Science: Martian Monsoons

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Seen through the cameras of Mariner 9, the parched surface of Mars presents a scientific puzzle. Twisting gullies cut across the stark landscape like the traces of a mighty river system; yet scientists agree that there is now no free-flowing water on the Red Planet. How were the "riverbeds" made?

The 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."

Gyroscope. The possibility of monsoons is not as farfetched as it seems. Mars has an eccentric orbit that causes large variations in the planet's distance from the sun. The 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.

But 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 of the caps release most of their carbon dioxide and any water into the atmosphere.

If 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 at which the north polar summers occur at a maximum distance from the sun. Thus no Martian monsoons could occur for another 10,000 years.