Clues to climate history
In 1666, Gian Domenico Cassini, the Italian-French astronomer, first observed how the white poles of Mars wax and wane with the planet's seasons. He assumed they were covered by water ice like the Earth's poles. Now we know that frozen carbon dioxide is also present and that an unusual area of what is called "layered terrain" surrounds both poles.
The two poles, though, are far from identical. The residual cap at the north pole - the cap left in summer after the winter deposits have melted - is larger than that in the south, but the area of layered terrain is smaller. Sand dune fields, which surround the layered terrain at both poles, are more extensive in the north. The north residual cap consists almost entirely of water ice, whereas frozen carbon dioxide seems to predominate in the south. During winter, both poles expand enormously in surface area and consist of frozen water and carbon dioxide.
Three images showing layers at the north pole (NASA/JPL/Malin Space Science Systems)
The polar layered terrains are thought to hold the key to understanding Mars's climate history. "At the south and north pole, the layers are a few tens or hundreds of metres thick. And it's quite evident that they alternate - light and dark. This layering is due to wind blown debris and sedimentation for sure," says Gian Gabriele Ori from the Universita d'Annunzio, Pescara, Italy and a Co-Investigator on the Mars Express High Resolution Stereo Camera.
Each season, freezing water and carbon dioxide trap wind blown dust and other debris at the poles. Gradually, layers are built up that preserve a record of water, carbon dioxide and dust over timescales ranging from seasonal to millions of years. Both polar regions are considerably higher than the surrounding areas, lending support to the notion that they are made of layered terrain deposited over aeons.
The extension of the layered terrains beyond the present residual polar caps suggests that the poles have moved over time, which is not surprising as the inclination of Mars's axis of rotation varies cyclically over thousands of years. Cyclical variations in the shape of Mars's orbit around the Sun also affect the climate, which influences the growth of layered deposits.
Images of the residual polar caps taken by the Mars Orbiter Camera on board NASA's Mars Global Surveyor spacecraft.
"No-one knows why the layered terrain in the south is larger than in the north," says Ori. "But if an ocean was once present in the northern hemisphere, it could have limited the extent of the polar deposits there." Another piece of circumstantial evidence for an ocean is the extensive sand dune field around the northern layered terrain. "Sand is a component of the sediment on the ocean floor. The fact that there is a lot of sand in the north is supporting evidence for an ocean there at some time," says Ori.
These uncertainties will be resolved only when better data is returned on the topography and physical appearance of the polar regions. Mars Global Surveyor has been sending back the data since the late 1990s and now the OMEGA and HRSC instruments on Mars Express are helping to build up an even clearer picture.
||Morphology & Erosion
Last Update: 08 Aug 2006