(left) Cassini mosaic of Iapetus, showing the bright trailing hemisphere with part of the dark area appearing on the right; the large crater Engelier is near the bottom.
Iapetus, is the third-largest moon of Saturn, and eleventh largest moon in the Solar System.
Discovery
Iapetus was discovered by Giovanni Domenico Cassini, an Italian/French astronomer, in 25th October 1671.
He had discovered the moon on the western side of Saturn and tried viewing it on the eastern side some months later, but was unsuccessful. The pattern continued the following year as he was able to observe it on the western side, but not the eastern side.
Cassini finally observed Iapetus on the eastern side in 1705 with the help of an improved telescope, finding it two magnitudes dimmer on that side.
Cassini correctly surmised that Iapetus has a bright hemisphere and a dark hemisphere, and that it is tidally locked, always keeping the same face towards Saturn. This means that the bright hemisphere is visible from Earth when Iapetus is on the western side of Saturn, and that the dark hemisphere is visible when Iapetus is on the eastern side. The dark hemisphere was later named Cassini Regio in his honour.
Naming
Iapetus is named after the Titan Iapetus from Greek mythology.
In Greek mythology, Iapetus, also Iapetos or Japetus, was a Titan, the son of Uranus and Gaia, and father (by an Oceanid named Clymene or Asia) of Atlas, Prometheus, Epimetheus, and Menoetius and through Prometheus, Epimetheus and Atlas an ancestor of the human race. He was the Titan of Mortal Life, while his son, Prometheus, was the creator of mankind.
Stats
Diameter: 1,469 km
Semi-major axis: 3,560,820 km
Orbital Period: 79.32 days
Orbit
The orbit of Iapetus is somewhat unusual. Although it is Saturn's third-largest moon, it orbits much farther from Saturn than the next closest major moon, Titan.
It has also the most inclined orbital plane of the regular satellites; only the irregular outer satellites like Phoebe have more inclined orbits. The cause of this is unknown.
Because of this distant, inclined orbit, Iapetus is the only large moon from which the rings of Saturn would be clearly visible; from the other inner moons, the rings would be edge-on and difficult to see.
Physical characteristics
Unlike most moons, its overall shape is neither spherical nor ellipsoid, but has a bulging waistline and squashed poles. Also, its unique equatorial ridge is so high that it visibly distorts the moon's shape even when viewed from a distance. These features often lead it to be characterized as walnut-shaped.
Equatorial Ridge
A mystery of Iapetus is the equatorial ridge that runs along the center of Cassini Regio, about 1,300 km long, 20 km wide, 13 km high. It was discovered when the Cassini spacecraft imaged Iapetus on December 31, 2004.
Peaks in the ridge rise more than 20 km above the surrounding plains, making them some of the tallest mountains in the Solar System. The ridge forms a complex system including isolated peaks, segments of more than 200 km and sections with three near parallel ridges.
The ridge system is heavily cratered, indicating that it is ancient. The prominent equatorial bulge gives the moon a walnut-like appearance.
Two-tone coloration
The difference in colouring between the two Iapetian hemispheres is striking. The leading hemisphere and sides are dark (albedo 0.03–0.05) with a slight reddish-brown coloring, while most of the trailing hemisphere and poles are bright (albedo 0.5-0.6, almost as bright as Europa).
Thus, the apparent magnitude of the trailing hemisphere is around 10.2, whereas that of the leading hemisphere is around 11.9.
The pattern of coloration is analogous to a spherical yin-yang symbol or the two sections of a tennis ball.
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.
NASA scientists now believe that the dark material is lag (residue) from the sublimation (evaporation) of water ice on the surface of Iapetus, possibly darkened further upon exposure to sunlight.
Because of Iapetus's slow rotation of 79 days (equal to its revolution and the longest in the Saturnian system), the moon would have had the warmest daytime surface temperature and coldest nighttime temperature in the Saturnian system.
Even before the development of the color contrast; near the equator, heat absorption by the dark material results in a daytime temperatures of 129 K in the dark Cassini Regio compared to 113 K in the bright regions.
The difference in temperature means that ice preferentially sublimates from Cassini Regio, and deposits in the bright areas and especially at the even colder poles. Over geologic time scales, this would further darken Cassini Regio and brighten the rest of Iapetus, creating a positive feedback thermal runaway process of ever greater contrast in albedo, ending with all exposed ice being lost from Cassini Regio.
However, a separate process of color segregation would be required to get the thermal feedback started. The initial dark material is thought to have been debris blasted by meteors off small outer moons in retrograde orbits and swept up by the leading hemisphere of Iapetus.
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