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Saturn's third largest moon Iapetus has a fascinating appearance. Not only does it have a strange walnut-like shape but its surface is dramatically two-toned. One face of the moon, Cassini Regio, is dark and the other, Roncevaux Terra, is white as new snow. The difference in albedo (reflectivity) of the surface is striking. When Giovanni Cassini first observed it in 1671 he noted that it disappeared from view for half of its orbit and correctly guessed its two-toned nature.

A proposed explanation of the process creating the difference in surface colouration is that a positive albedo feedback promotes sublimation of ice from the dark face of the moon and subsequent enhanced deposition on the light face. Since Iapetus is tidally locked in its 79 day orbit of Saturn the dark, leading, face has (and may continue to do so) swept debris up preferentially from its orbital path further darkening the Cassini Regio and strengthening the feedback.

Some trivia about Iapetus

  • Radius = 736 km
  • Mean density = 1083 kgm-3
  • Albedo of Cassini Regio = 0.03--0.05
  • Albedo of Roncevaux Terra = 0.5--0.6
  • Daytime equitorial temperatures
    • Cassini Regio = 128 K
    • Roncevaux Terra = 113 K



UVic / SEOS / Climate Group / About Front Page Picture / Iapetus and Albedo Last updated: Wednesday, 07-Apr-2010 15:19:34 PDT