This may be my favorite picture of Saturn's moon Enceladus yet:
Quick! It's trying to get away! My first thought (well, my real first thought was that this is Enceladus on full afterburners) was that this was a composite: two images combined. The moon is icy and very bright, while the plumes of water shooting away from cracks in its surface are faint. But actually, there are two separate light sources here! The face of the Moon we see here is being lit by a full Saturn (think of Saturn as being behind you as you look at the picture). But the Sun is behind Enceladus (not directly behind, but mostly behind and off to the side of the picture), and is illuminating the geysers. That's why the fainter plumes are visible; we're looking at the dark side of the moon! If Saturn weren't lighting it up, the moon would look much darker, and the plumes would be even more visible. I love how the plumes look like a rocket exhaust. With the moon itself just a little offset in the picture, a little above the center, our brains make it look like it's jetting up, up, and away! But don't worry about it actually rocketing into the wild black yonder. I've seen various estimates for the amount of water erupting away in the plumes, but a reasonable amount is about 250 kg/sec, or a ton of water every 4 seconds. That may sound like a lot, but a ton of water is only a cubic meter*! Enceladus has a mass of about 10^20 kg, or 400 quadrillion times the mass it loses from the geysers per second! So even though this picture looks dramatic, the amount of mass Enceladus is losing is actually, and almost literally, a drop in the bucket... but still, enough to resupply the ice particles in Saturn's diffuse E-ring. Amazing.
^* And while some of the material does escape into space, acting like an extremely weak rocket, a lot of it falls back onto the surface of Enceladus. Those particles can't move the moon, any more than pulling up on your shoes can make you fly.
Credit: NASA/JPL/Space Science Institute. Tip o' the cosmic umbrella to Carolyn Porco.
Related posts: - Enceladus is erupting! - Cassini dances with Enceladus once again - Enceladus! - Midnight on a ringed world
