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Saturn's moon, Iapetus, which the Cassini spacecraft is currently exploring, has revealed one of its secrets. Why is it bright white one one side and pure black on the other? Read on....

Click the image to see the story.

Dusty material spiraling in from outer moons hits Iapetus head-on, and causes the forward-facing side of Iapetus to look different than the rest of the moon. Once the leading side is even slightly dark, thermal segregation proceeds rapidly. A dark surface absorbs more sunlight and warm up, so the water ice on the surface evaporates. The water vapor then condenses on the nearest cold spot, which could be Iapetus's poles, and possibly bright, icy areas at lower latitudes on the side of the moon facing in the opposite direction of its orbit. So the dark stuff loses its surface ice and gets darker, and the bright stuff accumulates ice and gets brighter, in a runaway process.

Click the image to see the story.

Convinced? I'm sure we'll encounter many more mysteries as we explore the Solar System and discover many more surprises. Cassini is currently flying over Saturn's largest moon, Titan....

Best,
Chris

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