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.
None of these findings are dispositive, but their combined weight persuaded NASA scientists to summarize their findings in unusually explicit language. "We have concluded that the rocks here were soaked with liquid water," said Squyres flatly. "The ground would have been suitable for life."
Does that mean that there was or still is life on Mars? The fossil record on Earth suggests that given enough time and H2O, life will eventually emerge, but there's nothing in the current findings to prove that this happened on Mars. Without more knowledge of such variables as temperature, atmosphere and the length of time Martian water existed, we can't simply assume that what happened on our planet would necessarily occur on another.
Opportunity and its twin robot Spirit are not equipped to search for life. Their mission is limited to looking for signs of water. But there's still a lot for them to do. Just knowing that rocks were wet doesn't tell you if the water was flowing or stationary, if it melted down from ice caps or seeped up through the ground. And if water was once there in such abundance, where did it go? Opportunity, which is very likely to exceed its planned 90-day mission, is already looking for those answers, toddling off to investigate other rocks farther and farther from its landing site. Spirit is conducting its own studies in Gusev Crater, on the opposite side of the planet.