Saturn: The Ringed Planet from Galileo to Cassini-Huygens

by Christopher Seddon

A concise history of Saturn from Galileo's discovery of of the rings, to the robotic probes of the Space Age. This 12,000 word Kindle Short Read describes how the Cassini-Huygens mission has transformed our knowledge of Saturn, its spectacular ring system, and its many moons. These include smog-shrouded Titan with its dense atmosphere; Enceladus, where life could exist an a subsurface ocean; and enigmatic Iapetus, darkened on one side with a mysterious ridge running along its equator.Did you know?*Saturn is the least dense planet in the Solar System. Its density of 0.687 gm cm-3is less than that of water, meaning that if placed in a (very large) ocean, Saturn would float.*Saturn presents a more serene appearance than Jupiter. Its atmosphere has a banded appearance like that of Jupiter, but the bands are far more subdued in colour. There are no features corresponding to Jupiter’s Great Red Spot, but short-lived white spots do periodically become prominent, such as the Great White Spot of 1990, which was previously observed in 1876, 1903, 1933, and 1960. Like the Great Red Spot, the Saturnian white spots are storms. However, unlike Jupiter, Saturn’s storms are linked to a seasonal cycle, arising from its more pronounced axial tilt and greater orbital eccentricity.*Before the Space Age, Saturn was known to have just three the outermost A ring, separated from the brighter B ring by the Cassini division, and the fainter, innermost C ring, or Crepe ring, discovered in 1850. The picture was transformed by the Voyager fly-bys. A D ring was discovered inside the C ring and E, F, and G rings were found exterior to the A ring. In addition, there are five very faint rings or ring arcs associated with small moons. The structure of the main rings is highly complex, made up of numerous small ‘ringlets’ and narrow gaps, to give an appearance that has been likened to a vinyl record. Although the three major rings extend from 74,650 to 136,750 km (47,000 to 85,000 miles) from the centre of Saturn, their thickness is thought to be typically no more than 10 m (33 feet).*Reports of rings exterior to the A ring predate the Space Age. French astronomer Georges Fournier (1881-1954) reported a ‘dusky ring’ in 1907, but the E, F, and G rings are way beyond the resolution of the 280 mm (11 inch) refractor he used and can only be observed from space probes. There were also reports of a D ring, interior to the C ring for decades before its confirmation by Voyager 1. Again, though, the ring reported by Voyager 1 is too faint to have been observed from Earth with the telescopes of that time.*Saturn’s total of 82 known moons is three more than Jupiter’s 79 and the most of any planet in the Solar System.*Titan is the only moon in the Solar System to possess a dense atmosphere. Consisting mainly of nitrogen, the atmosphere is denser than that of Earth. Other than Earth, Titan is the only known Solar System body where stable bodies of liquid have been found, albeit these are liquid hydrocarbons rather than water.*That’s no moon - yes, it is. The landscape on Mimas is dominated by the crater Herschel, which stretches a third of a way around the moon. When viewed from the right angle, Mimas resembles the moon-sized Deathstar space station that featured in the first Star Wars movie. The film was released in 1977, three years before Mimas was first imaged in detail during the Voyager flybys.

100 pages, Kindle Edition

Published June 3, 2021