Word: polarity
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Dates: during 1970-1979
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...answer may lie in a theory suggested by Astronomer Bradford A. Smith of New Mexico State University and others long before Mariner 9 took off. Smith says that water may be stored as ice in the planet's northern polar cap under a thin layer of frozen carbon dioxide, or dry ice. That hidden water, he says, may be released periodically into the Martian atmosphere, producing regional rains and perhaps floods to erode the arid Martian surface. Bemused scientists at Caltech's Jet Propulsion Lab are now calling Smith's rains Martian "monsoons...
...Martian north pole is currently tilted toward the sun only when the planet is also at its greatest distance, or aphelion, from the sun. In contrast, the southern hemisphere is tilted toward the sun at the planet's closest approach, or perihelion. As a result, the southern polar cap gets warm enough to evaporate almost completely each summer, releasing most of its dry ice into the atmosphere. The northern polar region, however, remains cold enough during summer to retain a large and relatively thick polar cap throughout the year...
...like a spinning gyroscope, Mars slowly wobbles as it travels around the sun. Though this motion, or precession, is barely perceptible, the Martian axis leans in the opposite direction every 25,000 years, or halfway through a complete precessional cycle. When that happens, the northern polar area is angled toward the sun at the planet's closest approach, while the southern polar area, tilted away, freezes and traps the moisture. What interests Smith, however, is the orientation of the poles in between those extremes. Then, both polar regions receive equal heating from the sun. The warmer outer portions...
...glacier-like quantities of frozen water have accumulated in either polar cap, says Smith, enough water might be released to keep the monsoons going for centuries, and possibly millenniums, until the slow precession of the planet's axis causes one pole to begin cooling enough to draw water back out of the atmosphere and into the Martian deep freeze for another 25,000 years. Even if the monsoon theory is correct, however, many centuries will pass before visitors to Mars will have to shoot Martian rapids or brave unearthly downpours. Mars has just passed the point in its precession...
...that also is going, and the settlement's 400 snowmobiles have entirely replaced the dog sled. About the only thing that has survived from the old days is the hunt. The men still hunt whales from fragile little boats made from animal skins. They also stalk walrus, seal, polar bear and caribou. But now they use high-powered rifles to bring down their prey...