I have sat in many workshops where the rationale for human exploration has been discussed. We've talked about stimulating the economy. We've talked about national pride. We've talked about the effect on education. And I don't think any of those reasons are the real reasons that we will ultimately go to Mars. I think we go to Mars because it inspires us. It fills us with awe and pride and it lifts us above the humdrum everyday concerns of food and shelter. I think this spiritual driving force will ultimately take us to Mars.

Having said that, I think there is a lot that we must do before we ultimately go to Mars. The main interest in going to Mars is the possibility that life may have started there, and there may still be extant life there on the planet. The reason for that hinges on the story of water on Mars. In 1971, the Mariner 9 mission returned pictures, and we were astonished to see evidence of large floods, of erosion by dry river valleys all over the planet. And the reason we were so surprised is, at that time, we knew that conditions on Mars were very harsh that it was too cold for liquid water to be there, much, much too cold. Something had to have happened in the past, we thought, that had changed, and the planet had evolved from an Earth-like place to the dry, cold desert that we know today.

Much of the last 30 years has been spent trying to understand better the evidence for water, and understand better the reasons or mechanisms whereby the planet may have changed. And we're still in a quandary over those things. We still don't know how one can change the climate of Mars. Atmospheric modelers have tried various tricks and with no success. We simply don't know how it might have happened. So the whole question of liquid water on the surface of the planet has been questioned. People have questioned the formation, the mode of formation of those valleys. People have questioned whether there really were large floods, whether they were caused by water. Recently, of course, with the Mars rovers, we have just acquired very strong, almost totally compelling evidence that, in fact, there were in the past bodies of water there.

What we have found at the Opportunity site is a thick sequence of evaporites. These are salts that form when lakes or seas evaporate, and leave behind these beds of evaporites. The evidence is unambiguous. This is extraordinarily stimulating because, indeed, we now have absolute firm evidence that on Mars there were places in which life could flourish. Prior to the discovery of these evaporites, people have thought, "Well, perhaps the water is only underground and seeps out in local places." And there is contrary evidence of warm conditions on the path from mineralogy and so forth. But what this latest discovery confirms unambiguously is that there were lakes and seas on Mars.

That has enormous potential for exobiology. In this sense, Mars is a biological treasure: something to be exploited but something to be carefully taken care of. We have to understand this planet - understand whether, indeed, biology did start on Mars and whether biology is present today before we contaminate the planet with terrestrial organisms, and so confuse the signal and perhaps destroy the evidence that is there. That's why I say we need a very strong aggressive robotic program before we send people to Mars.

What should that program consist of? It should consist of rovers just like we have on Mars today well-instrumented rovers not only with geologic instruments, but with biological instruments and with instruments to determine organic chemistry. But we also need sample returns. And sample returns are extraordinarily important, because with samples in hand on Earth we can utilize the full analytical capability of all terrestrial laboratories to analyze the samples. We can also adapt the analytical strategy to the results as they come in. And we can also develop instruments to attack problems that the samples present, problems that we didn't anticipate. I think the availability of the lunar samples or the acquisition of the lunar samples demonstrates this potential for samples well.

The combination of mobility, being able to get around the planet with rovers, and the sample return, is very important. It has been demonstrated recently with Opportunity. For example, Opportunity landed in a small crater. Out of reach was an outcrop, and this outcrop is where these evaporites are found. Had we landed there just with an arm, and reached out and dug up the soil, we would have totally missed the exciting materials that are just a few meters away.

So we have to have sample returns that are combined with mobility that is, Mars rover sample returns. We've been working on these kinds of missions for 30 years ever since Viking - and we still have not had a Mars rover sample return. I believe a number of Mars rover sample returns are essential before we send people there, before we jeopardize the evidence that might be there of former life and prebiotic chemistry.

What's Next

The 2009 Mars Science Laboratory is planned as the first set of biological experiments in the current exploration strategy. As the NASA Office of Space Science noted however, there has been considerable debate about when to time a sample return: "We note with concern that there appears to be a growing division within the Mars community between scientists seeking early Mars Sample Return and those who believe it is best to delay it."

Dr. Michael Carr is an astrogeologist in with the U.S. Geological Survey in Menlo Park, California. His specialty is Mars, which he has studied for over 30 years. He testified before the President's Commission on the Moon, Mars and Beyond on April 16, 2004.

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