Among the Jovian planets, Saturn stands out not only for its enormous size and striking atmosphere but also for its spectacular planetary rings—a feature that sets the giant planets apart from the terrestrial worlds. Although Saturn’s rings are by far the most prominent, faint ring systems have now been observed around all four Jovian planets. Of these, only the rings of Uranus were first definitively discovered from ground-based observations.
In 1977, astronomers attempting to measure the chemical composition of Uranus’s atmosphere during a stellar occultation (when a star passes behind a planet) noticed that the star briefly disappeared several times both before and after passing behind the planet. The pattern revealed the presence of at least five narrow rings. A year later, four more rings were detected by similar occultation events. These discoveries marked the first time planetary rings were found beyond Saturn.
Jupiter’s ring system was discovered next, in 1979, when the Voyager 1 spacecraft flew past the planet and detected a faint halo of dust. In 1986, Voyager 2 captured the first direct images of Uranus’s rings, confirming and expanding the earlier occultation results to a total of eleven. Finally, during its 1989 flyby of Neptune, Voyager 2 made the definitive discovery of that planet’s system of five distinct, thin rings.
These observations confirmed that all four Jovian planets possess ring structures. Yet only Saturn’s are bright and extensive enough to be seen from Earth through small telescopes. Saturn’s main rings extend roughly from 7,000 km to 80,000 km above its cloud tops and are composed almost entirely of water ice. Individual particles range in size from about 1 cm to 10 m across, while the rings’ vertical thickness is typically less than 1 km—thinner, in scale, than a sheet of paper compared to its width. The rings lie within Saturn’s Roche limit, the region where tidal forces from the planet prevent material from coalescing into a moon.
Beyond the main rings are several more diffuse ring systems associated with Saturn’s outer moons. The large, faint E ring (see Figure 8-3) is maintained by active ice volcanoes on the moon Enceladus, which eject plumes of water vapor and ice from fissures near its south pole. Many of Saturn’s more tenuous rings were discovered during the Voyager 1 and 2 encounters in 1980–81, but numerous additional ringlets were identified later by the Cassini–Huygens mission, which orbited Saturn from 2004 to 2017. The outermost Phoebe ring—so vast that its diameter would appear twice that of the Moon as seen from Earth—was discovered in 2009 by NASA’s Spitzer Space Telescope.
The Phoebe ring is thought to originate from sputtering—micrometeorite impacts that eject dark dust from the surface of Saturn’s irregular outer moon Phoebe. The ring’s retrograde motion and 27-degree inclination relative to Saturn’s equator match Phoebe’s orbit, supporting the idea that Phoebe is a captured Kuiper Belt object. Interestingly, the existence of this ring had been predicted decades earlier to explain the two-tone coloration of another Saturnian moon, Iapetus.
Iapetus, Saturn’s third-largest moon, is tidally locked, always showing the same face toward Saturn—just as our Moon does toward Earth. Giovanni Cassini discovered Iapetus in 1671, but for years he could only see it when it lay on one side of Saturn. When it appeared on the opposite side, it was two magnitudes dimmer. Cassini correctly inferred that Iapetus has one bright hemisphere and one dark hemisphere (see Figure 8-4). Modern data confirm that the leading side of Iapetus’s orbit is coated with dark material—likely dust from the Phoebe ring—while the trailing side remains bright. The resulting contrast may have been amplified by a thermal feedback process: darker regions absorb more sunlight, warm up, and lose surface ice, while cooler, brighter regions accumulate it. Over time, this runaway albedo feedback deepened the stark two-tone pattern we see today.
cosmiCave.org 