April 22, 2011

Vast, Frozen CO2 Deposit Altered Climate On Mars

New research shows that hostile conditions on Mars were much worse 600,000 years ago than they are today, with evidence suggesting the red planet once had a much dustier, stormier atmosphere.

NASA's Mars Reconnaissance Orbiter (MRO) discovered that the total amount of atmosphere on Mars changes significantly as the tilt of the planet's axis varies.  The process can affect the stability of liquid water -- if it exists on the surface "“ and can also increase the frequency and severity of Martian dust storms.

"It was an unpleasant place to hang out," said Roger Phillips of Southwest Research Institute, deputy team leader for MRO's Shallow Radar instrument and lead author of the study, during an interview with the Associated Press.

The climate on Mars climate was likely similar to the American Dust Bowl of the 1930s "” only worse, he said.

The researchers used the MRO's ground-penetrating radar to identify a large, buried deposit of frozen carbon dioxide, or dry ice, at Mars' south pole.  The scientists suspect much of this carbon dioxide swells the atmosphere's mass when the planet's tilt increases.

The newly found deposit has a volume similar to that of Lake Superior, or about 3,000 cubic miles. The deposit contains up to 80 percent as much carbon dioxide as today's Martian atmosphere.   Collapse pits caused by dry ice sublimation and other clues suggest the deposit is in a dissipating phase, adding gas to the planet's atmosphere each year.

Mars' atmosphere is about 95 percent carbon dioxide, in contrast to Earth's much thicker atmosphere, which is less than .04 percent carbon dioxide.

"We already knew there is a small perennial cap of carbon-dioxide ice on top of the water ice there, but this buried deposit has about 30 times more dry ice than previously estimated," said Phillips.

"We identified the deposit as dry ice by determining the radar signature fit the radio-wave transmission characteristics of frozen carbon dioxide far better than the characteristics of frozen water," said Roberto Seu of Sapienza University of Rome, team leader for the Shallow Radar and a study co-author.

Additional evidence came from correlating the deposit to visible sublimation features typical of dry ice.

"When you include this buried deposit, Martian carbon dioxide right now is roughly half frozen and half in the atmosphere, but at other times it can be nearly all frozen or nearly all in the atmosphere," said Phillips.

An occasional increase in the atmosphere would strengthen winds, lofting more dust, resulting in more frequent and intense dust storms. 

Another result is an expanded area on the planet's surface where liquid water could persist without boiling.  Modeling based on known variation in the tilt of Mars' axis suggests dramatic changes in the total mass of the planet's atmosphere can happen within 100,000 years or less.

The changes in atmospheric density caused by the carbon-dioxide increase also would also serve to amplify some of changes caused by the planet's tilt.

Researchers took the mass of the buried carbon-dioxide deposit, and entered it into climate models for the period when Mars' tilt and orbital properties maximize the amount of summer sunshine hitting the south pole.  The results showed that at such times, global, year-round average air pressure is approximately 75 percent greater than the current level.

"A tilted Mars with a thicker carbon-dioxide atmosphere causes a greenhouse effect that tries to warm the Martian surface, while thicker and longer-lived polar ice caps try to cool it," said co-author Robert Haberle, a planetary scientist at NASA's Ames Research Center in Moffett Field, Calif.

 "Our simulations show the polar caps cool more than the greenhouse warms. Unlike Earth, which has a thick, moist atmosphere that produces a strong greenhouse effect, Mars' atmosphere is too thin and dry to produce as strong a greenhouse effect as Earth's, even when you double its carbon-dioxide content."

The findings are published in the journal Science.


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