The 914-mile-wide Iapetus turns its darker side towards Cassini’s cameras in this photo, taken on March 3, 2009, while a hint of its brighter face shows along its northern edge.
This moon of Saturn has the distinction of being coated in dark material on one side, and bright white material on the other. Theories as to why this is range from fallen material ejected from another moon, Phoebe, to ice volcanism that sends brighter material to the surface from within the moon, to the more recent idea that heating by the sun melts off brighter ices on one half while making them develop on the other. Whatever the case is, this phenomenon has been in play for a very long time…it was noted by astronomer Giovanni Cassini in 1671 when he observed that Iapetus was only visible when it was on the western side of Saturn. He concluded then that one half of the moon must be darker than the other.
Now, 338 years later, the spacecraft that bears his name is there helping today’s astronomers learn more about why this is. Fantastic.
Image credit: NASA/JPL/Space Science Institute