Saturn

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

Saturn is the sixth planet from the Sun, named after the ancient Roman god of agriculture, believed to have ruled the earth during a time of happiness and virtue. He dethroned his father Uranus and was in turn dethroned by his son Jupiter. Literally, Saturn is "The Lord of the Rings," the most spectacular to see through a telescope. The rings were first seen by Galileo through his telescope in 1610, but his angular resolution was so low that they appeared as lobes around the planet. Fifty years later, Christian Huygens explained that the "ears" were rings around the planet. Saturn is second in size of all the planets, and the least dense of all, it could float on water!

Saturn has at least 22 moons, with seven large enough to be spherical. The largest of these moons is Titan.

Titan

False color image of Titan. NASA Cassini Spacecraft

The atmosphere is primarily composed of nitrogen (N2): 82-99% and methane (CH4): 1-6%, the amount varies with altitude. Argon may also be present (less than 6%) but it is not detected directly and it may be absent all together. Other trace constituents are observed: hydrogen gas (H2), hydrocarbons such as ethane and propane, and nitrogen and oxygen compounds (carbon monoxide and some carbon dioxide). This type of atmosphere resembles Earth's primordial atmosphere, before the evolution of life. Amino acids can form in it, and it is thrilling to speculate about the possible formation of the firsts steps of life elsewhere, beyond our own world. In addition, the chemical and photochemical reactions taking place in Titan's atmosphere, along with the low temperatures, could allow the presence of large lakes or seas of liquid ethane on the surface. Liquid and vapor water cannot exist on Titan because of the cold average temperature of 94 K (–179°C), but large amounts of water ice are present in the crust and mantle.

The atmosphere of Titan probably formed the same way as the atmospheres of the terrestrial planets, i.e. by outgassing from the solid material in Titan. Some gases were also accumulated from comets colliding with it. Why then, do Callisto and Ganymede, the largest moons of Jupiter lack atmospheres? Their sizes are similar to Titan's, and their surfaces are also made up of dirty ice. The answer is that water ice traps many gases at low temperatures, but not at higher temperatures. Thus, the warmer temperatures of Ganymede and Callisto prevented the trapping of gases by the water ice, so no outgassing could occur to form an atmosphere.

Saturn's rings lie on the plane of the equator. Because of the tilt of the equator to the orbital plane, and to a lesser amount because of the inclination of Saturn's orbit to the ecliptic, the rings change orientation as seen from Earth. In 1996 the ring system was edge-on and we couldn't see much of it. At other times the rings reach their maximum inclination and display their splendor to us.

Like Jupiter, Saturn rotates differentially, but the rotation periods are much harder to measure because the patterns on Saturn's atmosphere are not as obvious as the zones and belts on Jupiter. The rotation speeds were measured with cameras aboard the Voyager spacecraft, which detected circulation patterns on the atmosphere. As with Jupiter, the rotation period of the magnetic field is taken to be the rotation of the planet's interior.

The rapid rotation of Saturn, and its low density, make it oblate, or "out of round," more than any other planet in the solar system. Oblateness = 0 means the object is round (like Venus), and oblateness = 1 means it is a straight line.

Astronomers can infer the chemical make up of a planet by knowing its density and comparing it to the known densities of compounds. For example, the density of water is 1 g/cm3, and the density of rock is about 3 g/cm3. Since Saturn's density is less than water, it is made up mostly of gases, and liquids less dense than water.

The table below gives a heat ratio of about two, i.e. Saturn emits more than twice the energy that it absorbs from the Sun. This observation, together with the fact that Saturn's atmosphere seems to be depleted of helium with respect to the Sun's and Jupiter's atmospheres, suggests that helium droplets form in Saturn's atmosphere and fall to the interior. As they fall, gravitational energy is converted into heat that is radiated into space. From observational data, astronomers can calculate the rate of helium precipitation needed to provide the excess energy observed, and compare it to the amount of the helium missing in the atmosphere. The two amounts are consistent with each other, thus the explanation of helium droplets falling to the interior is probably correct.

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