Saturn's two largest moons, captured in tandemS

Saturn has 62 known moons. Pictured here are the biggest of the lot, arranged as two crescents, one sitting atop the other.

The one you've probably heard of before is Titan [click here for hi-res]. Within our solar system, the massive orb is second only to Jupiter's Ganymede in size. It's also the only moon with a dense, fully developed atmosphere, the haze of which is clearly visible here, even from a photographed distance of 1.2-million miles.

Much less substantial is the "atmosphere" of Rhea, shown here looming 400,000 miles closer than its sibling. But Rhea will surprise you. In 2010, measurements made by NASA's Cassini orbiter (the same spacecraft that took this photograph), revealed what researchers described as a tenuous, oxygen/carbon dioxide atmosphere. Rhea is made up mostly of water ice; when this ice is irradiated by charged particles from Saturn, it decomposes into hydrogen and oxygen. But don't plan on popping the hatch on your spacecraft next time you find yourself marooned there — Rhea's atmosphere may be 70% O2, but it's still trillions of times less abundant than what you'll find here on Earth.

Like the researchers said: when it comes to wimpy atmospheres like Rhea's, the key word is tenuous.

[Via CICLOPS]