The Blueberries of Mars

Was the Red Planet once a wet planet? A plucky Martian rover finally delivers some hard evidence

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Giovanni Schiaparelli could have told you there had been water on Mars. It was Schiaparelli who peered through his telescope one evening in 1877 and discovered what he took to be the Red Planet's famous canals. As it turned out, the canals were an optical illusion, but as more powerful telescopes and, later, spacecraft zoomed in for closer looks, there was no shortage of clues suggesting that Mars was once awash in water. Photographs shot from orbit show vast plains that resemble ancient sea floors, steep gorges that would dwarf the Grand Canyon and sinuous surface scars that look an awful lot like dry riverbeds.

Given all that, why were NASA scientists so excited last week to announce that one of their Mars rovers, having crawled across the planet for five weeks, finally determined that Mars, at some point in its deep past, was indeed "drenched"--to use NASA's term--with liquid water?

Part of their excitement probably stems from sheer failure fatigue. NASA has had its share of setbacks in recent years--including a few disastrous missions to Mars. So it was with some relief that lead investigator Steve Squyres announced that the rover Opportunity had accomplished its primary mission. "The puzzle pieces have been falling into place," he told a crowded press conference, "and the last piece fell into place a few days ago."

But there was also, for the NASA team, the pleasure that comes from making a genuine contribution to space science. For despite all the signs pointing to Mars' watery past, until Opportunity poked its instruments into the Martian rocks, nobody was really sure how real that water was. At least some of the surface formations that look water carved could have been formed by volcanism and wind. Just two years ago, University of Colorado researchers published a persuasive paper suggesting that any water on Mars was carried in by crashing comets and then quickly evaporated.

The experiments that put that theory to rest--and nailed down the presence of water for good--were largely conducted on one 10-in.-high, 65-ft.-wide rock outcropping in the Meridiani Planum that mission scientists dubbed El Capitan. The surface of the formation is made up of fine layers--called parallel laminations--that are often laid down by minerals settling out of water. The rock is also randomly pitted with cavities called vugs that are created when salt crystals form in briny water and then fall out or dissolve away.

Chemical analyses of El Capitan, performed with two different spectrometers, support the visual evidence. They show that it is rich in sulfates known to form in the presence of water as well as a mineral called jarosite, which not only forms in water but also actually contains a bit of water trapped in its matrix.

The most intriguing evidence comes in the form of the BB-size spherules--or "blueberries," as NASA calls them--scattered throughout the rock. Spheres like these can be formed either by volcanism or by minerals accreting under water, but the way the blueberries are mixed randomly through the rock--not layered on top, as they would have been after a volcanic eruption--strongly suggests the latter.

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