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Saturn

Moons - Orbital and Rotational Detail - Physical Data - Atmosphere - Interior - Magnetic Field - The Ring System - Origin of the Rings

 

Atmosphere

The main components of Saturn's atmosphere are, by volume: hydrogen (96.7%), helium (3%), methane (0.2%), ammonia (0.02%), and water vapor (unknown percentage, estimated at 0.4%). Traces of ethane, phosphine, acetylene and other compounds, in less amounts, are also observed. The section Physical Data explains why Saturn's atmosphere is depleted of helium compared to the Sun and to Jupiter.

As Jupiter, Saturn's atmosphere contains layers of clouds that form at different altitudes. The lowest cloud layer is made up of water ice and extends some 10 km in height at temperatures of 250 K and pressures greater than 10 bars. The next cloud layer, about 50 km higher, is made up of ammonium hydrosulfide ice (NH4HS), at a temperature of about 180 K and a pressure of 5 bars. About 80 km above it, ammonia ice clouds form, where the temperature and pressure are, respectively, about 120 K and 1-0.5 bars. Above these are the visible cloud tops, then a haze layer and finally a pure hydrogen and helium atmosphere, extending from about 200 to 270 km over the water clouds. Auroras form at heights greater than 260 km above the water clouds, in the mesosphere.

As seen through a telescope, Saturn doesn't look as colorful as Jupiter. The methane haze layer above the cloud tops blurs the colors, and the colder temperature prevents chemical reactions from occurring; these reactions contribute to Jupiter's vivid colors.

The circulation of the wind patterns in Saturn were detected by Voyager instruments. An equatorial jet stream extends to latitudes +40º and –40º and reaches speeds of 1300 km/hr. A high-latitude current is observed north of latitudes +40º.

Interior

Saturn's interior is composed of the same materials as Jupiter, but in different proportions. A rocky core with a temperature of 15,000 K extends up to about 7,500 km; it is surrounded by an icy core that goes up to 16,000 km from the center, where the temperature has dropped to 10,000 K. As with Jupiter, the rock and ice core is composed of a mixture of silicon, oxygen, metals and heavy volatile elements, and is about 10-15 times as massive as Earth, or 13% of Saturn's mass. Then a layer of metallic hydrogen extends up to 30,000 km, it is here where the magnetic field originates. Above the metallic hydrogen is a layer of liquid hydrogen that extends up to 60,000 km, where the temperature is only 95 K.

Magnetic Field

Saturn's magnetic field is aligned with its rotation axis, which is surprising because the theories for the generation of magnetic fields in planets require that the field is inclined to the planet's rotation axis. Saturn's field strength at the equator is 0.2 gauss, compared to 0.3 gauss on Earth.

Radio noise from Saturn was first detected by Voyager in 1980, it wasn't detected before because it falls in the AM region of the spectrum, which is reflected by the Earth's ionosphere. The radio emissions from Saturn arise when charged particles (electrons, protons and heavy ions) interact with the magnetic field. The charged particles arise from the solar wind, the planet's atmosphere and the surfaces of the satellites. For example, protons and electrons come from the solar wind, nitrogen ions arise from Titan's atmosphere, and oxygen ions come from sputtering from the surfaces of the icy moons and the rings.

When charged particles from the magnetosphere spiral down the magnetic field lines into the atmosphere, they make the atmospheric molecules glow and produce auroras over the polar regions. Auroras should be present in Saturn, but were not photographed by Voyager because light reflected by the rings makes the night side glow some.

Saturn's Rings

Saturn Rings
A snapshot of Saturn's rings as seen by the NASA Cassini spacecraft.
Galileo first saw the rings of Saturn through his telescope in 1610, without knowing what they were. In 1659, Christian Huygens was the first to explain that Saturn was surrounded by a thin flat ring. Sixteen years later, Giovanni Cassini discovered that there are at least two rings. The separation between the two main rings is now called the Cassini Division. In the second half of the nineteenth century it was proven that the rings were not solid, but composed of billions of small particles. It wasn't until 1970 that the particles were known to be composed of water ice, and until 1973 that the sizes of the particles were measured by bouncing radar waves. The sizes range from grains of sand to boulders as big as houses (larger than 10 meters across), and they are loosely distributed. The most abundant particles are 10 cm across.

Most of what we now know of the rings, and the most spectacular photographs, come from the Voyager encounters in 1980 and 1981. The rings are extremely long and narrow. Their width is only about 20 meters, yet the main rings stretch from 7,000 km above the atmosphere of Saturn to more than 70,000 km away! The distance between one end of the rings to the other is about the distance from the Earth to the Moon. The plane of the rings lies on the plane of Saturn's equator, and almost all particles' orbits are circular and follow Kepler's laws of planetary motion. Therefore, the orbital period of the closest particles to Saturn is 5.6 hours, while the outermost particles take 14.2 hours to orbit the planet.

The rings are labeled by letters. The Cassini Division and rings A, B and C are different from each other and can be seen from Earth. The rest of the rings, and other divisions, were discovered by spacecraft. Voyager photographs show tens of thousands of ringlets; each ringlet consisting of different concentrations of ring particles. Most gaps are not empty, but consist of smaller and darker dust particles. Photographs also reveal structures in the rings like waves, kinks, braids, eccentric rings and spokes.

The rings are kept in place by the gravitational forces of some of the satellites. Without them, over hundreds of millions of years the ring system would fade away because some ring particles would fall into Saturn and others would go into orbits further away from the planet. For example, the thin F Ring is kept tightly confined and with an eccentric shape by the two shepherd satellites.

Prometheus and Pandora, one inside and one outside the ring. When the inner moon (Prometheus) catches up with a ring particle close by, it gives the particles energy and causes it to move slightly outward. And when a particle on the outer edge of the ring catches up with the outer moon, it looses energy and moves inward. So the moons force the particles together.

Origin of the Rings

The rings of Saturn were probably formed by a satellite that came too close to Saturn and was ripped apart by tidal forces. From the total mass in the rings, it is estimated that an icy satellite about 250 km in diameter might have been the originator of the rings.

Bibliography

Moons and Planets, W. Hartmann, 1993, 3rd edition, Wadsworth Publishing Co.
Astrophysical Data: Planets and Stars, K. R. Lang, 1991, Springer-Verlag
The Planetary System, D. Morrison and T. Owen, 1996, 2nd edition, Addison-Wesley Publishing Co., Inc

 

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