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The image on the right is a cropped and scaled up version of one of Cassini’s last images of Saturn’s moon, Iapetus.
The moon is unique in that its east and west hemispheres have completely opposite albedos, with one being very dark and the other very bright. It also has a very distinctive large crater, seen in this image. Scientists do not quite understand what causes the dichotomy, though they have models that partly explain it, partly from material being deposited on the moon’s leading hemisphere combined with the temperature differences at different latitudes.
The cause of the extreme brightness dichotomy on Iapetus is likely to be thermal segregation of water ice on a global scale. Thermal effects are usually expected to act latitudinally. That is, polar areas are colder than equatorial terrain in most cases due to the more oblique angle of the solar irradiation. Therefore, an additional process is required to explain the longitudinal difference as well. In one model, dark, reddish dust coming in from space and preferentially deposited on the leading side forms a small, but crucial difference between the leading and trailing hemispheres, which is sufficient to allow the thermal effect to evaporate the water ice on the leading side completely, but only marginally on the trailing side.
It was this moon’s strange dichotomy that had Arthur Clarke use it in 2001: A Space Odyssey. While Cassini’s images clearly prove that the brightness difference was not created by an alien civilization, as imagined by Clarke, those images have not really provided us a full explanation for its cause. The uncertainty of science marches on!