The secrets of the universe are almost entirely invisible.

Dione (pictured in front) is a moon of Saturn and the fifteenth-largest moon in the solar system. Made up almost entirely of water ice, Dione is denser than all the moons smaller than it—combined. It’s shown here as it transits Titan, the second-largest moon in the solar system and the only moon known to have a dense atmosphere. It’s also the only other object in the solar system other than Earth that has shown clear evidence for liquid at the surface. Although Titan is composed of rocky material and water ice, the temperatures are too cold there for liquid water to exist. The lakes on Titan are filled with liquid methane.

(via NASA Image of the Day)