polar caps of Mars
Figure 1. The north polar cap of Mars as seen by Mars Global Surveyor on Sep. 12, 1998.
Figure 2. South polar cap. Image: Mars Global Surveyor, Apr. 17, 2000.
Mars has ice caps at both its north and south poles. The perennial or permanent portion of the north polar cap consists almost entirely of water ice. In the northern hemisphere winter, this gains a seasonal coating of frozen carbon dioxide about one meter (three feet) thick.
The south polar cap also acquires a thin frozen carbon dioxide coating in the southern hemisphere winter. Beneath this is the perennial south polar cap, which is in two layers. The top layer consists of frozen carbon dioxide and about 8 meters (27 feet) thick. The bottom layer is very much deeper and is made of water ice. Data collected by the Marsis radar instrument aboard Mars Express has indicated that enough water is locked up at Mars' south pole to cover the planet in a liquid layer 11 meters (36feet) deep.
How we learned about the martian poles
The knowledge that the martian polar caps consist almost entirely of water ice goes back only a few years. Until recently, it was thought that both polar caps consisted largely of frozen carbon dioxide, with a small amount of water ice. This idea dates back to 1966, when the first Mars spacecraft determined that the martian atmosphere was largely carbon dioxide. Scientists at the time argued that the ice caps themselves were solid carbon dioxide and that the caps regulate the atmospheric pressure by evaporation and condensation.
Later observations by the Viking orbiters showed that the north polar cap contained water ice underneath its dry ice covering; however, experts continued to believe that the south polar cap was made of dry ice. In 2003, California Institute of Technology researchers Andy Ingersoll and Shane Byrne argued, on the basis of high-resolution and thermal images from Mars Global Surveyor and Mars Odyssey, respectively, that the martian polar ice caps are made almost entirely of water ice – with just a smattering of frozen carbon dioxide at the surface. These images showed flat-floored, circular pits 8 meters deep and 200 to 1,000 meters in diameter at the south polar cap, and an outward growth rate of about one to three meters per year. Infrared measurements from Mars Odyssey showed that the lower material heats up, as water ice is expected to do in the martian summer, and that the polar cap is too warm to be dry ice. Based on this evidence, Byrne and Ingersoll concluded that the pitted layer is dry ice, but the material below, which makes up the floors of the pits and the bulk of the polar cap, is water ice. This shows that the south polar cap is similar to the north pole, which was determined, on the basis of Viking data, to lose its one-meter covering of dry ice each summer, exposing the water ice underneath. The new results show that the difference between the two poles is that the south pole dry-ice cover is slightly thicker – about eight meters – and doesn't disappear entirely during the summertime.
These findings present a new scientific mystery to those who thought they had a good idea of how the atmospheres of the inner planets compared to each other. Planetary scientists had assumed that Earth, Venus, and Mars are similar in the total carbon dioxide content, with Earth having most of its carbon dioxide locked up in marine carbonates and Venus's carbon dioxide being in the atmosphere and causing the runaway greenhouse effect. By contrast, the 8-meter layer on the south polar ice cap on Mars means the planet has only a small fraction of the carbon dioxide found on Earth and Venus.
The new findings further pose the question of how Mars could have been warm and wet to begin with. Working backward, one would assume that there was once sufficient carbon dioxide in the atmosphere to trap enough solar energy to warm the planet, but there's not enough carbon dioxide locked in the poles for this to clearly have been the case. There could be other explanations. It could be that Mars was a cold, wet planet; or it could be that the subterranean plumbing would allow for liquid water to be sealed off underneath the surface. In one such scenario, perhaps the water flowed underneath a layer of ice and formed the channels and other erosion features. Then, perhaps, the ice sublimated away, to be eventually redeposited at the poles.