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 one.

I think the most important reason to search for life on Mars is the possibility of finding a second genesis of life. And I say "possibility," because life on Earth and Mars may share a common origin. It's not certain that, just because there's life on Mars, it represents a second genesis. But a second genesis is what we're interested in.

We're interested in it because it would allow us to do comparative biochemistry, just as we're interested in comparative planet logy. As you know, it's very difficult to study objects when you only have one example. Imagine how limited our knowledge of the universe would be if we only had one to study.

In addition to comparative biochemistry, if we found a second genesis on Mars, if we discovered that life had two independent starts here in our solar system, we would know right away that life is common in the universe. So it would have a practical scientific implication as well as a deeper philosophical implication.

Of course, searching for life is also important in the planetary environmental sense. It gives us information about the early martian environment. And it certainly gives us information relevant to understanding the origin of life on Earth. But I think these are secondary objectives.

The search for life on Mars began with the Viking landers. The Viking missions had two separate approaches to searching for life. One was the three biology experiments. They sought to detect life directly. And then we had the GCMS (gas chromatograph-mass spectrometer), which sought to detect what we now would say are signs of life. There's an important distinction here, between trying to detect an actual living organism and trying to detect some indication, some sign that life is present.

Well, looking back now, with 20-20 hindsight, how would we assess the Viking approach? The Viking biology experiments detected life by doing incubation: taking a sample, putting it in three chambers, seeing if anything would grow. But that presupposes that you have knowledge of the conditions and nutrients needed for growth. We now know that that's actually a bad approach to searching for life

We now know that we couldn't do the Viking experiments successfully even here on Earth. For example, 90 percent of terrestrial soil bacteria cannot be grown in culture. If you took the Viking experiments out into your back yard, they would fail to detect 90 percent of the organisms in the soil. It would detect the other 10 percent, so you'd get a positive signal. But anything that fails to detect 90 percent is probably not a good approach.

As a result, it's not surprising that the Viking biology experiments gave ambiguous answers. And it's also not surprising that in the astrobiology community now there is no longer a focus on culturing, trying to grow organisms, as a way of detecting life, because we don't know the conditions and nutrients needed for them to grow. And if we can't make it work for 90 percent of organisms on Earth, we're not going to make it work on Mars and Europa, where we know even less.

So the Viking biology experiments' approach is not an approach we want to follow.

But the GCMS approach is a much more powerful approach, where what we look for are more robust signs of life that require us to know less about the organisms we want to detect.

There is good evidence for water on Mars. In images taken from orbit we see good evidence that at least some of Mars' fluvial features were carved by water. But I want to make the point that on early Mars, when Mars was wet, it wasn't warm, it was cold. And I think this is an important point that we're starting to come around to as a community. I'd say there are several lines of evidence that argue that Mars was cold when it was wet.

One, most importantly, is the evidence of very low erosion. Erosion on Mars is low even compared to places on Earth where erosion is at its minimum, such as the Antarctic dry valleys. Also, the sporadic distribution of the valley features we see in images of Mars is a powerful argument that Mars is not Earth-like in the sense of having a warm, Earth-like climate with average annual temperatures of 15 degrees Celsius (59 degrees Fahrenheit).

There's also some weaker evidence suggesting it was cold. One is the climate modelers have difficult getting Mars's surface temperatures near or above zero - but that's a sort of a theoretical data point.

Another line of evidence is that no massive surface carbonates have been detected by remote sensing. The fact that they're not detectable by remote sensing doesn't mean they're not there, but still this is consistent with Mars being very cold.

Well, cold is very interesting. I want to talk a little bit about the environments on Earth that are the most Mars-like, in the sense of being cold, and look at the evidence for life in those environments. And I would argue that the most relevant environment is the dry valleys of Antarctica. This is the largest ice-free region in Antarctica. The average temperature is minus 20 C (minus 4 F). Summertime temperatures are higher.

Precipitation is equivalent to one or two centimeters per year of moisture. And the pressures are well above the triple point of water. [The triple point of water is a combination of temperature and pressure that enables water to exist in all three states: solid (ice), liquid and gas (water vapor). When atmospheric pressure is too low, as on the surface of Mars today, water cannot exist in liquid form, regardless of the temperature.]

In the Antarctic dry valleys it never rains, only snows, and yet there are large rivers and lakes, such as Lake Vanda and the Onyx River, the largest river in Antarctica. Typically the Onyx River flows a couple months a year. When the temperatures in the summer get above freezing, the glaciers melt and that water forms the Onyx River, which collects in the lake.

Teachers Experiencing Antarctica and the Arctic, Rice University

What's interesting about this environment, is that because it's so dry, it's dead. There's nothing growing on the surface soil. Precipitation is too slight. It evaporates, blows away. This is one of the most lifeless places on Earth. But in the water in Lake Vanda, underneath the ice, there are thick mats of algae and bacteria.

So here in this Mars-like environment, where the average temperatures are minus 20 C (minus 4 F), there's still a hydrological cycle, based on snow, glaciers, melt and accumulation in ice-covered lakes. And it hosts a rich community of microorganisms, similar to what you would expect on the early Earth and would hope to find on Mars.

On Earth, the hydrological cycle averages a meter (39 inches) a year at a temperature of plus 15 C (59 F). On Mars, you could guess that if the average temperature were, say, minus 35 C (minus 31 F), you'd get a hydrological cycle orders of magnitude lower than Earth's. That's similar to what we see in the dry valleys, where the temperature is minus 20 and the hydrological cycle is about 1 to 2 centimeters a year.

The point in making this analogy is that cold can still be wet. Very cold can still be wet - and can still be alive. You can see this on Earth. We have this bias for warm places because we're warm creatures and most of Earth is warm. In fact, Earth is on the warm edge of the habitable zone.

But warm is not really a requirement for life. Life would do perfectly well in cold water. So a very cold Mars in not uninteresting biologically. It could be very interesting biologically. And we can quantify that. Models of the Martian atmosphere show that ice-covered lakes could have existed for perhaps a billion years of its early history. Even as the average temperature got quite cold.

So we don't have to say that early Mars was 0 degrees C (32 degrees F) to be interesting biologically. I think you could easily see that - and I'm talking just the surface, not even discussing the subsurface - temperatures could get quite low, minus 40 C (minus 40 F), and there could still have been a significant hydrological cycle that could support ice-covered lakes.

So Mars was cold when the valley features that we see now were formed. Temperatures could have been as low as minus 35 C (minus 31 F) and Mars could still have had enough water activity to create the features that we see. And these features could have been habitats for life.

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