Space Topics: Cassini-Huygens
The Year in Pictures: 2007
Cassini Flies By the Yin-Yang Moon
Why one side of Iapetus is dark and the other side is bright has been a mystery
since Giovanni Domenico Cassini discovered this Saturnian moon more than 300
years ago. Cassini's namesake spacecraft is now exploring the Saturn system
on a primary mission that will include nearly 80 orbits of Saturn. Cassini
swings by Titan on more than half these orbits and has frequent reasonably
close encounters with most of the other moons. Iapetus orbits Saturn at three
times Titan's distance, so Cassini had only
one chance—on September 10,
2007—to swing out on a specially
planned, highly elliptical distant orbit
and view the yin-yang moon up close.
This global portrait of the bright side
of Iapetus shows the complexity of the boundary between the bright and dark
material. Close inspection of the image reveals that there is no "gray" on
Iapetus; moving from the dark to the bright regions, the dark stain on Iapetus'
leading hemisphere breaks up into smaller and smaller patches, concentrated
on crater floors and equator-facing crater walls.
The image below, captured on November 27, 2006, shows the opposite hemisphere
of the moon.
Iapetus' leading hemisphere and "snowman"
Cassini took this distant photo of Iapetus on November 27, 2006. On the left is the dark terrain of Cassini Regio, on Iapetus' leading hemisphere; toward the right, the dark material breaks up into a complicated boundary with the light terrain of Iapetus' trailing hemisphere. The eastern boundary of Cassini Regio is marked by a feature of three overlapping craters outlined in dark material that look a little like a snowman. The image has been enlarged by a factor of two, and the contrast adjusted to reveal more detail within the dark terrain, including two very large impact basins.
Credit: NASA / JPL / SSI / Emily Lakdawalla
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