1659

Saturn Has Rings

Christiaan Huygens (1629–1695), Giovanni Domenico Cassini (1625–1712), James Clerk Maxwell (1831–1879)

Among the wonders that Galileo was the first to glimpse by telescope in 1610 was the planet Saturn. Through his modest astronomical telescope, the planet appeared to be a round disk with two bright blobs on both sides that he referred to as “ears.” The nature of these features, which came and went over the years, remained an unresolved puzzle to Galileo for the rest of his life.

In 1659, the Dutch astronomer Christiaan Huygens trained his more powerful telescope on Saturn and became the first person to recognize the “ears” as a disk, or “thin, flat ring,” surrounding Saturn. In 1675 the Italian-French mathematician and astronomer Giovanni Domenico Cassini discovered a dark gap in the rings (now called the Cassini Division) and suggested that the rings are actually a series of many narrower, separate rings. Astronomers and mathematicians considered the rings to be solid disks until the Scottish physicist James Clerk Maxwell hypothesized that the rings must instead be made of huge numbers of individual particles because solid rings would be ripped apart by gravitational and centripetal forces.

A 1659 drawing of Saturn and its rings from Christiaan Huygens’s Systema Saturnia.

Maxwell’s hypothesis was confirmed by the Voyager 1 and 2 flybys of (and through) Saturn’s rings in the 1980s, which, along with the Cassini Saturn Orbiter, have revealed the rings to be an intricate structure of thousands of separate ringlets composed of countless dust- to house-size “particles” of nearly pure water ice, with impurities of silicate dust and possibly some simple organic molecules. The main rings are 174,000 miles (280,000 kilometers) wide but, astonishingly, are less than about 328 feet (100 meters) thick. The “gaps” in the rings aren’t really gaps but are areas where ring particles have been greatly depleted by gravitational interactions with small moons that orbit within the rings. Planetary scientists debate the origin and age of Saturn’s rings. Are they primordial or “young,” perhaps formed only a few hundred million years ago from the catastrophic disruption of an icy moon?

Hubble Space Telescope composite of photos of Saturn obtained from 1996 (bottom) to 2000 (top), as our view of the tilt of the planet’s rings changed from nearly edge-on to much more open.