In a commentary in Nature Geoscience, Vincent Chavrier, of the University of Arkansas' Center for Space and Planetary Science, noted that NASA's twin rovers, Spirit and Opportunity, found sulfates in the Meridiani Planum in 2004 - evidence of water acting in a highly acidic environment.
The European Mars Express orbiter later found clay, which typically forms in much less acidic (and more life-friendly) conditions.
Some of the clay deposits clearly formed early, in the Martian highlands.
An MRO image shows a winding river channel that once carried water and sediment from a large highland watershed.
The river breached the rim of a large crater, then entered a broad crater lake, leaving a sprawling deposit of clay-rich sediments that resemble the Mississippi Delta in southern Louisiana.
Elsewhere on Mars, clay deposits were buried by subsequent layers of rock and lava that are unaltered by water, evidence of drier days on the planet.
Some of the clay was exposed again by erosion and meteor impacts.
As for the time that elapsed between the clay's formation in water and the period when Mars finally became too dry, Murchie said: "We're probably talking about a 400 million-year time span."
Was that enough time for life to evolve ?
"The oldest fossils on Earth date back to that period of time," Murchie said. "So if we use Earth as an analogy, the answer would be yes."
"But this is the oldest stuff we see through electron microscopes and as chemical traces on Earth," he said of the fossils. "Just the most primitive things. And in some cases, it's debatable whether they are fossils."
Still, it's motivation enough for astrobiologists.
The MRO findings have added at least one potential landing site for NASA's Mars science laboratory, scheduled for launch in the fall of 2009, Murchie said.
After that, he said, "For landers that have a somewhat higher accuracy, there will be dozens of sites, representing different preserved environments, accessible to rovers. ... That can expose a whole further level of detail of [Mars'] mineralogical history."
frank.roylance@baltsun.com
