Chris McKay, a planetary scientist with NASA's Ames Research Center, is a co-investigator for the Phoenix Lander, as well as for the Mars Science Laboratory, which is scheduled for launch in 2009. In this interview with Astrobiology Magazine editor Leslie Mullen, McKay explains why future Mars missions must dig deeper to learn about the martian potential for life.

Astrobiology Magazine (AM): You want to drill on Mars in order to search for evidence of life?

Chris McKay (CM): Right. The surface of the planet has been bleached clean by ultraviolet radiation and superoxides. The bones are gone. Microbes don't have any bones, unfortunately, but the evidence is gone. So to find the preserved bugs, we need to go deeper.

AM: And the water ice we think is located in the north polar region is a promising target in the search for life because it's a source of water, or because it's a source for preservation? Or both, I guess.

CM: Mostly because it's a source for preservation. It may have been a source of water in the past. The good thing about ice is that it might block off the diffusion of the oxidant that's destroying the organics on the martian surface. So if the ice plays that role, and if in the last period of high obliquity the northern plains were warmer, there could have been water at the surface, and there could have been biology making organic matter. Then as the water froze, it would protect the organic matter from destruction by oxidants.

AM: Why are the plans to search the north pole and not the south?

CM: The north is lower and smoother, so it's easier to land there. The south is higher and has heavily cratered terrain. The south also has remnants of the ancient magnetic fields, and those qualities indicate that the south has an older surface. So it's the place to go for really ancient, 3.5 billion-year-old ice -- but you have to drill deep down to get at it. If I had a drill that could go a kilometer, I'd want to go to the south. But if we only can dig a few meters, we're not going to get down into the old stuff. Then the north is more interesting, because it is more likely to have had liquid water at the surface in the more recent past. In the north I think we're seeing young ice.

AM: NASA plans to launch Mars missions every two years. There's Phoenix in 2007, Mars Science Laboratory in 2009, and then a Mars Scout mission in 2011. All potentially could find evidence for life on Mars.

CM: Yes, but Mars Science Laboratory and also the European ExoMars rover are landing in the equatorial regions instead of the poles. So all they're going to find are mineral remains. I don't think they're going to find organic material.

Viking landed in the equatorial region and didn't find any organic material. There was some concern that the gas chromatograph mass spectrometer didn't work, but I think that's been put away - it worked. A separate issue is how to interpret the labeled release experiments. Of course, Gil Levin still thinks that they found life, but I'm not so optimistic.

I'm sure that Mars Science Laboratory and ExoMars would like to land at the poles if they could, but they're constrained by practical reasons to land near the equator, and at low elevation.

Phoenix is landing in the north polar region, but it doesn't have a drill, it has an arm. So it'll scratch the surface, but it won't get down into the ice. We're now working on the next step. I'd like to drill a kilometer deep into the ice, but the depth we could drill is limited by what kind of technology we could put on a lander, and how much mass and cost we can afford. We plan to be able to drill down to 2 meters and maybe even 10 meters.

AM: How would such a drill work? You'd have an autonomous drill that can only be a certain size because of launch weight and size restrictions...

CM: The drill would be stowed on the side of the deck, and when the vehicle landed on Mars, the drill would mount into an upright position. It would drill off the side of the lander, and when it drilled down to its full length another segment could attach itself and push the drill further down. The drill would be an auger, so the cuttings would move up the stem. The whole system would pull out of the hole carrying the auger cuttings, and then the cuttings would be dropped into instruments to analyze them.

AM: Do drilling missions face any opposition from the planetary protection office?

CM: No, we'll follow clean procedures. For instance, the arm for Phoenix is stored in a bio-protection bag that won't be opened until it lands on Mars. We would treat any drill the same way, sterilizing the drill and keeping it rigorously clean.

AM: Drilling on Earth is a notoriously dirty process, where you contaminate the subsurface with surface materials and other things.

CM: That's not an issue for planetary protection, because they're only concerned about contaminating Mars with stuff from Earth. But in terms of mixing the materials -- that will happen if we drill. We'll form a pile of material around the drill hole, and that will be mixed with material from various depths. But we'll still be able to get samples from known depth even while we're making a mess around the drill. We'll drill, then bring the drill up and clean it, and then send it back down and drill for a little bit, and then bring it up again. The stuff that's on the drill at that point will be from that known depth, and we can study it.

AM: So in your estimation, when is our best chance of finding evidence for life on Mars? Not until we have a mission that will drill down into the ice?

CM: Well, I don't know. Phoenix might get lucky, or we might get lucky with Mars Science Laboratory or ExoMars. We have to push in all directions. I wouldn't say that I have the answer, and that we'll find evidence for life 2 meters down into the ice. But I do think the ice is the most promising target.

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