Thursday, in the journal Science, 11 articles by 122 researchers supported the idea of a sometime-wet red planet.

Liquid water was once intermittently present at the Martian surface at Meridiani, and at times it saturated the subsurface, wrote 50 of the group in a collective article, led by Steve Squyres of Cornell University, chief scientist for the twin Mars Exploration Rovers, Spirit and Opportunity.

Opportunity completed an air-bag-assisted landing last Jan. 25 at a spot on the Martian equator called Meridiani Planum, halfway around the planet from where Spirit had landed Jan. 3 at Gusev Crater, a large, flat area resembling a dry lake bed.

Thus far, Spirit's discoveries have suggested the presence of water on Mars, but Opportunity's finds have been the real show-stoppers. After nearly 11 months on the Martian surface, the instruments aboard the golf-cart-sized, solar-powered Opportunity have located and analyzed rocks that clearly preserve a record of environmental conditions different from any on Mars today, wrote Squyres and his co-authors.

Because liquid water is a key prerequisite for life, the article continued, we infer conditions at Meridiani may have been habitable for some period of time in Martian history.

In the typically understated way scientists tend to describe findings large and small, Squyres and colleagues concluded something very large in human history: there once was a habitable world beyond our own.

Michael Meyer, NASA's chief scientist for Mars exploration, was less restrained.

Formal review and publication this week of these amazing discoveries further strengthens the need for continued exploration by orbiters, surface robots, sample-return missions and human explorers, Meyer said in a statement. There are more exciting discoveries awaiting us on the red planet.

The evidence presented in the articles boils down to the following:

-- Rocks found by Opportunity at Meridiani - both in a small crater where it landed and in Endurance - retain a high proportion of bromine to chlorine. The ratio is the result of a well-known chemical process that proceeds only in the presence of liquid water.

In fact, this chemical signature indicates not merely some water, but the past presence of large amounts of water, wrote Rudi Rieder and Ralf Gellert of the Max Planck Institute for Chemistry in Mainz, Germany, and co-authors.

-- The article by Rieder and Gellert, along with another one by Phil Christensen of Arizona State University in Tempe and his collaborators, suggests an abundance of sulfur-rich minerals in the rocks - something else that points solidly to a watery past on Mars.

-- An article by Ken Herkenhoff, of the U.S. Geological Survey in Flagstaff, Ariz., and co-authors, details how cavities in the rocks found at Meridiani - each about the size of a shirt button - represent crystals that once formed inside the rocks, then dissolved.

-- Minerals carried by water formed blueberries, peppercorn-size gray spheres rich in iron and embedded in some of the rocks examined by Opportunity.

-- Patterns of fine layers in some of the rocks reveal a past flowing body of surface water that shaped the sediments that became the rocks.

-- Several characteristics of the rocks even suggest the water appeared and disappeared repeatedly, as it does in some shallow lakes on Earth.

-- The chemical composition of the rocks indicates water of high acidity and saltiness - in other words, a Martian equivalent of seawater.

-- As reported by Goestar Klingelhoefer at the University of Mainz, Germany, Richard Morris of NASA's Johnson Space Center in Houston, and co-authors, the rocks contain a hydrated iron-sulfate salt called jarosite - yet another mineral that forms in the presence of water.

Rocks rich in mineral forms of sulfur, iron and bromine, a giant crater that looks like a lakebed, sedimentary rock that seems to have been sculpted by wave action - all beg the obvious question: The case for water seems firm, but have fossils been observed in any of the rocks?

The answer, unfortunately, is no, not yet.

The problem is the twin rovers were not designed to look for fossils. They were designed to uncover evidence of liquid water, something they have done magnificently in 11 months of exploration.

We cannot determine whether life was present or even possible in the waters at Meridiani, Squyres and colleagues wrote in Science, but it is clear that by the time the sedimentary rocks in Eagle crater were deposited, Mars and Earth had already gone down different environmental paths. Sample return of Meridiani rocks might well provide more certainty regarding whether life developed on Mars.

Unless the rovers happen to stumble onto something visible, that next step will have to wait until the Mars Science Laboratory, the next-generation rover, arrives sometime in the fall of 2010.

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