11. Uranus and Neptune

Storm clouds on the outer giants

Mild weather on Uranus

With no appreciable heat rising from the interior to drive the weather system, Uranus presents a dull and placid face to the world. In addition, a cold and hazy atmosphere obscures our view. Even at close range, looking at Uranus is something like gazing down into a bottomless ocean.

Fig. .. 

Yet, some zonal banding was extracted from the Voyager 2 images. The clouds are arranged in bands that circle the planetís rotation axis, running at constant latitudes parallel to the equator like the more vivid bands seen at Jupiter and Saturn. The features at different latitudes on Uranus move in the same east-west direction as the planet rotates, but at faster speeds. The difference is greatest at high latitudes, where the clouds circle the poles in 14 hours, and it gets progressively smaller toward the equator, closer to the internal rotation period of 17.24 hours.

Stormy weather on Neptune

Fig. .. 

The Voyager 2 flyby in 1989 forever changed our view of Neptuneís weather. Despite its great distance from the Sun, the dimly lit atmosphere of Neptune is one of the most turbulent in the solar system, with violent winds, large dark storms and high-altitude white clouds that come and go at different places and times.

The wind pattern on Neptune lacks Jupiterís multiple zonal winds that flow in opposite directions. Neptune has just one westward air current at low latitudes, like the Earthís trade winds, and one meandering eastward current at mid-latitudes in each hemisphere, resembling the Earthís jet streams. And like Uranus, the polar and equatorial temperatures on Neptune are nearly equal.

Fig. .. 

You wouldnít want to forecast the lively, variable and unpredictable weather on Neptune. When the Hubble Space Telescope took another look at the planet, in 1994 to 1996, the violent storms seen by the Voyager 2 cameras had vanished without a trace, and other storms had appeared.

Fig. ..  Fig. .. 

The largest dark storms on Neptune are probably high-pressure systems that come and go with atmospheric circulation. The most prominent one was the Great Dark Spot, a vast, circulating storm almost as large as Earth. It is called the Great Dark Spot because it resembles the Great Red Spot of Jupiter. Both storms are found in the planetary tropics - at about one-quarter of the way from the equator to the south pole, both rotate counterclockwise, in the direction of high-pressure anticyclones, and both are about the same size relative to their planet. As on Jupiter, some of the small dark spots on Neptune may be whirling in the opposite direction to the bigger one, perhaps indicating that they are little cyclones with descending material at their centers.

There are some important differences between the two Great Spots. The Jovian red spot has survived for centuries, while Neptuneís dark one disappeared from view within a few years of its sighting from Voyager 2. And Jupiterís whirling storm lies above the clouds while Neptune's seems to form a deep well in the atmosphere, providing a window-like opening to the deeper, darker clouds below.

White, fleecy cirrus-like clouds cast shadows on the blue cloud deck below, indicating that they are high-altitude condensation clouds that rise about 100 kilometers above the surrounding ones. They form as atmospheric gas flows up, over and around the storm center, without being consumed by it. When the rising methane gas cools, it forms white clouds, fashioned from crystals of frozen methane. Water in the Earthís atmosphere freezes in a similar way into ice crystals that form cirrus clouds. When strong upwelling carries the wispy white clouds to great heights in Neptuneís atmosphere; they are sheared out like anvils of terrestrial thunderstorms.

Although the global wind pattern on Neptune resembles the Earthís trade winds and jet streams, they cannot be energized in the same way. Solar heating of the atmosphere and oceans drives the terrestrial winds. At Neptuneís distance the Sun is 900 times dimmer and the winds should be correspondingly weaker if they are driven by the feeble sunlight. The fast winds on Neptune and the planetís complex stormy weather must instead be energized by heat generated in the planetís core. The internal heat warms Neptune from the inside out, producing convecting currents of rising and falling material, somewhat like a pot of boiling water on a stove. Uranus, on the other hand, shows no signs of substantial internal heating, and this may explain why its atmosphere is relatively benign and inactive.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University