What is the name of the dark region on Saturn's moon Iapetus?

The dark region on Saturn’s moon Iapetus is called Cassini Regio. It covers much of Iapetus’s leading hemisphere and forms one half of the moon’s striking two-tone appearance.

The region is named for Giovanni Domenico Cassini, who discovered Iapetus in 1671. Cassini noticed that the moon appeared brighter on one side of Saturn than the other, an early clue that Iapetus was tidally locked and had contrasting hemispheres. The darker surface has a very low albedo, while the opposite side is bright and icy.

Cassini Regio is not simply a painted-on dark patch of uniform thickness. The dark material can be only a few tens of centimetres thick in places, with bright ice exposed where small impacts disturb it. Scientists think external dust, possibly associated with Saturn’s outer moon Phoebe, helped initiate the contrast. Heating of the darker surface then encouraged ice to sublimate and migrate toward colder, brighter regions, amplifying the difference over time. The bright areas are named Roncevaux Terra and Saragossa Terra.