The Cassini spacecraft has been orbiting Saturn since 2004, and has been featured frequently both on my blog and in these Top Ten lists. Its views of the ringed planet and moons has been nothing short of spectacular time and again.
This shot reminds me why. The moons of Saturn orbit it almost all on the same plane, so Cassini sometimes sees them near each other in the sky. And if things play out just right, they even pass directly in front of each other.
That's exactly what happened here
: icy Rhea, Saturn's second largest moon, is seen here superposed on the giant moon Titan. Despite catastrophic appearances, they were in no danger of collision: 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.Get the high-res version here.Image credit: NASA/JPL/Space Science Institute