Saturn’s third largest moon, Iapetus (eye-AP-eh-tuss), was discovered at the then-new Paris Observatory in 1671 by Italian-French astronomer (and observatory director) Giovanni Domenico (Jean-Dominique) Cassini (1625-1712). Upon further observation, Cassini noted that he could only see Iapetus when it was on the west side of Saturn, never the east. His telescope was not large enough to detect Iapetus on the east side of Saturn because it was much fainter then. He correctly reasoned that, “it seems, that one part of his surface is not so capable of reflecting to us the light of the Sun which maketh it visible, as the other part is.” He also must have realized that Iapetus was locked in synchronous rotation—as is our Moon—with the same side facing Saturn all the time, with its rotation period being equal to its orbital period. Today we know these periods to be 79.3215 days.
The leading hemisphere of Iapetus has a visual albedo of only about 5%, whereas most of the trailing hemisphere is much brighter, having an albedo around 25%. Thus, when Iapetus is on the west side of Saturn, its apparent visual magnitude is around 10.2, but on the east side of Saturn Iapetus is 1.7 magnitudes fainter at 11.9. Without a doubt, Iapetus is one of the most outlandish places in the solar system, and the Cassini Saturn orbiter flybys certainly amplified the strangeness.
Cassini made one close targeted flyby of Iapetus on September 10, 2007, passing within 762 miles of the surface. Here are a few of the best photos of Iapetus from Cassini.
The dark material appears to have been deposited from elsewhere in the Saturnian system, but sublimation of water ice may also play a role. In any event, the dark material is a relatively thin veneer, significantly less than a meter thick in many places.
The warm day on Iapetus sees a surface temperature of -227° F on the dark terrain and an even colder -256° F on the bright terrain. Inhospitable, to say the least!
