Currently, Mars has a thin atmosphere dominated by carbon dioxide, with pressures at most of the planet's surface so low that liquid water will immediately boil. But a variety of features we've discovered argue that the planet has once supported copious amounts of water, indicating that the planet's atmosphere must have differed considerably in the past. Using radar data from the Mars Reconnaissance Orbiter, scientists have now found a potential resting place for some material that was once in the Martian atmosphere: a huge deposit at the south pole that holds nearly as much CO 2 as the planet's current atmosphere.

Mars' south pole has extensive ice deposits, but most of that material is thought to be water, with only a thin coating of carbon dioxide on top. However, the MRO's radar instrument identified several reflection-free zones, where most of the radar signal went entirely through the icy material to the planet's surface itself. Based on the authors' calculations, this can't be water ice, but it does have very similar reflective properties to dry ice, or frozen carbon dioxide. The area also has features that indicate that some of the dry ice has sublimated to a gaseous form, resulting in areas where the surface has collapsed.

If the area is dry ice, then the total amount present is huge. The authors estimate the total volume of the non-reflective material at somewhere between 9,500 and 12,500 cubic kilometers. That's 30 times more than had previously been estimated to reside at the poles, and is about 80 percent of the current CO 2 content of the entire atmosphere. If all the dry ice were heated up, Mars' atmospheric pressure would nearly double.

Like the Earth, Mars undergoes orbital variations that alter the distribution of sunlight across the planet. One of these involves changes in the orientation of its axis of rotation relative to the plane of its orbit, called the obliquity. Mars undergoes more dramatic changes in obliquity than Earth and, as a result, its poles see more significant changes in sunlight at the extreme. The authors argue that this can help explain why the reflection-free zones lack any material from the planet's famous dust storms, which should reflect the radar effectively.

Mars' atmosphere needs to be above a certain density to support the particles that make up its dust storms. As the poles undergo extended cold periods, the authors suggest "the atmosphere collapses onto the polar caps." So much of the planet's dry ice winds up frozen at the poles that the atmosphere becomes even thinner than it is at present, and incapable of supporting dust storms.

As of now, however, the amount of sunlight at the poles is increasing, leading to the loss of some of the material from these areas, which is bulking up the atmosphere. The authors run a simplified global circulation model of Mars' atmosphere to see what happens as the planet reaches the opposite extreme, and all of the polar dry ice is liberated into the atmosphere. Pressure on the surface would nearly double, and the increased CO 2 would enhance the planet's existing greenhouse effect. However, it would also increase the formation of seasonal dry ice deposits that reflect sunlight and offset this warming, leaving Mars slightly cooler.

However, the simplified model leaves out some other factors. For one, the denser atmosphere could support more significant dust storms, changing the planet's ability to absorb sunlight. Some of the water ice at the poles would probably also melt, adding water vapor to the atmosphere and further enhancing the greenhouse effect. However, the increased atmospheric pressure would allow some of the liquid water to remain on the surface without boiling, meaning we could see some pools of water on Mars.

Sorting out exactly what would happen will apparently require a more complete climate model for the red planet. "Given the complex interplay between the dust, water, and CO2 cycles, additional changes in the climate system are very likely," the authors conclude. Still, even with the possible melting of the polar ice caps and enhanced greenhouse effect, the total of the changes don't seem to be sufficient to get us to anything like Mars' watery past, which suggests that some of the planet's carbon dioxide and water may now be trapped in geological features.

Science, 2011. DOI: 10.1126/science.1203091 (About DOIs).