A satellite orbiting Mars has found widespread deposits of clay - mineralogical evidence that very early in its history, the red planet was a watery place with broad lakes and flowing rivers.
While the findings provide no direct evidence that life ever thrived in those Martian waters, clay on Earth is very good at preserving traces of organic matter.
The deposits identified and mapped by a Maryland-built instrument aboard NASA's Mars Reconnaissance Orbiter are already influencing the selection of landing sites for the space agency's next Mars lander, which will search for evidence of past or present life.
"It's very exciting to me because it's [revealing] that Mars is a very diverse place. As we look at smaller and smaller scales, we see that all these different environments existed on different parts of the planet. It bodes well for life," said Bethany Ehlmann, a Brown University doctoral candidate who was the lead author on one of two recent papers on the discoveries.
Scientists reported the findings in today's edition of the journal Nature and in the June 2 issue of Nature Geoscience.
Among the instruments that revealed the broad distribution and diversity of the clay minerals was the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), developed at the Johns Hopkins University's Applied Physics Lab near Laurel.
The Mars orbiter began studying the surface mineralogy in November 2006. Its instruments have now mapped 60 percent of the planet and snapped 7,200 high-resolution images, according to APL's Scott L. Murchie, principal investigator for CRISM.
The Martian clays appear to have formed in the earliest, or Noachian, period of the planet's history, 4.6 billion to 3.8 billion years ago, as widespread water flushed through rocks and soil.
Some clay formed deep underground; other deposits formed in sands and river deposits on the surface.
CRISM found clay deposits across roughly half the planet's surface. Their chemical diversity suggests they formed in a wide variety of rocks, temperatures and other environmental conditions.
"The occurrences and diversity tell you the alterations by water were much more widespread during this period that was thought," Murchie said.
"It's a story of a Mars more pervasively wet than previously suspected during its earliest history."
Mars today is frigid and dry. Scientists interested in the question of life there, or the availability of water for future astronauts, have struggled to understand its early, more watery history.
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."