Saturn, the ringed planet
Saturn, the large gas giant beyond Jupiter and sixth planet from the Sun, is one of the five (Mercury, Venus, Mars and Jupiter) planets visible from Earth using only the naked-eye. Galileo, with his rather primitive refracting telescope, was the first to observe Saturn's complex ring system. Since then Saturn has been seen as a symbol of the majesty, mystery, and order of the physical universe. Although many other planets do indeed have ring systems (Jupiter, Uranus and Neptune) they are not nearly as extensive nor brilliant. While the origin of these great rings is at present unknown, scientists hope to learn more through studying the planet's history with the aid of space probes such as Cassini.
Saturn, being a giant, gaseous planet, has an interesting atmosphere. Alternate jet streams of east-west and west-east circulation can be traced in the motions of the cloud tops, the speeds of these jet streams reach speeds of hundreds of metres per second and are the cause of the banded appearance of the clouds. Hydrogen and helium make up the bulk of the atmosphere, but it also includes trace amounts of other elements. Electrical processes and heat from internal planetary sources enrich the chemical mixture of the atmosphere, which most likely transitions from superheated water near the core to the ammonia ice clouds that are observed at the cloud top. Not to be outdone by its larger neighbour, Jupiter, Saturn's atmosphere also features large storm structures similar to Jupiter's famous Great Red Spot.
Saturn's ice cloud have variety of features. Time-lapse images of cloud features not only provide information on how these massive storms evolve with time, but provide a way to measure atmospheric wind speeds. On our Solar System's gas giants (Jupiter, Saturn, Uranus and Neptune), winds blow mainly along lines of constant latitude. The winds of Saturn blow eastward (the same direction as Saturn rotates), near the equator at speeds of around 1100 m/sec.
Galileo was the first to observe the rings of Saturn (in 1610), but it wasn't until Christian Huygens (a Dutch astronomer) used an improved telescope in 1659 to observe that the rings actually are separate from the planet. The French-Italian astronomer Jean Dominique Cassini first observed in 1676 what appeared to be a division between the rings now known as the Cassini division. Much has been revealed about Saturn over the centuries due to improvements in telescopes: the banded atmosphere, the storm 'spots', and a very apparent 'flattening' at the poles, these three features were also common to Jupiter.
Over the past two decades, a series of spacecraft (Voyager, Pioneer etc.) flew by Saturn, giving scientists their first close-up looks of the planet and revealing a magnetic field 1,000 times stronger than Earth's. New rings and moons were also discovered. Some moons were found to be covered with very smooth ice. Also, visible and infrared observations of Saturn showed us a surprising mix of thermal patterns among the cloud bands, suggesting internal processes yet to be understood. The Voyager spacecraft discovered hundreds of ringlets within Saturn's major rings. Some ringlets were found to be 'braided', some had small moons flanking them (called 'shepherding' moons), and all gave the impression of great dynamism. Shadowy 'spokes' were seen to develop and dissipate in the rings. Ring particles were found to be composed mostly of ice crystals and to range in size from a few centimetres to a few metres.
Today it is known that Saturn has 7 major ring divisions and 18 major moons (the figure for the number of moons is always rising as more and more get discovered, however the newer moons are usually very small and of minor significance). It is thought that Saturn's rings may be the remnants of moons destroyed by tidal interaction with its gravity. They may include remnants of comets that passed too close to it and were likewise destroyed. Of more interest however is Saturn's largest moon, Titan. Titan is a particularly large moon (being around the same size as Mercury) and is shrouded by a thick nitrogen atmosphere that might be similar to what Earth's was like billions of years ago. Further study of this moon promises to reveal much about planetary formation, and perhaps about the primordial Earth as well.
Cassini, a joint U.S-European orbiter/probe mission to Saturn and Titan, was launched in October 1997, and completed its first four-year mission in 2008. Cassini's 4-year scientific mission had two purposes: to complete a multispectral, orbital surveillance of Saturn and to investigate Titan. Cassini discovered much concerning Titan's atmosphere and the Huygens probe revealed methane lakes and an internal water-ammonia ocean on the satellite.
Author: Astronomy Today Staff