"Viking was the first lander to go to Mars. It presented us with a certain view of the planet, and provided us with a tremendous amount of information. It also represents the start of an intellectual understanding of Mars.

Historically in science, we tend to move from a pure discovery mode of just finding what is there, to a survey mode in which we find out in general how things are distributed. Eventually we get to a point where we can form a hypothesis, and then we go out and test that hypothesis. That's how we're going to learn about Mars.

Our exploration of Earth gives us an idea of how science progresses. In the later half of the nineteenth century, the theory was put forward that below a certain point in the ocean there's too much pressure and no light, so there's no possibility for life there.

This became a debate in the Royal Society, and so the Society formed a mission in conjunction with the military to sample the deepest depths of the ocean and find out whether or not life is there. The Challenger expedition sailed off in December 1872. This mission started the discipline of oceanography -- over 100 scientists were involved, and they generated 50 volumes of oceanographic research, basically a bible that took 23 years to complete after the expedition was over. This formed the basis of our understanding of the oceans and the unbelievable spread of life throughout the depths.

So how does that relate to Mars? Like the Challenger expedition, Viking had instruments to measure whether or not there's life, and it set the stage for our understanding of the planet. The information we gained through Viking is more complex than we can tell from just looking through our telescopes, or even by spacecraft crashing into the planet's surface.

Interestingly, about the same time that Viking was on the surface of Mars, oceanographic vessels on Earth discovered the hydrothermal vents. They found another type of life on this planet that was dependent not upon photosynthesis, like everything else on our surface, but living off of chemistry. So our understanding of life, at least on this planet, suddenly grew, and we now recognize that life is in places that we thought were impossible, such as in boiling hot water in the deepest depths of the ocean.

This shows how far we've come in oceanography -- from not knowing if anything was there, to doing survey work and understanding how the oceans work, to finding things that we never even expected despite a hundred years of research of the world's oceans.

In the same way, Viking, here at the 30th anniversary, has set the stage and has created a discipline of Mars scientists. Now we're going from survey mode, with our orbiters and landers, to understanding the planet as a whole, to the point where we can start forming hypotheses and testing them.

A good example is how we decided where to send the Opportunity rover. We saw the mineral hematite in the orbital data, and hematite usually forms in water. So we sent Opportunity to that site to see whether or not water was ever there. And, in fact, that's what we did discover.

The Mars program and missions has been organized around water, which I think is appropriate. With this organizing principle, we're able to address whether or not life could've started on Mars.

We also found evidence for water from Mars Global Surveyor, Odyssey, and Mars Express. To what extent and for how long the water was there is a big question. The margin of error of hundreds of millions of years makes it difficult to say anything definitive. But we're switching from wondering if there was water and where it's been, to whether or not Mars has been habitable. I think this will lead us to the question of whether or not life ever got started, and if so, how long was the life there.

How do we start looking for signs of life? We have an easy prospect here on Earth -- we see something and we usually recognize it right away. We also have a reasonable idea of our geologic history, and what physical chemical processes look like. So when you're looking for some evidence of life, such as a fossil or chemistry, you have things that you know life can do, and what sort of remnants might be left behind by living things. On the other hand, we also have things that can be created by physical chemical processes, so you can get some beautiful symmetric things like crystals that are not life.

After looking at the early history here on Earth with the fossils that we have, we're finding that there's quite a range of ambiguity of things that look like they could've been formed by life. How do you determine whether it's made by the living organism or by physical chemical process?

So far on Mars, the cameras haven't found anything that would end all doubt whether or not life ever got started on that planet. I suspect that when we're looking for evidence of life on the Red Planet, we're going to be facing the daunting problem of looking at something like a crustous lichen.

A lichen is a plant and animal living symbiotically, and on top is a mineral crust. And the problem is, if a robot is wandering around on the surface, how does it recognize something such as lichen as evidence of a living process?

So working here on Earth helps us better understand the vagaries, and recognize the things that are more consistently determined to having been done by a living process compared to a physical chemical process.

We also hope to learn more about Mars with upcoming missions. The Mars Reconnaissance Orbiter is in orbit now, and by November we hope to be in a science mapping orbit. In 2007, the Phoenix lander will be going to the polar regions of Mars. Then in 2009 we'll have the Mars Science Laboratory, and that will be our first roving analytical lab to go to the Red Planet."

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