In one of the less subtle episodes of the original Star Trek series, hippies hijack the Enterprise to get to a planet that looks like paradise but turns out to be a grim, acid-soaked purgatory. Over the past two years, a similar allegory has played itself out in Mars science. Drawn to Mars largely by signs of past Earth-like conditions, researchers have finally found definitive relics of gently lapping seas and balmier skies: in particular, deposits of sulfate salts. To form those sulfates, though, the ancient seas must have been acidic enough to burn off skin.

But a different tale is told by another class of minerals, fully mapped only recently: clays. They suggest that even before the era of the sulfates, Mars was drenched in water safe enough to dunk a hand in. "The clays indicate alteration with a lot of water," says Franois Poulet of the University of Paris-South, a member of the discovery team. "The sulfate indicates a second step in the climate of Mars."

The Viking missions of the mid-1970s and subsequent ground-based telescopic observations saw hints of clay, but they were ambiguous, and mid-infrared spectrometers on NASA's orbiting Mars Global Surveyor and Mars Odyssey probes came up blank. The OMEGA spectrometer on the European Space Agency's Mars Express orbiter has gone where no spectrometer has gone before, covering near-infrared wavelengths and offering 10 times the resolution of earlier instruments. It first detected clays (technically called phyllosilicates) last year, but the data were spotty, and some scientists wondered whether the clays were merely superficial layers, the result of gradual weathering rather than thorough soaking.

At a September meeting of the American Astronomical Society, Poulet described how clays now show up on numerous small and widely dispersed outcrops, as well as in crater debris and rock strata--evidence for a substantial deposit. The terrain appears to be the oldest on the planet, arguing against a gradual process. "They're located in the older, more battered-looking exposures," says geologist Ray Arvidson of Washington University in St. Louis. These areas are devoid of sulfates. Almost all the chemical varieties of clay are represented, notably smectites, which on Earth form in neutral to alkaline water. In addition to the ancient deposits, OMEGA saw thin coatings of dark clay of unknown origin.

To be sure, spectra measured from orbit can be tricky to interpret, and the clays have yet to be confirmed by rovers on the ground. Mineralogist Richard Morris of the NASA Johnson Space Center says, "For something as important as phyllosilicates, I'd like to see two things point to it"--that is, two independent lines of evidence. Tantalizingly, Diana Blaney of the Jet Propulsion Laboratory, a member of the Mars Exploration Rover team, announced at the September meeting that a rock found by the Spirit rover, christened Independence, is claylike. Next March, NASA's Mars Reconnaissance Orbiter is due to arrive, carrying a spectrometer that covers the same wavelengths as OMEGA but at 10 times the resolution--and, crucially, a different viewing angle. Combining the two instruments' data will help isolate the confounding effects of atmospheric dust.

Another question is whether novel chemistry, unknown on Earth but common on Mars, could be fooling geologists. NASA plans to launch a lander in 2007 to conduct experiments on the Martian soil. Phoenix, so named because it is basically a copy of the ill-fated Mars Polar Lander of 1999, carries four small vials of water to wet the soil and measure its pH and composition.

If researchers confirm that clays exist, then one of the greatest mysteries in Martian science will deepen--specifically, the lack of carbonates. "If you have an alkaline solution, you should get carbonates, so where are the carbonates?" Poulet asks. "It's clearly a problem." Maybe it will just take an even more sensitive instrument to find them, or maybe early Mars, for some still unknown reason, was less of an Eden than scientists suspect it was.