Traces of a mineral indicative of water have been found all over Mars’ surface, suggesting that 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 examination of surface dust by NASA’s orbiting Mars Global Surveyor also sheds light on a longstanding atmospheric mystery.
SCIENTISTS SUSPECT 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.
“Wow,” Bandfield said of what he considered the most important result of the study. “You can store part of the atmosphere in the rock.”
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.”
Though vast amounts of frozen water exist on the red planet, convincing evidence for running or standing water remains elusive.
The new study is important for confirming discovering magnesite, a so-called carbonate.
“We’re finding low levels of carbonates just about everywhere on Mars,” said Bandfield, lead author of a report on the findings that appears in Friday’s issue of the journal Science. He said about 3 percent of the surface dust is made of carbonates, most of it magnesite.
Carbonates form when carbon dioxide gas meets other minerals in the presence of water. But the science on that is not totally firm, Bandfield cautioned, meaning the results are not 100 percent conclusive in proving the existence of liquid water.
If Mars harbors even trickles of liquid water, most biologists agree the stage is set for the presence of microbial life.
SORTING OUT THEORIES
Many scientists agree that some sort of flowing material has worked in the past to sculpt 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.
One theory of ancient Mars holds that it was once significantly warmer and wetter, its atmosphere laden with heat-trapping carbon dioxide. If so, some theorists say, then the planet could have retained oceans of water. Studies have found surface features that look like former sea floors or lake beds, and even apparent shorelines have been spotted.
But some critics have never bought this scenario or the photographic evidence for it. They will feel some vindication in the new study.
If Mars had oceans or large lakes, more carbonates — including limestone structures — should have developed, Bandfield said. Visible limestone formations akin to the famed White Cliffs of Dover should remain, even despite eons of erosion. Other evidence of relatively recent erosion on Mars implies such structures would be exposed, at least in some places.
Nothing of the sort has been spotted.
“It leads one to believe that you didn’t have longstanding bodies of liquid water sitting on the surface of Mars,” he said. “They probably froze pretty quickly.”
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.
“We believe that the relatively small amounts that we see probably did not come from oceans, but from the atmosphere interacting directly with dust,” said Philip Christensen, another Arizona State researcher who worked on the study.
“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
The research points to a similar starting point for planetary atmospheres, with scenarios that unfold in wildly different ways.
Venus and Mars have atmospheres made mostly of carbon dioxide. Christensen said Earth’s early air was probably loaded with the stuff, too.
“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 evolution of air resulted in a dry, frigid 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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