extent the choice of a landing site for the Phoenix mission is driven by that discovery. The actual landing site will be chosen to optimize both the amount of ice and the depth below the surface at which that ice occurs.

AM: Do all three qualities -- water, organics, energy -- need to be present for a site to be considered habitable? What if we only find two out of the three?

CS: Well, of the three, liquid water is the one that has the most discrimination between different landing sites. Sunlight is the dominant energy source for most places where one would look for evidence of life on Mars, and sunlight is going to be available pretty much equally anywhere.

While the chemical building blocks of life is certainly an issue, they also are available nearly everywhere. We have measurements from four landing sites on Mars which show basically the same global uniform soil unit, and the soils contain those elements. So the presence of liquid water is going to be the big difference between sites on Mars. In the MEPAG analysis, they stated that unless you have a high habitability index -- in other words, that these three probabilities combined suggest there was ever a habitable environment -- you shouldn't send a life detection mission. But a factor they didn't consider is the detectability of biosignatures. One of the things you need to know is whether biosignatures are preserved in that environment, or whether there are processes that destroy biosignatures. A model based on the non-detection of organic compounds at the Viking landing sites says there are oxidants in the soil that destroy organics. So that factor would take away from your probability of detecting life.

Another factor to consider is time. The Mars Exploration Rover sites are both in locations that have a high habitability index, but the factors of habitability, like liquid water, occurred 3 billion years ago or more. That's really a long time ago. If you look at the history of life on Earth, the further back in time you go, the harder it gets to prove you've found evidence of life. In fact, the only evidence scientists have accepted as definitive about ancient life on Earth is by having a modern analogue to compare to. You're not going to get that on Mars.

So my argument is that you've got a much higher probability of finding a detectable form of life if that habitable environment was recent -- actually happening right now or in the recent past. I think the habitable conditions in the northern plains are likely to be more modern, and don't rely on something that happened 3 or 4 billion years ago. So for that reason, the biological potential of the northern plains, from the point of view of detectability, is higher.

AM: So you think the northern plains had liquid water in a more modern time period? Maybe there's liquid water even today underneath the surface ice?

CS: It's not modern in the sense that it's happening today; it's modern in the sense that it has happened in the last million years. The climate conditions on Mars change as a function of the cycling of solar insulation due to several orbital parameters that vary over time.

The orbit of Mars is eccentric. It makes a difference if you're close to the sun or far from the sun in the summer. Currently, in the northern summer, Mars is furthest from the sun, and in the southern summer, Mars is closest to the sun. That amounts to a 50 percent difference in the solar insulation between the southern hemisphere in the summer and the northern hemisphere in the summer.

The southern hemisphere in the summer is warm enough that water should be rapidly evaporating out of the southern high latitudes. But because the mean elevation of the southern hemisphere of Mars is 2 kilometers higher than it is in the north, the atmospheric pressure never gets high enough for there to be liquid water in the south.

In order to get liquid water, you need to have pressure and temperature above a certain threshold called the triple point. The triple point conditions are never exceeded in the southern hemisphere in the summer, even though it's very warm, because the pressure is too low.

In the north, during periods when Mars happens to be closest to the sun during northern summer, solar insulation is 50 percent higher so it also gets very warm. The lower elevation of the north means the atmospheric pressure is higher, so it is possible to have liquid water during the north's warm periods.

Right now, we're in a period where the solar insulation in the northern plains in the summer is at a historically low point. But 25,000 years ago, it was 50 percent higher. And 500,000 years ago it was 300 percent higher. So the triple point conditions were exceeded in the north during that warmer climate epic.

That climate epic happens on 50,000 year time scales, which is a long time by our experience. But you could have habitable conditions where life grows in those periods when conditions are good, and then freezes and remains frozen for 50,000 years. Life is perfectly adapted to doing that on Earth. We have many examples of permafrost sediments that have been frozen and metabolism-prohibited for hundreds of thousands of years, and even in some cases millions of years, and life has survived and can be revived the next time it gets warm. That kind of environment is not happening in the northern plains of Mars today, but in the last 50,000 years liquid water could have been periodically available.

AM: Phoenix has a rotor to grind up the ice, and those ice crystals will go into a scoop for analysis. Can we tell from that sample if there's any life present?

CS: Phoenix can't actually detect life, but it can tell you if there are organic compounds. The presence of organic compounds is another one of those factors that goes into detectability. Finding out if biosignatures are preserved has to do with finding out if they are being destroyed faster than they're being produced. If life has to sit around for 3 billion years, it doesn't have to be destroyed very fast to not be preserved. But if life only sits around for 50,000 years between growing periods, then you may have a chance.

The availability of solar energy cuts both ways, because on the one hand, solar energy is by far the most plentiful and attractive energy source. But on the other hand, it's highly laden with ultraviolet light, which can destroy biological signatures. You need solar energy for life to metabolize and create organics, but then you want to bury those organics so they're not being destroyed by the solar energy. You want the organics to be buried faster than they're destroyed.

The other factor comes from the Viking results. Viking poured soil into different instruments and added water, and then reactive chemistry occurred. Although the results duplicated many of the features of a biological reaction, it was interpreted as not being biological but instead as active chemistry resulting from oxidants.

There was a big difference between the reactions for the Viking 1 and the Viking 2 landing sites. Viking 1 was at low latitudes, while Viking 2 was within about 10 degrees of latitude of where Phoenix is going to land. So Viking 2 might have been sitting on ground ice, but that ice would have been under a thicker blanket than what Phoenix will be sitting on. The Viking 2 lander saw less evidence of oxidants.

Models have shown that many candidate oxidants react with water vapor, and so the more water vapor you have, the less oxidants you'll have. That's one line of evidence that suggests there won't be oxidants at the Phoenix site, or at least they won't be as dominant.

So the Phoenix site not only may have habitable conditions, it also could have conditions that will preserve biological signatures. The problem Phoenix will face is we don't know the deposition rate of sediments that might have buried the surface when it had habitable conditions.

AM: We don't know how far down 50,000 years is.

CS: Exactly. Hopefully 50,000 years is not very far down. But we'd really like to go back a million years, or 10 million years, so we'd get a record of a lot of these climate cycles. For that, we'd probably have to drill down at least a few meters.

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