When Jean-Dominique Cassini discovered Iapetus in 1671, he was surprised to find it visible on just one side of its orbit around Saturn. The moon’s orbit had to be synchronous, he correctly inferred, with its leading hemisphere far darker than its trailing one. More than 300 years later, Voyager 2 revealed that the charcoal dark and frosty bright surfaces interleave, like two halves of a tennis ball. But the pattern and sharpness of the dark–bright boundaries remained mysterious. Using data collected by Cassini–Huygens since 2004, John Spencer (Southwest Research Institute), Tilmann Denk (Free University of Berlin), and colleagues have now confirmed a hypothesis first proposed in 1974. Micrometeorites swept up on the leading hemisphere during the moon’s orbit, they argue, darken it enough to trigger the thermal migration of ice: sublimation from dark, warmer patches centered around the equator and subsequent recondensation at bright, colder areas near the poles and trailing side. Visible-spectrum images such as the ones shown here indicate that the dust coating Iapetus’s leading side is redder than the dirt presumed intrinsic to the moon. Judging from IR data, the dust reaches temperatures up to 129 K during Iapetus’s 79-day rotation. Sublimed ice molecules can travel ballistically hundreds of kilometers before recondensing at cold (113 K) traps. An enormous gossamer ring around Saturn detected last year by the Spitzer Space Telescope is the putative source of reddish dust. (J. R. Spencer, T. Denk, Science, in press, doi:10.1126/science.1177132; T. Denk et al., Science, in press, doi:10.1126/science.1177088.)—R. Mark Wilson
