10. Saturn: lord of the rings

Saturnís winds and clouds

Fig. .. 

The wind speeds of Saturnís equatorial jet streams reach 500 meters per second, almost four times the speed of Jupiterís fastest winds and ten times hurricane force on the Earth. The dominant winds on Saturn blow eastward, in the same direction as the planetary rotation, at almost all latitudes, with the most powerful nearest to the equator. Reversals in wind direction are only found near Saturnís poles, where the clouds counter flow in the eastward and westward direction. They form banded belts and zones similar to those observed almost everywhere on Jupiter.

Despite its raging winds, Saturn lacks the dynamic and colorful storm clouds of Jupiter. Stormy weather on Saturn is apparently masked by an upper deck of dirty, smog-coated particles that give the planet a pastel, butterscotch hue. Jupiter, being warmer than Saturn, has less of this smoggy haze, and its cloud features are more distinct.

Fig. .. 

When Voyager 2 passed behind Saturn, its homebound radio signals penetrated the upper atmosphere, and alterations in these transmissions have been used to deduce the pressure and temperature below the clouds. Because there is no solid surface directly below the clouds, altitudes are referred to the level in the atmosphere where the pressure is equal to 0.1 bars, or one-tenth the sea-level pressure on Earth. This is the approximate level where the temperature bottoms out, at about 82 degrees kelvin, and the obscuring veil of haze may be formed.

Under the assumption that Saturnís gas mixture is in chemical equilibrium, with a uniform composition like that of the Sun, ammonia is expected to condense and form clouds at about 100 kilometers below the reference level, where the pressure has risen to about 1 bar. These clouds of ammonia ice presumably rise to form the bright, white storms that are occasionally seen above the global haze. Water clouds may form much lower in the atmosphere, where the pressure rises to almost 10 bars, but no one has ever seen them.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University