The atmospheres of the giant planets are thick with clouds and haze, so thick that we can’t see the surfaces of these worlds. The clouds on all four giant planets are organized in systems of dark belts and bright zones that run parallel to their equators. These can best be seen on Jupiter and Saturn. The clouds of the Jovian planets are believed to be composed primarily of ice crystals - frozen ammonia, ammonium hydrosulfide, and water. These compounds don’t represent the overall composition of hydrogen and helium. They’re minor constituents that condensed out of the atmospheres, in the same way that a minor constituent, water vapor, condenses out of Earth’s atmosphere to form terrestrial clouds.

Colors and patterns reveal conditions in planet atmospheres. Differences in the cloud patterns and colors of the giant planets are explained by differences in atmospheric temperature, chemistry, and structure. On Jupiter and Saturn, the bright zones are whitish or yellowish, and the darker belts show gray-brown and reddish tinges. The colored compounds in the clouds - including polysulfides, phosphorus, and organic compounds - stain the clouds these colors. The word organic does not imply the presence of life, but merely describes the complex chemistry of carbon-hydrogen bonds in the clouds of these planets. Keep in mind that many of the published images of planet atmospheres have had their colors amplified for cosmetic reasons. The true colors of the giant planets are quite delicate and subtle.


The clouds on Uranus are indistinct, because they are hidden beneath a thick, hazy upper atmosphere. The blue colors of Uranus and Neptune are caused by two phenomena: first, the absorption of red light by methane, which is an important constituent of the haze above their clouds. Second, the same scattering of blue light that exists in Earth's atmosphere also occurs in these giant planets. The blue color is less prominent for Jupiter and Saturn because their uppermost clouds have less methane haze above them.

 
The clouds on the Jovian planets are dynamic - they are in constant motion. Even a backyard telescope reveals that Jupiter's cloud belts change. The clouds change from month to month and year to year due to turbulent movements of the atmosphere, just as clouds on the Earth change from day to day. The Voyager probes measured winds on Jupiter blowing eastward along the equatorial and temperate zones at speeds 150 meters per second (338 mph) faster than the movements of the clouds in the dark equatorial belts. Saturn has even higher differential wind speeds: the eastward jet stream along the equator moves up to 450 meters per second (1010 mph) faster than the neighboring belts. The colors, wind shear patterns, and clouds upwelling from below combine to produce fascinating, swirling patterns of color, reminiscent of abstract paintings.

The most famous and dramatic feature on any of the giant planets is Jupiter’s Great Red Spot, which is an oval storm system. It has existed for at least 330 years, since G. D. Cassini first reported it in 1665. The Great Red Spot became more strongly visible around 1887, when it was rediscovered and given its current name. It’s three times the size of the Earth! This and other smaller transient spots are probably hurricane-like systems. Small clouds approaching the Red Spot get caught in a counterclockwise circulation, like leaves in a giant whirlpool. In recent years smaller red spots have formed (starting out white in some cases), and then merged together and faded away. This shows that Jupiter is still a planet with atmospheric surprises.

There is also intense electrical activity in the violent storm systems of Jupiter. Voyagers' photos of the night side revealed mighty blasts of lightning playing among the clouds. The parachuted probe also detected radio static from distant lightning. Typical lightning strokes were more energetic than on Earth, but there were fewer per unit area than on Earth. Voyager also photographed enormous auroral displays like the northern lights and southern lights of Earth, flickering high above the clouds in Jupiter's polar regions. Saturn's atmosphere has less lightning and less auroral activity, and the atmospheres of Uranus and Neptune are still less active.

Modeling weather systems of giant planets is a good test of our understanding of climate and air circulation on our own planet. A particularly active area of current scientific research uses theoretical models to compare the atmospheric circulation of the Jovian planets with the terrestrial atmosphere. Why are cyclonic swirls more prominent than belt patterns on the Earth? Why do storms last for hundreds of years on Jupiter, but only weeks on Earth? How does energy flow through atmospheres of different compositions? The answers to these and similar questions may help us clarify our understanding of the Earth.
Author: Chris Impey
Editor/Contributor: Ingrid Daubar-Spitale
Taken from: http://www.teachastronomy.com/astropedia/article.aspx?qid=43
最后修改: 2021年08月30日 星期一 10:33