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The bright and dark side of Iapetus

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The bright and dark side of Iapetus
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Iapetus is one of Saturn’s most mysterious moons: this icy moon with a diameter of 1468 kilometres presents the observer with two hemispheres that have developed in completely different ways.

The hemisphere facing the opposite direction of the moon’s orbit around Saturn, the so-called “trailing side”, as well as the polar areas, consist of white ice and reflect the light of the Sun almost 100 percent; on the surface of the hemisphere facing the direction of orbit, the so-called “leading side”, however, carbon compounds cover the ice – cyanide and other carbon compounds make the surface as black as tar.

This false-colour image shows the first mosaic of high-resolution image data from bright side of Iapetus; it consists of 60 separate images, which were recorded in September 2007 from a distance of 73 000 kilometres.

Ever since the Voyager mission of 1981, it has been suspected that dark material from the neighbouring moon Phoebe has coloured Iapetus’ leading side black. The ongoing analysis of image data obtained during the close flyby of Iapetus in September of this year, during which the separate images of this mosaic were recorded as well, will show if this interpretation is correct. This flyby had been planned in great detail by staff members of the Freie Universität Berlin and DLR.

The transition zone between both extremes is especially interesting. In the lower half of the image, you can see an impact crater which is 450 kilometres in size.

Credit: NASA/JPL/Space Science Institute.
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