For more than a decade, orbiters and landers have assaulted Mars, their handlers driven by the mantra "follow the water."
Now, scientists have pulled the results together in the most comprehensive look yet at what the rocks and minerals on the red planet are saying about its climate history and the potential that life may have briefly appeared there.
Their conclusion: If the red planet ever raised a "life welcome" sign, it would have been during its first billion years. After that, the planet's environment grew increasingly hostile. By 3.5 billion years ago, Mars had devolved into the frigid, arid orb humans are exploring today - "not a pleasant place for any form of life, even a microbe," notes John Mustard, a Brown University scientist and a member of the team conducting the analysis.
That said, the team also concludes that if life ever gained a foothold, the best places to look for evidence would be in three clay-rich regions on the planet's surface.
The analysis, which appears in Friday's issue of the journal Science, comes from an international team of planetary scientists. The team is led by Jean-Pierre Bibring, with the Institut d'Astrophysique Spatiale in Orlay, France, and drew heavily on data gathered by the European Space Agency's Mars Express Orbiter. The orbiter arrived Christmas Day in 2003.
Its initial mission length has been extended, and the orbiter has mapped the distribution of minerals over some 90 percent of the planet's surface.
The study also draws on information from US orbiters and the two Mars Exploration Rovers currently hunting for winter hibernation spots on Mars.
Geologically speaking, the planet's watery period was brief, the team found.
For Mars' first 600 million years, it had plenty of water, hospitable temperatures, and low acid levels. The team gleaned this from glimpses of the planet's oldest rocks, laid bare through erosion, cratering, and large temblors.
Of particular interest are the clays they found. Yet some uncertainty remains about how the clays formed.
The team leaves open the possibility that the Martian surface may never have had large amounts of water. The exposed clays may have formed beneath the surface - which would imply that the planet has always been cold and dry.
For the next 500 million years, the team found, the planet's mighty volcanoes erupted in a series of events that filled the atmosphere with sulfur. This sulfur fell back to the surface as sulfuric acid. At the same time, it began to lose its atmosphere - either blasted free by collisions with large leftovers from planet formation, or perhaps when the planet's internal dynamo finally gave out and its magnetic field vanished.
Over the next 300 million years, the planet arrived at the frigid, rust-red configuration that holds today. Dr. Mustard notes that the clay deposits in particular should be tempting targets for the US Mars Reconnaissance Orbiter, which currently is working its way into its final science-gathering orbit around the planet.
Ultimately, these may be the best locations to send landers hunting for signs that Mars might have once harbored simple life-forms.