Iapetus ( /aɪˈæpɨtəs/;Greek: Ιαπετός), occasionally Japetus ( /ˈdʒæpɨtəs/), is the third-largest natural satellite of Saturn, and eleventh-largest in the Solar System. Compared to Earth's moon, its radius is about 42% and its density is about 32.5% resulting in a mass of about 2.5% of Earth's moon. It was discovered by Giovanni Domenico Cassini in 1671. Iapetus is best known for its dramatic 'two-tone' coloration, but discoveries by the Cassini mission in 2007 have revealed several other unusual physical characteristics, such as an equatorial ridge that runs about halfway around the moon.
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... Iapetus has been imaged multiple times from moderate distances by the Cassini orbiter ... However, its great distance from Saturn makes close observation difficult ...