Mars Finding Shifts Focus to Inner Planets ; Evidence of Abundant Water Is Whetting Astronomers' Appetites for Future Missions to Mars, Mercury, and Venus

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This week, a mechanical geologist the size of a golf cart and nearly 156 million miles away galvanized the world with news that Mars bears unequivocal evidence of once-watery conditions capable of supporting life as we know it.

Yet for all the excitement surrounding the discovery, the value of the Mars exploration program may lie as much in what it suggests about the early history of Earth and about the prospects for habitable planets around other stars as it does about Mars.

Thus, the results announced Tuesday are whetting astronomers' appetites for coming missions to Mercury, and perhaps later to Venus. These missions are expected to help fill in the outlines of a planetary story whose plot - at least in general terms - is being repeated throughout the universe. And they are highlighting a renewed focus on the inner planets of our solar system.

Scientists are trying "to understand the diversity of outcomes from a common set of physical and chemical building blocks," says Sean Solomon, director of the department of terrestrial magnetism at the Carnegie Institution of Washington and lead scientist on the upcoming Messenger mission to Mercury, scheduled for launch in May.

Where Mars represents a potential reference point for environmental and possibly biological comparisons with Earth, Mercury represents "one of the most extreme outcomes of planetary formation," Dr. Solomon says. "It has much to tell us about how the inner planets assembled themselves" from the disk of dust and gas that surrounded the young sun some 4.6 billion years ago. Venus, by contrast, "is the most Earth-like planet" when looking at mass, density, and distance from the sun. And its searing, noxious atmosphere is an example of the greenhouse effect run amok.

This broader view represents what a top panel of astronomers two years ago called "a new paradigm for solar system exploration." The flyby views of other worlds in our cosmic neighborhood are yielding to a deeper search for answers to "fundamental questions about our place in the universe," the panel said. "Exploration of the inner solar system is vital to understanding how Earth-like planets form and evolve and how habitable planets may arise throughout the galaxy."

Little doubt remains that Mars once hosted habitats suitable for at least simple life forms. Tuesday's announcement that the rover Opportunity scored, drilled, and analyzed rocks containing minerals that form only in very wet environments is adding fresh impetus to a reexamination of the Mars exploration program. …