Iapetus is the third largest satellite in the Saturnian system. It is composed mostly of ice with some rocky material and has a heavily cratered surface. The moon even shows an unusual shape--not a typical sphere or ellipse--with bulging along its equator and squashing at its poles, sometimes described as a walnut shape. A prominent equatorial ridge runs along the planet that is large enough to be seen at a distance.
Iapetus is perhaps best known for its unusual two-tone coloration between hemispheres. The leading hemisphere of Iapetus has a dark brownish surface that absorbs nearly all incident light, while the trailing hemisphere has a bright surface that reflects half or more of the light shining on its surface. This contrast was so striking that Iapetus nearly disappears when its dark side is showing. The distribution of this light-dark coloration is sometimes compared to a Yin-Yang symbol or the markings on a tennis ball, and this pattern seems to remain consistent and without much variation.
One factor that contributes to the environment on Iapetus is it’s synchronous rotation with Saturn. Because of strong gravitational influences, one day on Iapetus (i.e. the time required for Iapetus to rotate once around) is equal to one year (i.e. the time required for Iapetus to orbit once around Saturn). This means that one side of the moon always faces Saturn, while the other side always faces outward. This in itself is not enough to explain the two-tone coloration--after all, all parts of the moon will still be illuminated by the sun at some point in orbit. However, the slow and steady orbit of Iapetus around Saturn may allow for more dust, or impactors, to fall onto one side than the other--a possible clue to explaining the Yin-Yang coloration.
Current observations from the Cassini orbiter suggest that the color contrast is indeed related to dust accumulation on the trailing dark side of Iapetus. This dust is probably from remnants of Saturn’s rings, impactors from space, and perhaps eruptions within Iapetus itself. This explains the source of the dust, but why is this color contrast so striking? As the sun shines on slowly rotating Iapetus, the dark regions warm up so that its water ice evaporates and is transported to the colder and brighter side. This loss of water makes the dark side even more absorbing, so that it warms even more and the rate of water loss increases. Likewise, the bright side of the planet increases in reflectivity as water ice accumulates, so that the color contrast between dark and light is reinforced. Although some aspects of the Yin-Yang pattern remain to be explained, these recent hypotheses seem to be more consistent with observations than any ideas in the past.
The persistent Yin-Yang pattern would likely be noticed by future colonists in the Saturnian system, with the bright trailing side an order of magnitude more visible in the sky than the darker leading side. Anyone venturing to Iapetus will certainly need to be prepared for the lack of atmosphere, but perhaps the strong color contrast could be used to help moderate the temperatures of artificial structures. Coupled with the availability of water ice, Iapetus may find a use as an occasional destination for Saturnian colonists.