The Atmosphere

Viewed from Earth, Saturn has an overall hazy yellow-brown appearance. The surface that is seen through telescopes and in spacecraft images is actually a complex of cloud layers. Like the other giant planets, Saturn's atmospheric circulation is dominated by zonal (east-west) flow. This manifests itself as a pattern of lighter and darker cloud bands similar to Jupiter's, although Saturn's bands are more subtly coloured and are wider near the equator. So low in contrast are the features in the cloud tops that it was not until the Voyager flyby encounters that Saturn's atmospheric circulation could be studied in any detail.

When defined with respect to the rotation of its magnetic field, virtually all of Saturn's atmospheric flows, or winds, are to the east—in the direction of rotation. Measured against the slower magnetic rotation rate observed by Cassini, the eastward flows are even more pronounced. The equatorial zone at latitudes below 20° shows a particularly active eastward flow having a maximum velocity close to 500 metres per second (1,800 km [1,100 miles] per hour). This feature is analogous to one on Jupiter but extends twice as wide in latitude and moves four times faster. By contrast, the highest winds on Earth occur in tropical cyclones, where in extreme cases sustained velocities may exceed 67 metres per second (240 km [150 miles] per hour).

The zonal flows are remarkably symmetrical about Saturn's equator; that is, each one at a given northern latitude usually has a counterpart at a similar southern latitude. Strong eastward flows—those having eastward relative velocities in excess of 100 metres per second (360 km [225 miles] per hour)—are seen at 46° N and S and at about 60° N and S. Westward flows, which are nearly stationary in the magnetic field's frame of reference, are seen at 40°, 55°, and 70° N and S. After the Voyager encounters, improvements in Earth-based instrumentation allowed observations of Saturn's clouds at distance. Made over many years, these tended to agree with the detailed Voyager observations of the zonal flows and thus corroborated their stability over time. Some high-resolution observations of Saturn's atmosphere showed a large drop in the velocity of the equatorial jet from 1996 to 2002. Analysis of data from the Cassini orbiter, however, suggests that any such velocity drop is confined to superficial layers of the atmosphere.

The general north-south symmetry suggests that the zonal flows may be connected in some fashion deep within the interior. Theoretical modeling of a deep-convecting fluid planet such as Saturn indicates that differential rotation tends to occur along cylinders aligned about the planet's mean rotation axis. Saturn's atmosphere thus may be built of a series of coaxial cylinders aligned north-south, each rotating at a unique rate, which give rise to the zonal jets seen at the surface. The continuity of the cylinders may be broken at a point where they intersect a major discontinuity within Saturn, such as the core.

Saturn's atmosphere shows many smaller-scale features similar to those found in Jupiter's, such as red, brown, and white spots, bands, eddies, and vortices, that vary over a fairly short time. However, in addition to having a much blander appearance, Saturn's atmosphere is less active than Jupiter's on a small scale. A spectacular exception occurred during September-November 1990, when a large, light-coloured storm system appeared near the equator, expanded to a size exceeding 20,000 km (12,400 miles), and eventually spread around the equator before fading. Storms similar in impressiveness to this "Great White Spot" (so named in analogy with Jupiter's Great Red Spot) have been observed at about 30-year intervals dating back to the late 19th century. This is close to Saturn's orbital period of 29.4 years, which suggests that these storms are seasonal phenomena.

Initial analysis of data from the Voyager spacecraft indicated that the planet's atmosphere is 91 percent molecular hydrogen by mass and is thus the most hydrogen-rich atmosphere in the solar system. Helium, which is measured indirectly, makes up another 6 percent and is less abundant relative to hydrogen compared with a gas having the composition of the Sun. If hydrogen, helium, and other elements were present in the same proportions as in the Sun's atmosphere, Saturn's atmosphere would be about 71 percent hydrogen and 28 percent helium by mass. According to some models, helium may have settled out of Saturn's outer layers, but more-recent research has suggested that the Voyager analysis underestimated the helium fraction in Saturn's atmosphere, which may lie closer to the value in the Sun.

Other major molecules observed in Saturn's atmosphere are methane and ammonia, which are two to five times more abundant relative to hydrogen than in a gas of solar composition. Hydrogen sulfide and water are suspected to be major constituents of the deeper atmosphere but have not yet been detected. Minor molecules that have been detected spectroscopically from Earth include phosphine, carbon monoxide, and germane. Such molecules would not be present in detectable amounts in a hydrogen-rich atmosphere in chemical equilibrium. They may be nonequilib-rium products of reactions at high pressure and temperature in Saturn's deep atmosphere well below the observable clouds. A number of nonequilibrium hydrocarbons are observed in Saturn's stratosphere: acetylene, ethane, and, possibly, propane and methyl acetylene. All of the latter may be produced by photochemical effects

Astronomers on Earth have analyzed the refraction (bending) of starlight and radio waves from spacecraft passing through Saturn's atmosphere to gain information on atmospheric temperature over depths corresponding to pressures of one-millionth of a bar to 1.3 bars. At pressures below 1 millibar, the temperature is roughly constant at about 140 to 150 K (-208 to -190 °F, -133 to -123 °C). A stratosphere, where temperatures steadily decline with increasing pressure, extends downward from 1 to 60 millibars, at which level the coldest temperature in Saturn's atmosphere, 82 K (-312 °F, -191 °C), is reached. At higher pressures (deeper levels) the temperature increases once again. This region is analogous to the lowest layer of Earth's atmosphere, the troposphere, in which the increase of temperature with pressure follows the thermodynamic relation for compression of a gas without gain or loss of heat. The temperature is 135 K (-217 °F, -138 °C) at a pressure of 1 bar, and it continues to increase at higher pressures.

Saturn's visible layer of clouds is formed from molecules of minor compounds that condense in the hydrogen-rich atmosphere. Although particles formed from photochemical reactions are seen suspended high in the atmosphere at levels corresponding to pressures of 20-70 millibars, the main clouds commence at a level where the pressure exceeds 400 millibars, with the highest cloud deck thought to be formed of solid ammonia crystals. The base of the ammonia cloud deck is predicted to occur at a depth corresponding to about 1.7 bars, where the ammonia crystals dissolve into the hydrogen gas and disappear abruptly. Nearly all information about deeper cloud layers has been obtained indirectly by constructing chemical models of the behaviour of compounds expected to be present in a gas of near solar composition following the temperature-pressure profile of Saturn's atmosphere. The bases of successively deeper cloud layers occur at 4.7 bars (ammonium hydrosulfide crystals) and at 10.9 bars (water-ice crystals with aqueous ammonia droplets). Although all of the clouds mentioned above would be colourless in the pure state, the actual clouds of Saturn display various shades of yellow, brown, and red. These colours are apparently produced by chemical impurities; phosphorus-containing molecules are a prime candidate.

Even at the extremely high pressures found deeper in Saturn's atmosphere, the minimum atmospheric temperature of 82 K is too high for molecular hydrogen to exist as a gas and a liquid together in equilibrium. Thus, there is no distinct boundary between the higher atmosphere, where the hydrogen behaves predominantly as a gas, and the deeper atmosphere, where it resembles a liquid. Unlike the tropopause on Earth, Saturn's troposphere does not terminate at a solid surface but apparently extends tens of thousands of kilometres below the visible clouds, reaching temperatures of thousands of kelvins and pressures in excess of one million bars.

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