Which planet does the moon Iapetus orbit?
The Spitzer Space Telescope has discovered the biggest but never-before-seen ring around the planet Saturn, NASA’s Jet Propulsion Laboratory announced late Tuesday. The thin array of ice and dust particles lies at the far reaches of the Saturnian system and its orbit is tilted 27 degrees from the planet’s main ring plane, the laboratory said. JPL spokeswoman Whitney Clavin said the ring is very diffuse and doesn’t reflect much visible light but the infrared Spitzer telescope was able to detect it. Although the ring dust is very cold — minus 316 degrees Fahrenheit — it shines with thermal radiation. No one had looked at its location with an infrared instrument until now, Clavin said. The bulk of the ring material starts about 3.7 million miles from the planet and extends outward about another 7.4 million miles. The newly found ring is so huge it would take 1 billion Earths to fill it, JPL said. Before the discovery Saturn was known to have seven main rings named A through E and several
Iapetus orbits much farther away from Saturn than any other major moon, three times farther away than Titan and about ten times farther than all the rest of the medium-sized icy moons. It is also the only one of Saturn’s major moons with an inclined orbit, 15 degrees away from the ring plane. Iapetus is the highest-contrast body in the solar system. Its leading hemisphere is very dark, as dark as asphalt, while its trailing hemisphere is bright, similar to the solar system’s other icy satellites. The cause of the black smudge on one face of Iapetus is unknown. The common explanation from before the Cassini mission was that dust from Phoebe — the darkest object in the Saturnian system except for Iapetus’s smudge — spirals in to Iapetus and is deposited on the surface. However, Cassini’s spectrometetric measurements indicate that the dark stuff on Phoebe is a better match to the composition of Iapetus’ bright side. The dust-from-outside hypothesis has a second problem: if true, then on
