Iapetus is the third largest moon of Saturn with a diameter of 913 miles (1,469 kilometres). After many billions of years, we end up with a moon whose two hemispheres look like total opposites. Over the course of many millions of years, the darker side absorbed more sunlight and got even darker, meanwhile the bright side only got brighter. The two hemispheres may have once looked similar, with one side being only slightly darker than the other. This sublimation has resulted in the darker side getting darker and the brighter side getting brighter. This heating of the darker area causes any ice there to sublime out, eventually retreating to the colder side of the moon. The length of its temperature cycles allows the darker region to absorb more sunlight and become noticeably warmer than the brighter hemisphere. With such a slow rotation, Iapetus also experiences long cycles in its surface temperature. Iapetus has a fairly long rotation, with the moon taking approximately 79 days to rotate once. It is very likely that thermal segregation is responsible for the dark material on Iapetus. In 2007, NASA’s Cassini spacecraft discovered a new, more likely explanation. Volcanic eruptions could also supply darker materials by ejecting vast amounts of hydrocarbons onto the surface as well. Another explanation is that there might be ice volcanoes on Iapetus that have distributed darker material. If Iapetus formed with its two contrasting hemispheres, we would expect newly formed craters to be brighter or darker depending on what hemisphere they are on, yet craters on the dark hemisphere are also dark, which suggests that the material is being renewed over time. If this hypothesis is true, then the dark material is constantly being renewed over time, which could explain the lack of bright impact craters. Iapetus may be sweeping up darker particles from another nearby moon called Phoebe. Scientists have proposed numerous explanations. Since Giovanni Cassini first discovered Iapetus, scientists have wondered why the moon has a bright side and a dark side. doi: 10.1016/j.icarus.2010.01.025.The Two Sides Of Iapetus Images of the two hemispheres of Iapetus placed together to illustrate the difference in brightness. "Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission" (PDF). "The orbits of Saturn's small satellites derived from combined historic and Cassini imaging observations". "Satellites of Jupiter and Saturn" (naming the moon). Archived from the original (discovery) on. (Subscription or participating institution membership required.) Because the orbits of Prometheus and Pandora are chaotic, it is suspected that Atlas's may be as well. Ītlas is significantly perturbed by Prometheus and to a lesser degree by Pandora, leading to excursions in longitude of up to 600 km (~0.25°) away from the precessing Keplerian orbit with a rough period of about 3 years. This would mean that for any additional particles impacting the equator, the centrifugal force will nearly overcome Atlas's tiny gravity, and they will probably be lost. In fact, the size of the equatorial ridge is comparable with the expected Roche lobe of the moon. The most likely explanation for this unusual and prominent structure is that ring material swept up by the moon accumulates on the moon, with a strong preference for the equator due to the ring's thinness. High-resolution images taken in June 2005 by Cassini revealed Atlas to have a roughly spherical centre surrounded by a large, smooth equatorial ridge. In 2004 a faint, thin ring, temporarily designated R/2004 S 1, was discovered in the Atlantean orbit. However, now it is known that the outer edge of the ring is instead maintained by a 7:6 orbital resonance with the larger but more distant moons Janus and Epimetheus. It is also designated Saturn XV.Ītlas is the closest satellite to the sharp outer edge of the A ring, and was long thought to be a shepherd satellite for this ring. In 1983 it was officially named after Atlas of Greek mythology, because it "holds the rings on its shoulders" like the Titan Atlas held the sky up above the Earth. Atlas is an inner satellite of Saturn which was discovered by Richard Terrile in 1980 from Voyager photos and was designated S/1980 S 28.
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