From Bad Astronomy's Top 14 Astronomy Pictures of 2010 at Discover:
[Image credit: NASA/JPL/Space Science Institute]
As Saturn's moons orbit more or less in the same plane, these kinds of juxtapositions, captured here by the Cassini spacecraft, are fairly common. This is Rhea, the second largest moon, with Titan, the largest, in the background:
Titan was over a million kilometers from Rhea when Cassini snapped this shot. Note that Rhea is covered in craters big and small, while Titan's thick atmosphere blocks us from seeing its surface directly. Do you also see the ring of material apparently floating above Titan? That's a haze layer composed of hydrocarbons like methane, ethane, and even benzene. Titan's atmosphere is twice as thick as Earth's!
Note also that Titan is three times larger than Rhea and is in fact comfortably bigger than the planet Mercury; it's truly one of the most aptly-named worlds in the solar system.
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