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SYFY WIRE Bad Astronomy

Saturn’s Glorious Dark Side

By Phil Plait

I donât say this very often, but drop whatever it is youâre doing (unless youâre holding another human or a priceless crystal vase) and click this image to embiggen it. Because holy wow.

That is Saturn, as seen by the Cassini spacecraft on Oct. 17, 2012. Itâs a mosaic of 60 images (taken in violet, red, and infrared light seen here in false color), methodically stitched together to produce this jaw-dropping view, and you absolutely positively must grab the bigger, higher-resolution version. It's stunning.

Thereâs a lot going on here, so let me explain. In fact, let me number these items for you to make it easier!

1) Cassini was almost directly behind Saturn when these pictures were taken; that is, Saturn was directly between the spacecraft and the Sun. Cassini was deep in Saturnâs shadow, and the visible half of the planet itself is almost entirely dark. In other words: Youâre seeing the night side of Saturn.

2) The rings are in full sunlight, and we see them from âbelowâ, looking up. The rings at the bottom of the picture are farther away; you can see the disk of Saturn blocking them.

3) The rings near the top are closer to us, coming around into Saturnâs dark side.

4) In fact, the shadow of the planet itself cuts across the rings!

5) The glow on the planetâs dark side (seen as green here) is sunlight reflected from the rings onto the planetâs atmosphere. If you were floating there, above Saturnâs clouds, youâd see the rings off to the side brilliantly illuminated by the Sun; that light is whatâs illuminating Saturn. Ringlight! Itâs like our own bright Moon lighting up the dark part of the Earth at night.

6) The dark bands going across the planet are the rings themselves, seen in silhouette. This is the part I had to wrap my brain around, and draw myself some diagrams. The cloudtops of Saturn are lit by the parts of the ring in sunlight (#5), but the arc of rings in Saturnâs shadow blocks our view of the gently illuminated cloud tops.

7) The bright arc of teal light (though remember, this is false color) going around the planet is sunlight scattered by Saturnâs clouds. Saturn isnât solid; itâs a gas giant, and sunlight can get through the thinnest, highest-altitude part of the cloud layer. It gets bent a bit toward Cassini, so we see it. This is the same as a spoon looking bent when it sits in a glass of water; light gets bent, or refracted, when passing from one medium to another, like air to water, or the vacuum of space to an atmosphere.

8) The outermost ring of Saturnâthe E ringâis faint and diffuse, but we can see it here as a fuzzy glow. Itâs normally difficult to spot, but with the glare of Saturn so diminished in this picture, itâs far easier to see.

9) Two moons are visible on Saturnâs left side, too (I put in lines pointing at them): Tethys, lower and to the left, and Enceladus, above and the right.

All in all, a helluva view, ainât it?

Iâll note that in September 2006, Cassini sent back a similar view. There, you could see the tiny dot of Earth. In this newer picture, the spacecraft was closer to Saturn than it was for the earlier shot, and the planet itself eats up more of the sky, blocking both the Sun and the inner planetsâincluding us.

These images are incredibly gorgeous, but also serve a scientific purpose. Seeing the planetâs atmosphere and rings this way makes it easier to see faint details. The way the light scatters in different colors can also give hints about the atmosphereâs physical characteristics, as well as the composition and size of the countless icy particles making up the rings.

And, it just goes to show you: Science is beautiful.

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