1671–1672

Iapetus and Rhea

Giovanni Domenico Cassini (1625–1712)

The discovery of Jovian moons in 1610 and a moon around Saturn in 1655 set off a frenzy of moon hunting by late-seventeenth-century astronomers. The solar system’s sixth and seventh new moons, eventually named after the Greek Titan lapetus and his mother, Rhea, were discovered in 1671 and 1672, respectively, by the prolific Italian-French mathematician and astronomer Giovanni Domenico Cassini. Strangely, Cassini could only see Iapetus when it was on the western side of Saturn—it seemed to disappear as it rotated around to the eastern side. In 1705, Cassini finally observed the moon on the other side of Saturn, when it was more than six times dimmer!

Cassini and others had surmised that, like our own Moon, Iapetus is tidally locked, with the same face pointed to its parent planet. Thus, half of Iapetus always faces forward in its direction of motion (the dark leading side), and half always faces backward (the bright trailing side). Rhea was also difficult to spot: it orbits 50 percent closer to Saturn than Titan; its discovery closer in to the glare of Saturn’s disk is a testament to Cassini’s prowess as an observer and the overall improvement in telescope and optics technology in the late seventeenth century.

A ridge 12.4 miles (20 kilometers) high, of unknown origin, circles much of the equator of Iapetus.

The strange two-toned surface of Iapetus was confirmed by closer encounters by the Voyager missions in the 1980s, which measured an ice-like density around 1.1 grams per cubic centimeter. More detailed studies by the Cassini orbiter mission have revealed that Iapetus has an ancient, heavily cratered surface and an equatorial ridge that gives the moon a walnut-like shape. Voyager also revealed Rhea up close, showing it to be bright (with a reflectivity higher than 50 percent), heavily cratered, and with an ice-like composition as well.

Why does Iapetus have such a strange, two-toned surface? The neighboring moon, Phoebe, the source of a faint outer ring of Saturn, may be the culprit: the leading side of Iapetus may have darkened early on while plowing into Phoebe’s ring dust.

A natural-color photo of the boundary between bright and dark material in the north polar region of Iapetus, taken in December 2004 by NASA’s Cassini Saturn orbiter spacecraft. BOTTOM RIGHT: A Cassini photograph of Rhea, silhouetted against the rings of Saturn in March 2010.