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Discovered by Giovanni Domenico Cassini Date of Discovery 1671 Distance from Saturn 3,561,300 km Radius 718 km Mass 1.6 × 10^24 g Orbital Eccentricity 0.02828 Orbital Inclination 14.72° Orbital Period 79.3301825 days Rotational Period Synchronous Density (gm/cm3) ???
Similar to Rhea, Iapetus is an icy moon believed to have a rocky core. One of the striking things about it is its appearance. It seems to have a huge birthmark. While part of the surface is bright, like Rhea, a large area of the leading hemisphere is covered with a reddish dust. It is unknown how the dust got there or what its composition is. Theories abound.
Could it have been swept up from space when meteors struck nearby Phoebe or from some other source? Perhaps, but the dust is not similar in color to Phoebe's surface. But the fact that the coloration is only on the leading face supports this theory. On the other hand, the patterns seem to resemble the volcanic flows of our moon and Mars, in that the dust seems to concentrate at the bottoms of impact craters and other low areas. Perhaps methane or ammonia from within the satellite erupted to the surface at some point in its history.
Sources: NASA.gov space.com