Not only would that have brought new water to the surface, it would also have vented an enormous amount of sulfur into the air. The result: acid rains and shallow lakes.
"It was a very acidic environment," Bibring said.
When the planet dried again, these acid waters left vast plains of sulfate minerals that produce infrared signatures that can be seen from space.
Since then—for the last 3.5 to 3.8 billion years—the planet has been slowly rusting, as iron in its rocks reacts with oxygen-containing molecules in the atmosphere to produce Mars's characteristic ruddy color.
Many scientists had expected that water had facilitated this oxidation process. But OMEGA has found no trace of water-altered minerals in the red planet's reddest sections.
"Water is not responsible for the planet being red," Bibring said.
Rover Findings Consistent
For more than two years NASA's Mars rovers have crawled over the surface of Mars in search of water-formed rocks. (News photo: "Mars Rover Still on Task Two Years Later.")
The latest OMEGA findings dovetail with what's been discovered so far with the Mars rovers, the scientists say.
Ray Arvidson is a professor at Washington University in St. Louis, Missouri, and a member of both the rover and OMEGA teams.
By luck, he said, the Opportunity rover wound up in a region where sulfate rocks produced in shallow lakes from the middle epoch poke through a covering of windblown sand.
The Spirit rover, on the other hand, landed in an area dating entirely from the third epoch, when the planet was cold and dry.
Thus the rovers are confirming what OMEGA saw from space for two of the three epochs.
Unfortunately, neither rover can examine clays from the oldest epoch, Arvidson said: "The rocks examined by the rovers are too young."
The next step, Bibring said, will be to land a rover on the surface, probably in 2011, equipped with an instrument called MicrOmega that can find individual clay grains.
If Mars ever had life, he said, the place to look for traces of it will be inside these grains. (Related news: "Mars Sea Discovery Ups Odds of Red Planet Life, Scientists Say.")
Meanwhile, OMEGA has confirmed prior beliefs that Mars was once warmer and wetter.
These beliefs were based on seeing terrain that looked like dried river channels and shorelines, Arvidson said.
"What we've done now is to add the mineralogical evidence," he said. "We're finding smoking-gun information that tells you about ancient conditions."
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