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Big changes in Mars’ atmosphere
Posted: 27 April 2011

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A massive reservoir of buried frozen carbon dioxide ice (dry ice) detected by the Mars Reconnaissance Orbiter (MRO) is intimately related to the mass of the red planet’s atmosphere as the planet tilts on its axis, which in turn could affect the stability of liquid water and the frequency and severity of dust storms.

A newly discovered buried deposit of frozen carbon dioxide (dry ice) near Mars’ south pole contains about 30 times more carbon dioxide than previously estimated. This map colour-codes thickness estimates: red=600 metres, yellow to about 400m; dark blue to less than 100m, tapering to zero. Image: NASA/JPL-Caltech/Sapienza University of Rome/Southwest Research Institute.

MRO made the detection using its ground penetrating radar instrument, revealing a frozen carbon dioxide ice deposit at Mars’ south pole that occupies a 12,000 cubic kilometre volume and holds 80 percent as much CO2 as today’s atmosphere. Planetary scientists already knew that a small cap of carbon-dioxide ice rested on top of the water ice there, but the newly detected deposit has about 30 times more dry ice than previously estimated.

“The discovery was very surprising to me and many other scientists, although theoretical models had predicted that the CO2 atmosphere would occasionally collapse onto the poles,” Roger Phillips of Southwest Research Institute and deputy team leader for MRO’s Shallow Radar instrument tells Astronomy Now. “To actually find evidence of this is another matter, so we were very careful in our analyses to make sure we got it right.”

This cross-section view of underground layers near Mars' south pole is a radargram based on data from the Shallow Subsurface Radar (SHARAD) instrument on MRO. Researchers interpret the zone that is nearly free of radio-wave reflections (hence dark in the radargram) to be composed of frozen carbon dioxide, or "dry ice." This cross section covers a transect about 330 kilometres long in a region from about 86 degrees to 87 degrees south latitude and 280 degrees to 10 degrees east longitude, and is approximately 1.7 kilometres deep. Image: NASA/JPL-Caltech/Sapienza University of Rome/Southwest Research Institute.

Features such as collapse pits, which are known to be caused by dry ice sublimation, suggest that the ice cap is dissipating, adding gas to the martian atmosphere. “There are two types of collapse pits indicating sublimation of the dry ice,” explains Phillips. “There are isolated sublimation pits, up to four kilometres in diameter, and there are linear troughs, along which sublimation collapse pits form. There are also erosional remnants of the CO2 deposit seen in the radar data.”

Phillips adds that if 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. When the dry ice is in its dissipating phase, much of the carbon dioxide enters the planet’s atmosphere, resulting in stronger winds, not only increasing the frequency of dust storms, but also their intensity. They even speculate that conditions might be stable enough for liquid water to exist in some locations on the red planet.

These images from orbit show an area near Mars' south pole where coalescing or elongated pits are interpreted as signs that an underlying deposit of frozen carbon dioxide, or "dry ice," has been shrinking by sublimation. The image on the left covers an area about 5.2 kilometres across, near 87 degrees south latitude, 268 degrees east longitude; the image on the right is an enlarged section of the rectangle in the left image. Image: NASA/JPL-Caltech/Univ. of Arizona.

The sublimating dry ice is likely linked to the tilt of Mars’ axis, which affects the amount of sunlight falling onto the polar regions. If Mars’ tilt and orbital parameters act to offer maximum exposure to the summer sunshine at the south pole, this could increase the atmospheric pressure by 75 percent its current level, and modelling suggests that the planet’s atmosphere could change several-fold on the order of 100,000 years or less.

“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,” says Robert Haberle of NASA’s Ames Research Center. “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.”