Much of ancient Martian atmosphere frozen at poles
A reworking of Mars Orbiter's satellite radar data claims to show that the Red Planet had a much thicker atmosphere - and more running water - in the past. But only when the tilt was just right. The polar ice caps of Mars have previously been interpreted as being made-up mainly of frozen water.
But in a paper published in this week's Science, a team of scientists are suggesting that a substantial slice of the southern ice cap is in fact solid CO2 - dry ice. It may hold as much 'frozen atmosphere' from the past as is found currently in the feeble Martian air
Mars is a somewhat smaller planet than the Earth, and so long ago lost much of its atmospheric gases to the vacuum of space; its weaker gravity couldn't hold onto them. It now has only 6 millibars of CO2 floating above its surface. But the ice caps, particularly at the southern pole, have always been thought to contain some CO2 - just not very much - being held as a thin layer over the much thicker frozen water ice. Now this study could change that consensus.
The Mars Orbiter satellite, which surveyed the planet back in 2005, used ground-penetrating radar to map out the structures lurking under the red dust of the planet's surface. It provided ghostly pictures of stacked layers, ripples and irregular surfaces. But working out the depths and make-up of these polar forms is a difficult task. For this paper, the multi-disciplinary team - led by Roger Phillips, from the Southwest Research Institute in Boulder - looked at a particularly interesting body which cropped up at depth across much of the southern pole's cap.
It showed little radar reflectivity, indicating it was free of dust. Previously, though, it had been considered to be frozen water. However, the team found that working to a new model of the body's radar properties showed that frozen CO2 was a much better fit for its make-up. And there would be a lot of it - some 9,500 to 12,500 cubic kilometers. That's enough to nearly double the atmospheric concentration of CO2 from its current level - to 10mbars or so - if it all 'sublimated' back into the atmosphere.
This can happen periodically with Mars because it has a major wobble cycle over a period of hundreds of thousands of years. Its axis of rotation regularly shifts from being at right angles to the orbit around the sun, to being tilted half-way over towards the sun. That's when the poles receive much more solar energy, and when their store of locked up 'frozen atmosphere' would be released. During those times of thicker atmosphere, the paper suggests that dust storms were more common and vigorous, and that running water could have flowed more easily in some parts of Mars.
At the margins of this 'dry ice' body, the team also found many round pits and troughs. They see these as evidence of the frozen CO2 ice sublimating - turning directly from solid 'dry ice' to gas - which would produce ice collapse structures seen. This all adds up to a complex dance between the atmosphere and frozen ice caps, and between CO2 and water - both solid, liquid, and vapor - as Mars tumbles its way around the Sun. Mars is increasingly showing itself as a planet on the go.