I often wonder what it would be like to stand on the surfaces of the celestial bodies that I study. I've imagined skiing on Enceladus, or sitting on the edge of a fissure in the midst of that moon's tiger stripes, with its plumes gushing and sparkling in the sunlight, much as I waited on a bench near Yellowstone's Old Faithful to see its glorious, predictable eruption. One moon's surface I have trouble visualizing is that of Iapetus, a large moon of Saturn. With a diameter of just under 980 miles, it is the third largest moon in Saturn's family, after Titan and Rhea. One side of Iapetus is covered with fairly fresh ice – similar to what you might see on the streets of New York City a day or two after a snowfall. The other side is pitch black – as black as tar or coal. Planetary scientists have long debated how the face of this moon acquired its strange countenance.

Iapetus was discovered by our old friend Giovanni Cassini, in October of 1671. He had just become director of the Observatoire de Paris, at the invitation of King Louis XIV (the “Sun King”), where he stayed until his death in 1712. Cassini noticed that he could see the moon only when it was on the east side of its orbit around Saturn. He stated that Iapetus has “a period of apparent Augmentation and Diminution, by which period it becomes visible in its greatest Occidental digression, and invisible in its greatest Oriental digression. It begins to appear two or three days before its conjunction in the inferior part and to disappear two or three days after its conjunction in its superior part.” Cassini correctly deduced that Iapetus keeps the same face toward Saturn, in the same way that our own Moon is locked toward the Earth, and that one side would have to be very bright and the other very dark for Iapetus to be visible on only one side of its orbit (see Figure 8.1). Cassini did not observe the dark side of Iapetus until 34 years later, with one of the ever-larger refracting telescopes he kept building at the Observatoire. Through a telescope, Iapetus appears over six times brighter when it is on the east side of Saturn.