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Astronomical Mystery near Saturn Solved
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The light and dark faces of Iapetus
A mystery involving Saturn's moon Iapetus that has confounded astronomers for over four centuries has apparently been solved.
Giovanni Domenico Cassini (Paris Observatory, France) discovered Iapetus on October 25, 1671. During the next few years, he quickly found that the moon was only visible when on the west side of the planet, but never on the east side. It wasn't until 1705 that Cassini finally managed to make the first observations of the moon when on the east side of the planet while using a new, more powerful telescope.
Cassini's observations revealed that Iapetus was 10 times fainter when on the east side of the planet than when on the west side. It was also concluded that Iapetus kept the same face toward Saturn at all times, just as our moon keeps the same face toward Earth. Cassini surmised that the moon was bright on its trailing side and dark on the leading side, a conclusion reached by many astronomers through the centuries that followed.
There was a lack of theories explaining Iapetus' condition until the latter half of the 20th century. During 1970, two astronomers suggested Iapetus was originally covered in ice, but the leading hemisphere was eroded through the millennia due to impacts from meteoroids being swept up by the moon as it circled Saturn. In 1974, another astronomer suggested just the opposite…that Iapetus was rocky but somehow managed to collect an icy material in the trailing hemisphere. He noted that there were a lot more icy bodies than rocky bodies in Saturn's environment to perhaps impact Iapetus and provide the ice.
Studies of Iapetus took a new direction in 1974, when S. Soter of Cornell University suggested that another one of Saturn's moons, named Phoebe, might be responsible for the color scheme of Iapetus. Phoebe orbits in the opposite direction to Iapetus and does not lie that far outside of Iapetus' orbit. It was suggested that when objects impact Phoebe, the ejected material initially stayed in Phoebe's orbit, but, in the course of 1-3 million years, it spiraled inward toward Saturn and some of it was swept up by Iapetus.
Hints toward which theory might be correct have come from the Cassini space probe, which has been orbiting Saturn since July 2004. Flybys of several moons have revealed that the dark area on the leading hemisphere of Iapetus has the same near-infrared spectral signature as Phoebe. But the predicted ring had not been found.
Astronomers Anne J. Verbiscer, Michael F. Skrutskie, and Douglas P. Hamilton aimed the Spitzer Space Telescope toward the orbit of Phoebe during February of 2009. The Spitzer is a different type of telescope than the more famous Hubble Space Telescope. While Hubble captures the visible light from objects in space, Spitzer captures the heat emanating from an object and can frequently detect objects that Hubble cannot see.
After obtaining several scans across Phoebe's orbit, an emission feature was detected which was consistent with the long-held idea of a ring. The dust glowed in the Spitzer images, even though the temperature of the dust was a cool -316 degrees F.
The new ring proves to be the largest planetary ring in the solar system. Saturn's already known rings have an inner edge 67 thousand kilometers from the planet and have an outer edge 483 thousand kilometers away. The inner edge of the Phoebe ring is located nearly 8 million kilometers from Saturn, while the outer edge lies over 12 million kilometers away. Interestingly, the dust is so sparse that probes could probably move through it without detecting anything that unusual!
