Magazine article American Scientist

Cassini and the Rings of Saturn

Magazine article American Scientist

Cassini and the Rings of Saturn

Article excerpt

The rings of Saturn are delicate, enigmatic, beautiful, and useful. NASA's Cassini spacecraft has been studying them (and the rest of the Saturn system) over the course of a spectacularly successful 13-year mission, but its concluding year has been the most spectacular of all-practically a whole new mission. In December 2016, Cassini commenced weekly plunges through the ring plane just off the outer edge of the main rings (activity termed the "Ring-Grazing Orbits"), and transitioned in April 2017 to weekly plunges between the rings and the planet's cloud-tops (the "Grand Finale" orbits). The mission concluded in September 2017 with a final descent into the planet's atmosphere, to preclude possible terrestrial contamination of Saturn's moons.

The science goals of the Ring-Grazing Orbits and the Grand Finale orbits included direct sampling of particles from Saturn's rings and atmosphere, detailed measurements of Saturn's gravity and magnetic field to probe the planet's interior, and unprecedentedly close-range imaging of Saturn and its rings with the spacecraft's main camera and similar instruments. These observations followed up on discoveries made through the prime and extended Cassini missions, continued monitoring seasonal or evolving phenomena, and took advantage of Cassini's unique proximity to Saturn in these closing stages of the mission.

Saturn's planetary ring system is made up of countless chunks of ice, each in its own orbit around the planet Saturn. The chunks range from marble-size to house-size. The ring system is confined to the plane of Saturn's equator and is arguably the flattest structure known to humanity, with an end-to-end dimension equivalent to circling the Earth seven times, but a vertical height about that of a house.

Saturn's rings are useful to scientists because they provide physical and chemical clues regarding the formation and history of the entire Saturn system, they serve as detectors and amplifiers for planetary phenomena around them, and they help us understand more generally how disk systems operate, providing clues about other kinds of disks, such as baby solar systems. A few of the most compelling science questions regarding Saturn's rings are: How do ring particles interact with one another, with moons embedded within them, and with moons farther away? Are ring particles made of ice that is fluffy or dense, pristine or sooty? What are their shapes and sizes? What structures do we see in the dusty parts of the rings, and what can we continue to learn from them?

The structure of Saturn's ring system is dominated by concentric bands and tightly wound spirals. Very few of these bands are empty gaps, so it is better to think of the entire system as a disk, rather than as "countless rings." Much of the structure is not well understood by scientists, but the most common type of understood structure are waves that propagate through the rings at locations where ring particle orbits resonate (or essentially "hum in tune") with the orbit of a moon orbiting beyond the rings. Cassini scientists discovered in recent years that a few of these waves are excited by structures within the planet Saturn.

Moons that orbit within the rings deflect nearby ring partides with their gravity and thus create a disturbance around themselves. …

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