Appearance of Saturn's moon Iapetus, illustration. This sequence starts at upper left, with Iapetus rotating on its axis as it orbits Jupiter (red arrows). Over time, tidal forces slow the rotation of Iapetus. At upper centre, micrometeorites are shown striking Phoebe, another moon of Jupiter. At upper right, the dust released by these impacts is shown striking the leading face of Iapetus. Across bottom, the effect of this dust is shown. At lower left, the dust settles on the icy surface of Iapetus. At lower centre, the heat of sunlight (yellow arrows) is better absorbed by the darker soot-covered areas, leading to evaporation. This exposes darker surface materials under the ice. At lower right, the darker areas absorb more energy from sunlight, leading to further evaporation and a runaway effect. This leads to the dark colouration observed on the leading face of Iapetus.