What is Iapetus?

Iapetus is the seventeenth moon of Saturn, approximately 2,200,000 miles from its primary.

It is about 800 miles across, about one-third that of our moon, has a density of about 1.2 grammes per cubic centimetre (just slightly higher than water), has no atmosphere and no gravity to speak of.

The Iapetusian "day" is just under 80 earth days and, like some other moons in the Solar System, including our own, the same side always faces the planet.

Iapetus is unusual in that it has two distinct "halves", one being smooth and reddish, the other rocky and lighter in colour. Why this should be is unknown, and will probably remain so until we can land there to look around.

It was discovered by Giovanni Cassini in 1671.

In Greek mythology, Iapetus, one of the Titans, was the son of Uranus (god of the sky) and Gaia (goddess of the earth).

He was the father, with his wife Clymene (also called Asia), of Atlas, Prometheus, Epimetheus, and Menoetius, the first three of whom are said to be the ancestors of humans.