Mars' Atmosphere: Dramatically Altered by Tilt of Its Axis
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April 22, 2011

Mars' Atmosphere: Dramatically Altered by Tilt of Its Axis

6a00d8341bf7f753ef014e5f595e3c970c-800wi A buried deposit of frozen carbon dioxide -- dry ice -- near the south pole of Mars contains about 30 times more carbon dioxide than previously estimated to be frozen near the pole. NASA's Mars Reconnaissance Orbiter has discovered the total amount of atmosphere on Mars changes dramatically as the tilt of the planet's axis varies. This process can affect the stability of liquid water, if it exists on the Martian surface, and increase the frequency and severity of Martian dust storms.

Researchers using the orbiter's ground-penetrating radar identified a large, buried deposit of frozen carbon dioxide, or dry ice, at the Red Planet's south pole, which they suspect enters the planet's atmosphere and swells the atmosphere's mass when Mars' tilt increases.

The newly found deposit has a volume similar to Lake Superior's nearly 3,000 cubic miles (about 12,000 cubic kilometers). The deposit holds 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 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 Roger Phillips of Southwest Research Institute in Boulder, Colo. Phillips is deputy team leader for the Mars Reconnaissance Orbiter's Shallow Radar instrument and lead author of the report.

"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 co-author of the new report. 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," Phillips said.

An occasional increase in the atmosphere would strengthen winds, lofting more dust and leading to more frequent and more 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 several-fold changes in the total mass of the planet's atmosphere can happen on time frames of 100,000 years or less.

The changes in atmospheric density caused by the carbon-dioxide increase also would amplify some effects of the changes caused by the tilt. Researchers plugged the mass of the buried carbon-dioxide deposit into climate models for the period when Mars' tilt and orbital properties maximize the amount of summer sunshine hitting the south pole. They found 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 loss of Mars' atmosphere has been an ongoing mystery," according to Doug McCuistion, director of the Mars Exploration Program at NASA Headquarters in Washington.

In 2014, if all goes as scheduled, the Mars Atmosphere and Volatile EvolutioN mission will examine the evolution of Mars' atmosphere, as well as underlining their dedication to acronyms even when they require almost random capitalization and a dictionary.  A 'maven' is an expert, by the way, and in late 2014 this orbiter will know more about Mars' atmosphere than anything ever has.

It should, because it'll be flying through it.  The MAVEN will be orbiting low enough to scoop out samples of the air not only to find out what's there, but also what isn't.

One theory is that Mars lost its magnetic field, without which it was defenseless against the brutal onslaught of solar radiation which stripped anything not nailed down (like air) off the planet.  You may remember this from an explanation in 'The Core', but we at the Galaxy must recommend against thinking about "The Core" and real science at the same time in the strongest possible terms.  It's like mixing matter and antimatter: not something you want to do inside your skull.

After arriving at Mars in the fall of 2014, MAVEN will use its propulsion system to enter an elliptical orbit ranging 90 to 3,870 miles above the planet. The spacecraft's eight science instruments will take measurements during a full Earth year, which is roughly equivalent to half of a Martian year. MAVEN also will dip to an altitude 80 miles above the planet to sample Mars' entire upper atmosphere.

During and after its primary science mission, the spacecraft may be used to provide communications relay support for robotic missions on the Martian surface. See, cooler people get to play with lots more money.

Meanwhile physicists from the University of Leicester, part of an international team that has identified the impact of the Sun on Mars' atmosphere, report that Mars is constantly losing part of its atmosphere to space due to pressure from solar wind pulses.

The researchers analysed solar wind data and satellite observations that track the flux of heavy ions leaving Mars's atmosphere. The authors found that Mars's atmosphere does not drift away at a steady pace; instead, atmospheric escape occurs in bursts.

The researchers related those bursts of atmospheric loss to solar events known as corotating interaction regions (CIRs). CIRs form when regions of fast solar wind encounter slower solar wind, creating a high-pressure pulse. When these CIR pulses pass by Mars, they can drive away particles from Mars's atmosphere.

The team found that during times when these CIRs occurred, the outflow of atmospheric particles from Mars was about 2.5 times the outflow when these events were not occurring. Furthermore, about one third of the material lost from Mars into space is lost during the impact and passing of CIRs.

Professor Mark Lester, Head of the Department of Physics and Astronomy at the University of Leicester said: "The main reason it happens at Mars and not at Earth is the lack of a magnetic field produced by the planet, which protects the atmosphere at Earth.

"One other aspect of this work is that the observations were made during a very quiet period in the eleven year solar cycle and so we would expect the effect of these and other large scale disturbances to be higher at other times in the solar cycle."

The Daily Galaxy via jpl.nasa.gov

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