The Moons of Saturn: Iapetus
by Dani Johnson
April 4, 2013
These two global images of Iapetus show the extreme brightness dichotomy on the surface of this peculiar Saturnian moon.
NASA/JPL/Space Science Institute
Iapetus (eye-APP-eh-tuss) is the third largest of Saturn's 62 moons and one of only about 15 moons that are tidally locked. The first thing that you'll notice about Iapetus is that one side of this icy moon is white, like dirty snow, and the other side is the color of dark chocolate. The dark region is named Cassini Regio, and the bright region is divided into Roncevaux Terra north of the equator, and Saragossa Terra south of it. Upon closer inspection, you'll also notice that this strange Saturnian moon also has a remarkable mountainous ridge running almost exactly along most of its equator. It is hard to imagine that the tallest parts of the ridge reach heights more than twice as tall as Earth's Mount Everest! Part of the ridge is named Tortelosa Montes and part of it is called Carcassone Montes. Iapetus is made mostly of ice and its density that is only 1.2 times more than liquid water.
Brightness and Color Dichotomy
Despite having Voyager and Cassini perform close flybys to Iapetus, we still don't know exactly what is causing its distinct brightness and color dichotomy. Its leading hemisphere has a reflectivity (or albedo) as dark as coal (albedo 0.03-0.05 with a slight reddish tinge) and its trailing hemisphere is much brighter at 0.5-0.6. (About Saturn & Its Moons, NASA/JPL) One theory is that debris from impacts on the darker moon, Phoebe, creates dust clouds that Iapetus plummets through during its orbit around Saturn. Another theory is that there have been cryovolcanic activities that erupted material that is darkened by solar radiation. Whatever the cause, we know that the dark material sits on top of the white ice and that it tends to pool in the bottom of craters.
The remarkable feature that gives Iapetus its walnut-like appearance also has a mysterious origin. Some scientists think the ridge may have been caused by an early ring system formed by a former sub-moon that may have once orbited Iapetus. Other scientists believe ancient cryovolcanoes or tectonic forces might have caused the thin mountainous ridge. Still others think it might be a remnant of the more oblate shape of the young Iapetus, when it was rotating more rapidly than it does today.
Name and Discovery
Iapetus was discovered in 1671 by Giovanni Cassini the French/Italian astronomer. He noticed that Iapetus (which he referred to as one of the Sidera Lodoicea) could only be seen while on the western part of its revolution around Saturn. Cassini was able to see the satellite to the right side of the planet for about 39 days but then it would disappear for about 39 days only to reappear on the western side again for another 39 days again and again. He correctly surmised that one side of the satellite must be really dark and the other side significantly lighter and that it is also tidally locked to its planet. As the technology for telescopes advanced Cassini was finally able to see Iapetus travelling on the western side of Saturn but he still only saw a small spec of light. It wasn't until the 1980 and 1981 Voyager I and Voyager II missions that we finally got a close-up look at the Saturnian moon. The English astronomer John Herschel suggested that since Saturn was already named after a mythical titan that the moons should be named after his brothers and sisters, but as many new moons were discovered scientists began selecting names from more mythologies, including Gallic, Inuit and Norse stories. In mythology, Iapetus is the son of Uranus and Gaia and he fathered Atlas and Prometheus. Some astronomers called Iapetus by its number in the order of the moons that were discovered at the time. Iapetus was referred to as Saturn VIII. Geological features of the satellite get their names from the French epic poem The Song of Roland. Nowadays the International Astronomical Union controls the naming of heavenly bodies.
Iapetus by Saturn Shine