Saturn's Moon Iapetus: Dichotomy

These two global images of Iapetus show the extreme brightness dichotomy on the surface of this peculiar Saturnian moon. 

The left-hand panel shows the moon's leading hemisphere and the right-hand panel shows the moon's trailing side. Image published Dec. 10, 2009. 

CREDIT: NASA/JPL/Space Science Institute

The light and dark faces of Saturn's moon Iapetus create a contrast that helped to hide the satellite for years, despite its distant orbit.

The moon, which keeps the same face perpetually turned toward Saturn, remained shrouded in shadows, with its bright side only visible from Earth when it traveled on the west side of its primary.

Discovery and naming

Iapetus is one of four moons discovered by Giovanni Cassini in 1672. But the icy moon challenged the Italian astronomer.

He first spotted it on the west side of the planet, but when he attempted to observe it 39 days later on the eastern side, no trace of the satellite could be found.

The pattern repeated itself over the course of the moon's 79-day orbit, leading Cassini to surmise that the moon was tidally locked, with one face darker than the other.

Cassini named the four moons that he discovered (Iapetus, Rhea, Dione, and Tethys) the Sidera Lodoicea, or the Stars of Louis, after King Louis XIV.

For years, the moons were referred to numerically, based on their distance from Saturn. Distant Iapetus started out as Saturn V, but the discovery of Mimas, Enceladus, and Hyperion bumped it down to Saturn VIII.

Nearly 200 years later, John Herschel suggested that the moons around the ringed planet be named for the Titans, the siblings of the Roman god Cronus, who was Saturn to the Greeks.

In Greek mythology, Iapetus is a son of Uranus and Gaia and the father to Atlas and Prometheus.

His relationship to Prometheus, the Titan who gave fire to humans, led early Greeks to regard Iapetus as the father of the human race.

Features on Iapetus take their names from the French epic poem "The Song of Roland."

Composition and orbit

Iapetus has a low density of only 1.083 grams per cubic centimeter, just a little more than liquid water, implying that it composed primarily of water, with rock making up less than a quarter of its composition.

The walnut-shaped moon is not spherical, but has a bulging equator with squashed poles. The squashed shape of the moon resembles a satellite that rotates once every 10 hours, rather than the 79 days it takes Iapetus to make a full revolution.

The angular structure of the moon has led to conspiracy-like suggestions that it is not a natural satellite but could have been built or modified by another civilization.

However, its odd shape could easily have a natural explanation. Early in its life, when the moon still spun rapidly, a thick crust could have frozen the moon into its odd shape.

Over time, the satellite's spin would have slowed until it eventually became tidally locked. Iapetus is the third-largest moon orbiting Saturn, with a diameter of 914 miles (1,471.2 km).

Although its radius is about two-fifths that of Earth's moon, its icy composition means that it is only about 2 percent as massive.