When Jean-Dominique Cassini discovered Saturn’s moon lapetus in 1671, he was surprised to find it visible on just one side of its orbit around the planet. The moon’s orbit had to be synchronous, he correctly inferred, with its leading hemisphere far darker than its trailing one. Some clever Earth-based IR radiometry 300 years later confirmed the extreme albedo difference, and images from the Voyager mission in the early 1980s revealed charcoal dark and frosty bright surfaces that interleave, like two halves of a tennis ball. But the origin of the pattern and sharpness of the dark-bright boundaries remained mysterious.
As early as 1974, Asoka Mendis and Ian Axford had proposed a plausible explanation: With its mean density close to that of water, lapetus is a dirty ice ball. Dust from micrometeorites hitting the leading hemisphere, the pair theorized, might darken it enough to trigger the thermal migration of ice: sublimation...
