Saturns Surface Appearance and Conditions

Though Saturn's weather patterns lack Jupiter's intense coloration, the planet still has violently fast winds, giant convective storms, and streaming ribbons of clouds.These weather patterns can be directly observed, unlike the planet's interior processes, but still little is known about them. Saturn's average temperature is —288°F (—178°C) at a pressure of about one bar. The planet's exceptionally cold atmosphere means that ice clouds form at low pressures, and liquid droplet clouds can only exist at greater pressures and hence higher temperatures.

Saturn consists of helium and hydrogen almost exclusively: Only 0.2 percent of its atmosphere by volume is heavier than helium.These heavier constituents create most of the clouds and color of the planet, however. Saturn's clouds are thought to consist mainly of ammonia ice (NH3), ammonium hydrosulfide (NH SH), and water (HO) in order of descending altitude.A few even rarer atmospheric components consisting of phosphorus, arsenic, and germanium each combined with hydrogen have been detected in infrared spectra from the planet. The relative abundances of molecules in Saturn's atmosphere are given in the table on page 120.

The upper clouds of ammonium compounds block the water clouds from observer's sight, but the cloud zone is thin and soon changes with depth to clear atmosphere of mainly helium and hydrogen.Aside from the thin layer of colored clouds that decorate the outermost shell of Saturn, there is also a hydrocarbon haze around the planet's poles, possibly caused by electrical interactions from the auroras with the upper atmosphere.

ABUNDANCE OF SELECTED MOLECULES

IN SATURN'S ATMOSPHERE

Number of molecules per

Molecule

Composition

molecule of hydrogen

hydrogen

H2

1

helium

He

0.035

methane

CH4

0.0045

ammonia

NH3

0.0003

ethane

C2H6

0.000003

phosphine

PH3

0.0000045

arsine

AsH3

0.000000003

carbon monoxide CO

0.0000000016

germane

4

0.0000000005

Saturn's atmosphere can be divided into two parts: the first, the troposphere, reaching from the planet's interior with temperature decreasing with height to a point called the tropopause; the second, the stratosphere, beginning at the tropopause and continuing upward with increasing temperature until the atmosphere merges into the vacuum of space. On Saturn the tropopause occurs at a pressure between 60 and 100 millibars and a temperature of —320°F (—193°C). Temperature increases in the stratosphere because of solar heating, and since each higher layer is hotter than its predecessor, the layers are stable to mixing: Higher temperatures create less dense material, so the higher material is buoyant with respect to all that lies beneath it. Because in the tropopause the temperature gradient is reversed, the troposphere is unstably stratified with respect to density, and it is prone to convection and creation of storms.

As with the other gas giant planets, Saturn's uppermost stratosphere, called the exosphere, is exceptionally hot. Its temperature cannot be explained by solar heating alone. The unusual heat may be caused by charged particles moving through Saturn's magnetic field that deposit energy in the uppermost atmosphere, or perhaps by electric currents dissipating in the upper atmosphere. Even gravity waves from deeper in Saturn have been proposed as the source of this energy. There is no single leading theory for this phenomenon at present. Though Saturn's clouds form spots and light and dark bands in parallel with its equator, these markings are much less well-defined than on Jupiter. Saturn has an especially intense equatorial jet with winds as fast as 1,100 miles per hour (1,800 km/hr) in the direction of the planet's rotation. This equatorial jet is four times as fast as Jupiter's. The high winds are not understood: On Jupiter, the Galileo probe measured strong winds down to pressures as high as 20 bars, which is well below the depths to which solar energy reaches. The winds cannot therefore be caused by solar heating alone.

Saturn's cloud surface has many visible patterns, including discernible storms similar to Jupiter's White Oval Spots. The Voyager 2 mission observed anticyclonic brown ovals in Saturn's northern hemi-

This vortex stream is in a cloud on Earth that is moving rapidly past an island in the north Atlantic, forming structures strikingly similar to those on Saturn. (NASA/ GSFC/LaRC/JPL, MISR Team)

sphere that were 3,100 miles (5,000 km) x 1,800 miles (3,000 km) in extent. Large storms on Saturn have been observed to appear and disappear over time since the 19th century, when telescopes were reliably good enough to discern individual storms with certainty. Brilliant white storms were seen by various astronomers in 1876, 1903, 1933, 1960, and 1990, establishing a 30-year cycle for their appearance.The appearance and disappearance of giant storms seems to be tied to Saturn's seasons, and thus to be driven by solar heating.They appear between about five and 60 degrees latitude, where solar heating is at a maximum, and are thought to consist of water clouds, something like thunderheads thousands of miles across. Ribbon structures in Saturn's upper atmosphere can be seen in some images from Voyager 2. Scientists think the ribbons are waves within a rapidly moving jet stream. These ribbons shed vortices to their sides that appear something like the vortices that shed from the sides of the trail of a fast airplane in a cloud, or that are formed in clouds moving rapidly past an island in the ocean (see figure on page 121). The forms of the ribbons and the vortices indicate that the ribbons are moving at about 335 miles per hour (540 km/hr).

Individual spots, mainly discrete storms, move with the flow of the latitude in which they form. Voyager observations in 1980 and 1981 were the first to accurately track the movement of these spots, and they provided the first measurement of Saturn's differential rotation: The planet's equator rotates faster than its poles, a phenomenon observed on all the gas planets and the Sun.Though atmospheric features are clearly visible in the image shown on page 107, compared to Jupiter, Saturn's visible surface has a relatively uniform color. Its uniformity is an ongoing mystery: Are many of the convective cells (cyclonic or anticyclonic storms) made of the same material? The material on Saturn may in fact be more uniform, but clouds on Saturn are also located deeper in the atmosphere than they are on Jupiter, because Saturn is in general colder at any given pressure than is Jupiter. Cloud patterns on Saturn are therefore more obscured by overlying hazes than they are on Jupiter.

Saturn's upper atmosphere is directly visible from the Earth and from space missions, but its observation has created more questions than answers. The polar hydrocarbon haze, exceptionally hot exos-phere, the depth of fast winds, and the uniformity of cloud color are all persistent questions without clear answers.

0 0

Post a comment