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The Moons of Saturn: Iapetus

by Dani Johnson

April 4, 2013

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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.

Equatorial Ridge

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.

Image Gallery:

Iapetus by Saturn Shine

The image shows Iapetus' surface illuminated by reflected light from Saturn (not by the Sun) and is the highest resolution view acquired to date of this part of Iapetus' surface.

Compared to the approximately one second exposure times used for imaging Iapetus' sunlit side, this view required a very long exposure time of 82 seconds. Cassini was designed to pivot while moving in order to keep its cameras and other remote sensing instruments pointed 'on target' with great precision. Consequently, despite the large relative speed between Iapetus and the spacecraft during this long exposure -- about 2 kilometers per second or almost 4,500 miles per hour at closest approach -- the image of the moon's surface is un-smeared (although the background stars are smeared).

Credit: NASA/JPL/Space Science Institute

Coated Craters

This image shows terrain in the transition region between the moon's dark leading hemisphere and its bright trailing hemisphere. The view was acquired during Cassini's only close flyby of the two-toned Saturn moon.

Credit: NASA/JPL/Space Science Institute

Inky Stains on a Frozen Moon

Dark material splatters the walls and floors of craters in the surreal, frozen wastelands of Iapetus. This image shows terrain in the transition region between the moon's dark leading hemisphere and its bright trailing hemisphere. The view was acquired during Cassini's only close flyby of the two-toned Saturn moon.

Credit: NASA/JPL/Space Science Institute

Flyby Follow-up

Following Cassini's highly successful flyby of Iapetus in September 2007, the spacecraft repeatedly glanced back at the two-toned moon for some time. As Cassini receded from Iapetus, more and more of the bright trailing hemisphere rotated into view.

Credit: NASA/JPL/Space Science Institute

Flight over Iapetus (VIDEO)

Click on image to view a VIDEO created out of images snapped by Cassini as it zips over Iapetus' surface.

Cassini sails low over the surface of Iapetus on approach to its close encounter with the enigmatic moon on Sept. 10, 2007.

Its flight takes it over the rugged, mountainous ridge along the moon's equator, where ancient, impact battered peaks -- some topping 10 kilometers (6 miles) in height -- are seen rising over the horizon and slipping beneath the spacecraft as it flies.

Frames used in this movie were acquired with the Cassini wide-angle camera on Sept. 10, 2007, as the intrepid robot soared past Iapetus (1,468 kilometers, or 912 miles across), within a few thousand kilometers of the surface. Additional simulated images were inserted between the Cassini images in this movie in order to smooth the appearance of the movement, a scheme called interpolation.

Credit: NASA/JPL/Space Science Institute

The View from Iapetus

While on final approach for its Sept. 2007 close encounter with Saturn's moon Iapetus, Cassini spun around to take in a sweeping view of the Saturn System.

Iapetus (1,468 kilometers, or 912 miles across) is the only major moon of Saturn with a significant inclination to its orbit. From the other major satellites, the rings would appear nearly edge-on, but from Iapetus, the rings usually appear at a tilt, as seen here.

Credit: NASA/JPL/Space Science Institute


by Dani Johnson

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