At a meeting of the American Astronomical Society's Division of Planetary Sciences, held in September 2003 in Monterey, CA, McKay gave a plenary talk in which he discussed the evidence for a cold, but wet, early Mars. McKay compared these early Martian conditions to Antarctica's modern-day dry valleys. And he laid out a strategy for searching for evidence of the organisms that may have inhabited Mars during its first billion years. His talk is presented here in two parts; this is part two.

As I mentioned earlier, Antarctica is very cold, but the pressure is high enough to support liquid. So when the glaciers melt, the water flows down the Onyx River, and it's stable against boiling and it flows into the lake.

That's the one requirement that's not met on Mars today. The reason that you couldn't see such systems on Mars today is not because it's too cold. Cold isn't really the issue here. It's because the pressure is too low.

So the key environmental factor for making Mars a better place for life, a kinder, gentler planet, is not making it warmer. The key factor is raising the pressure up from 6 to maybe 100 millibar. [One hundred millibar is one-tenth of the pressure on Earth at sea level.] Not much higher than that would be needed.

At that pressure, liquid water could exist on a very cold Mars. Lake Vanda in Antarctica could be an analog, for example, for Gusev Crater, which Nathalie Cabrol and many others have shown is likely once to have been full of water. If you look around at the terrain around Gusev, you can see that it would have been very cold at the time. So the remnant of an ice-covered lake could be what the MER-A lander "Spirit" is going to land in.

And what might it find? Probably the best thing it might find in a place like this is a fossil.

And now I want to go back to the question that I raised originally: Why are we going to Mars? We're going to Mars to search for a second genesis of life. A second genesis of life is not something we're going to get from a fossil. Fossils are not enough.

We'd be happy to find a good fossil on Mars. I'm sure it would make the cover of Science - or Nature, depending on whether it's NASA or ESA that finds it. It would tell us that there was life on Mars. But it wouldn't tell us the nature of that life, or its relationship, if any, to life on Earth. That's the key question: We want to know not just was there life on Mars, but how does it relate to us? Are martian organisms our cousins, or do they represent a second genesis?

Well, to do that, again I return to analogs on Earth, as a way of developing a strategy for searching for a second genesis on Mars. In Siberia, in old permafrost on Earth, we find frozen bacteria. This is 3.5-million-year-old permafrost, some of the oldest permafrost on Earth. And we find viable bacteria in this permafrost.

We're developing drills that can drill in permafrost without drilling fluid. There was some work done just a few months ago up in the Arctic, drilling in permafrost with air-supported drills, using a technique that lets us demonstrate that there's no contamination getting into the drill cores. We're learning how to use a drill as a microbiological instrument.

In Antarctica, we think we have ice that may be 8 million years old. Again, in that ice, we find viable bacteria. In the oldest and coldest ice on Earth, we find organisms still preserved. So we want to apply this logic to Mars. Could something remain preserved in the permafrost on Mars for a long period of time, perhaps billions of years?

Well, what limits long-term dormancy? There are two factors. One is the second law of thermodynamics, thermal decay. But this is not that important on Mars because it is so cold there.

The other, background radiation, from natural levels of the decay of uranium, thorium and potassium, even deep below the surface, is about 0.2 rads per year on Earth and would be roughly similar on Mars. This would deliver a lethal dose, even to the most radiation-resistant organisms, in about 100 million years.

So although it's cold enough in the martian permafrost for life to be preserved, over the time period that we're interested, in there would be hundreds of lethal doses delivered to any dormant organisms trapped there. So - "It's dead, Jim."

But it's there. And that's important. Because there's a big difference between something that's dead and a fossil. If you're searching for life and you find a corpse - that's what it would be, a corpse - you can do an autopsy. You can determine whether the corpse has the same genetic biological content that we have. You can't do an autopsy on a fossil. And the permafrost on Mars is where we have the best chance of finding these frozen, dead micro-martian corpses to do an autopsy on.

We have thought for awhile that there was a permafrost on Mars, that it had deep, cold ice-cemented ground. Now we have further indications that that's the case. The Mars Odyssey neutron spectrometer results show ground ice in the polar regions, and the magnetometer results indicate that in some of those regions, the ice is very old and very stable.

So I would argue that at longitude 180 degrees west and latitude about 80 south, far enough south that you're in deep permafrost but not so far south that you're in the younger polar deposits, you could find the oldest frozen material on Mars. And the magnetic striping in the terrain there, seen by the magnetometer onboard Mars Global Surveyor, confirm that this is a region that's likely to have been undisturbed by impacts for billions of years. Because you see where there have been large impacts, like in Hellas and Argyre Basins, that the pattern of magnetic striping has been erased.

So one strategy for searching for life is to go find fossils in Gusev. But then we also need to address the question of a second genesis, which is really the big question, the question that scientifically and culturally drives astrobiology. And to do that, we need to go drill deep into this permafrost where we will hopefully find an actual Martian organism.

Now, if we find a dead organism on Mars, how are we going to tell if it was once alive? How are we going to recognize life? One is to use a tricorder. You remember in episode 26, they adjusted the tricorder so they could not just detect life, but could detect silicon life, all done within a few seconds of the show - a great device.

Another approach for detecting life is to say, "Well, we'll just know it when we see it."

But maybe we can do better. And I want to make the suggestion that there's a general principle that we can use to detect life. I call it the LEGO principle. And it's based on the rather simple observation that life is built largely from a small number of components. Life is not just a hodge-podge of stuff all thrown together. It's certain bricks, used over and over again. The basic polymers of life, the proteins, the polysaccharides, the nucleotides, DNA and RNA, are all based on these few bricks, used over and over again. The same way a LEGO city is built out of identical bricks. And this is likely a common property of biology, as well as of mass-produced children's toys, throughout the universe.

There are, for example, the 20 amino acids that are used as the LEGO blocks in building up the proteins used by on Earth. Alien organisms might have a different set of LEGO blocks, but they would have a set of LEGO blocks. They would use certain molecules over and over again. And we would see these molecules show up in unusually high numbers.

And that's, I argue, a possible way to recognize a biological organic material from a non-biological one, even if it's alien and we can't amplify it with PCR. So I'd like to propose that on a future mission to Mars we send a 10-meter drill to the polar regions, to the permafrost, to look for martian LEGO blocks. I think that's where we have the best chance of digging up some ancient organic material from martian bugs, and of finding evidence of a second genesis of life.

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