杏吧原创

For 28 years, NASA astrobiologist Chris McKay has hunted for living things in the coldest and driest places on Earth. By studying Earth鈥檚 extreme environments, he hopes to better understand where missions like the Phoenix probe, due to land next month, should look for evidence of life on Mars. He talked to Anil Ananthaswamy after a trip to Antarctica鈥檚 Dry Valleys

What were you looking for in the ?

These are vast dry regions that were cut many millions of years ago by glaciers. We were trying to find frozen soil like that which the Phoenix probe will encounter when it lands on Mars on 25 May. Previous work had pointed us towards a place called Beacon valley at 1400 metres. So we dug three pits there but unfortunately never hit ice. It was as dry as a bone.

What did you do next?

Then we hiked up to University valley at 1700 metres, for nostalgic reasons: it鈥檚 one of the places I had camped in my first year there in 1980 with microbiologist Imre Friedman, who passed away recently. While we were up there we decided to dig another pit. Luckily, the site turned out to be exactly what we were looking for. We think it is analogous to Mars today, where the ground ice is in equilibrium with the water vapour in the soil.

How does that help the Phoenix mission?

We took samples and measurements similar to those that Phoenix will take. We also did something that Phoenix can鈥檛 do, which is to look at the biology. The lander will only look at chemistry and organic material. My job will be to extrapolate what we see on Mars, using this as a basis.

Scientifically, what鈥檚 the most important thing you have learned?

The main lesson that I have learned from my 28 years of chasing life in cold, dry places is that life on Earth is constrained by the existence of liquid water. We spent a lot of time searching for evidence of life that could get over that barrier. It鈥檚 fashionable these days for astrobiologists to talk about how rugged and hardy life is, and how it can survive in all the most amazing places. In fact, I have formed the opposite impression. Life is dangling on a fragile thread, and that thread is liquid water. We have searched for organisms that could do without it. No joy.

鈥淟ife is dangling on a fragile thread, and that thread is liquid water鈥�

So when I look at other worlds, I zoom in on the water. If there is no water, life is not really likely, based on what we have learned from life on Earth.

What else did you discover from these trips?

The other important lesson that I have learned over the years is that there is no such thing as cold and dead. If you freeze something, it can be preserved for a long, long time. In fact, this is a lesson that they teach us in snow survival school in Antarctica. If you find a body that鈥檚 cold and apparently lifeless, don鈥檛 assume that he or she is dead 鈥� warm them up and find out for sure.

It鈥檚 true for microorganisms, too. They get trapped in ice and snow and get frozen. Millions of years later they are still viable. Warm them up and they can come back to life. I know from studying polar regions and permafrost that cold can preserve life for a long, long time.

What does this preservation mean for life on Mars?

It makes me both pessimistic and optimistic. Pessimistic that there鈥檚 life growing anywhere on Mars today, because there鈥檚 no liquid water there today, but optimistic that if there ever was life, it鈥檚 preserved in the cold. It鈥檚 frozen, it鈥檚 still there. Even if it is not viable, it鈥檚 still intact. The corpses are still frozen 鈥� they are not fossils, they are actual corpses.

These insights inform my job, which I view as doing research that contributes to NASA鈥檚 concept development for future missions. Where do we go and why are we going there? I have been pushing for missions to go drill in ice and exhume the frozen remains of past life. I think that鈥檚 our best strategy for searching for life on Mars.

How did you get interested in life on the Red Planet?

It was a series of coincidences. I entered graduate school in 1976, the year Viking landed on Mars. The timing was critical. If I had entered grad school a year earlier, I would have been deep into another research project by the time Viking landed. Maybe I鈥檇 have looked up and said, 鈥淥h, Viking has landed,鈥� and gone back to my degree. On the other hand, if I had entered grad school a year later, it would have been old news. As it happened, the Viking results puzzled me. Here was a planet with all the elements that you need for life, but no life. It struck me as curious and I started working on it.

I was still in grad school when I applied for a NASA summer programme in Florida. That鈥檚 how I met Imre. I was still a student when I came down to Antarctica in 1980 with him and got involved in studying life in extreme environments. That was coincidence number two. But for a little bit of randomness, I could have been doing something completely different 鈥� neutrino physics at the South Pole, perhaps.

Did you ever want to go to Mars yourself?

Not really. There is an upper height limit for astronauts and I鈥檓 2 inches above it. That was never really a career option for me. Plus, I don鈥檛 think I鈥檇 like being an astronaut. When you are an astronaut, you are on a stage with microscopes pointed at you. I don鈥檛 think that would suit my personality.

When will Phoenix start sending in results?

Probably a month after landing. It takes a while to shake out the spacecraft, open the arms, dig a pit. The engineers proceed very cautiously, so it鈥檒l be at least a month before we start seeing results.

What鈥檚 next after Phoenix?

My focus is on a follow-up mission to the polar regions on Mars with a deep drill. We have been calling the mission Ice Breaker. Phoenix is an 鈥渋ce toucher鈥�. It will reach the ice and just scrape it. We want to break through the ice 鈥� 1 metre, 2 metres, 4 metres into the ice. It is not yet an approved mission, though. We tried to put in a proposal, but the technology for the ice drill just wasn鈥檛 there. So I am taking a step back and focusing on developing the drill, the actual ice breaker.

We are pushing two concepts. Carol Stoker is pushing a drill on a string, where the drill is self-contained and lowered on a string. I鈥檓 pushing a solid core drill, with a sampler built into it.

How do you feel about human exploration of Mars?

I鈥檓 at NASA because we fly spacecraft, and because someday we might send humans to Mars. I like to think that some of the science rationale we are developing for robotic exploration will also be the science rationale for human exploration. I would send humans to the same place to do the same things. Land in the polar regions of Mars, drill deep down into the permafrost, pull out the frozen remains of the Martian biosphere and let me know if there is any evidence for life. I鈥檒l be waiting in a rest home in California.

Astrobiology 鈥� Learn more in our out-of-this-world .