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. That reexamination was triggered last month when President Bush announced a reorientation of the US space program to focus more heavily on returning to the moon and eventually - if budgets permit - putting humans on Mars.
Indeed, Tuesday's announcement came on the eve of the first set of public meetings by a commission charged with developing a road map for accomplishing Mr. Bush's goals.
One of the critical tasks the two rovers on Mars are performing is providing ground-based data for the Mars Reconnaissance Orbiter (MRO), scheduled for launch next year, says Jim Gavin, lead scientist for Mars and the moon at NASA's headquarters in Washington, D.C. The MRO's cameras and spectrometers will be able to spot features far smaller than orbiters currently circling the planet. And armed with the latest information from Opportunity and Spirit as calibration points, it is likely to spot a large number of potential sites for landing robotic labs and additional rovers.
"We'll have a host of sites to choose from," says James Heard III, a Brown University planetary scientist who compares an embarrassment of landing-site riches on Mars with the scant information NASA had on potential moon landing sites during the Apollo program.
In 2007, NASA plans to send a lander to the north-pole region of Mars to help explore the history of water on the planet and search for potential "habitable zones" beneath the Martian surface. In 2009, the agency hopes to launch the first in a new generation of "smart" robotic science labs that could literally fly itself from one landing site to another over the course of a mission that could last for years instead of weeks or months.
The ultimate mission, however, would bring Martian rock samples back to Earth.
The twin rovers' principal investigator, Steven Squyres of Cornell University, notes that one of the big questions surrounding the new findings involves the ages of the rocks Opportunity examined. He and his colleagues can get a relative sense of age by looking for similar materials in crater walls and seeing where they appear in the walls' rocky layers. But the "best way to get age is to bring back samples," he says, where sophisticated dating and more-sensitive chemical analysis techniques can be applied in the lab.
Such analysis also could help answer the question of whether Mars ever hosted simple life forms.