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Friday, May 22, 2009

Unusual and awesome view of Saturn

by Phil Plait

Saturn, from Cassini. Click to embiggen

This takes some ’splainin’! But fasten your seat belt; this one is a bit of a wild ride.

First off, in this picture from Cassini we’re looking down on the rings from about a 41° angle. The sunlight is coming from the left, in a direction from below the rings as seen here. The part of the planet itself we see here is actually in shadow! That’s obvious from the top half of Saturn’s disk, which is dark. However, the bottom part of the disk is being softly illuminated by reflected light from the rings (rather like moonlight can illuminate the Earth). In that case, sunlight came from the left, hit the underside of the rings (underside as seen from this angle that is), reflected off, hit the planet, which then reflected that light back to Cassini’s camera.

The top half isn’t completely dark, though. Light from the Sun is passing through the rings, too. It gets scattered and diffused, and some of it hits the dark part of Saturn at the top of the picture. So we can see that as a faint illumination.

There’s more! The rings look like they’ve been sliced clean; that’s the planet itself blocking sunlight from the rings, so think of that as the shadow of Saturn on the rings. But if you look to the right of that cut, you can see still the rings! They look dark and thin, and you can only see them against the planet’s disk. Some of the light reflected off the planet’s southern hemisphere (which itself was reflected from the rings as mentioned above) backlights the rings where they are in shadow, so you can see them silhouetted against Saturn’s disk.

Wow. What a tortuous way to light an image! But it’s very cool that we have Cassini-on-the-spot to send us lovely images like this. We can simply enjoy their beauty, of course, or we can unravel the pieces of the puzzle (to mix a metaphor) to see what’s really going on here. Both are fun, and both are worth spending time doing.