Traces of a mineral -- indicative of water -- have been found all over Mars' surface, suggesting liquid water does or did exist on the red planet. But the low quantities of the mineral mean it's unlikely Mars ever had oceans or large lakes, a new study concludes.
The investigation of surface dust by NASA's orbiting Mars Global Surveyor also sheds light on a longstanding atmospheric mystery.
Scientists suppose Mars once had a thick atmosphere of carbon dioxide, but they can't figure out where it went. Today the atmosphere is thin, less than 1 percent of the pressure on Earth at sea level. Did the carbon dioxide escape into space? Or was it absorbed into the surface?
It appears to be trapped in the dust and rock, Arizona State University's Joshua Bandfield said in a telephone interview.
Water: The main goal
Liquid water is one of the two most highly sought things in any good Mars investigation. The other is life, which requires liquid water, insofar as biologists know. NASA's guiding principle for its massive financial investment in Mars is, therefore, to "follow the water."
Sorting out theories
Many scientists agree that some sort of flowing material has worked in the past to carve huge canyons on Mars. They disagree over whether it was water or carbon dioxide.
Whatever the material, a view has emerged in recent years that the erosion was catastrophic, coming in explosive bursts fueled by extreme variations of climate over relatively short periods of geologic time. Perhaps, according to one group of theorists, carbon dioxide was trapped and pressurized and then released like a gargantuan, shaken soda in volcanic eruptions.
The big question is whether things ever settled down long enough for water to sit still.
Back to the air
The conclusion dovetails nicely with the recent discovery of water ice embedded in just about the entire surface of Mars. Data from NASA's Odyssey spacecraft indicates there may be more ice than dirt in many areas.
"This really points to a cold, frozen, icy Mars that has probably always been that way, as opposed to a warm, humid, ocean Mars sometime in the past," Christensen said. "People have argued that early in Mars history, maybe the climate was warmer and oceans may have formed and produced extensive carbonate rock layers. If that were the case, the rocks formed in those putative oceans should be somewhere."
Mars vs. Earth vs. Venus
"On Earth, the vast majority of that early, thick carbon dioxide atmosphere has been subsequently locked up in the carbonate rocks, which are everywhere thanks to the Earth's oceans," Christensen said. "We went from a mostly carbon dioxide atmosphere to one where it is only a minor player. On Mars, it doesn't look like that happened."
And on Venus, the carbon dioxide led to what scientists call a runaway greenhouse effect that creates searing temperatures on the surface.
On Mars, a different development of air resulted in a dry, cold world.
"If you form enough carbonates, pretty soon your atmosphere goes away," Bandfield said. "If that happens, you can no longer have liquid water on the surface because you get to the point where liquid water is not stable."
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