8. Mars: the red planet
The polar regions
Seasonal polar caps
The seasonal caps were long thought to be composed of water ice, by analogy with the Earth’s polar caps. But the seasonally varying Martian caps are composed of frozen carbon dioxide, or dry ice. This is the same dry ice that is used on Earth to keep ice cream, lobsters and other things cold for days at a time. The winter on Mars is so cold that the carbon dioxide gas above each pole freezes and falls down to the ground.
The atmospheric carbon dioxide condenses in the winter when the temperature at the pole drops below 150 degrees kelvin, forming a large seasonal polar cap. It sublimates, or evaporates from solid ice to gas, when the polar temperature rises above 150 degrees in the spring and summer, returning to the atmosphere. This condensation of carbon dioxide during winter and its subsequent sublimation in the spring is what gives rise to the familiar waxing and waning of the Martian polar caps. The process is entirely analogous to the snowfall that blankets the Earth’s polar regions in the winter, and evaporates in the summer, except the “snowfall” on Mars consists of dry ice. It also accounts for the enormous seasonal change in the surface pressure on Mars; about 30 percent of the atmospheric carbon dioxide cycles into and out of the polar regions each year.
The residual, remnant, perennial or permanent caps
At both poles, the caps never completely disappear in the heat of the summer, when the temporary, seasonal deposits of dry ice sublimate back into the atmosphere. Residual, or remnant, polar caps are left behind. Since they remain throughout the Martian year, these residual caps have also been called perennial or permanent caps.
The residual caps at the two poles have a split personality, with different sizes and compositions. At the south pole of Mars, the frozen carbon dioxide never entirely disappears, and a residual deposit of dry ice persists throughout the summer’s warmth. All the seasonal dry ice disappears during the northern summer, and the part that survives is about three times larger than the southern residual cap. Instead of frozen carbon dioxide, the residual cap at the north pole is composed of water ice.
Evaporation of carbon-dioxide ice in the summer reveals laminated terrain that extends horizontally for several thousand meters along the edges of both residual caps. Up to 20 layers have been exposed, each a few tens of meters thick, alternating between dark dust and bright ice. The extensive, regular polar layers were probably deposited during periodic climate change. It is estimated that the layered material was laid down at the rate of about 0.001 meters per year. So a layer that is several tens of meters thick took ten thousand to one hundred thousand years to accumulate, which is roughly comparable to the periodicity of great ice ages on Earth. The laminated terrain on Mars might be attributed to astronomical rhythms that have similarly created long-term, periodic changes in the climate of Mars, at least for the past few million years and perhaps longer.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University