Saturn's E-Ring, and the Zodiacal Light

In this photo, shamelessly stolen from a friend's Facebook post, Earth is visible in the upper left portion of Saturn's outermost ring, or the E ring. Apparently this ring isn't all that visible except when Saturn itself blocks out the sun during an eclipse. Those don't happen on Earth, since Saturn is WAY out in the outer solar system cruising along it's nearly 30 year orbit.

Also visible in this photo is the faintest hint of something called the Zodiacal Light. Check the Wikipedia page for plenty of awesomeness, but in short, it's the dust cruising around the solar system on it's way from places like the Oort Cloud, Kuiper Belt, and random asteroid, planetary, and cometary impacts, towards the sun itself.

You'll notice that the glow gets rather bright just outside this E-Ring in the photo, then fades away gradually as you get further from Saturn in the image. The reason for this is twofold - first, Saturn plows through the solar system mopping up any debris that gets near it - it's gravitational field is quite efficient at either capturing (ring material), or ejecting (bye bye) comets, asteroids, and even dust particles, since gravity works pretty much the same no matter your mass. This leaves a distinct Saturn-shaped hole in the dust, and with no dust, there's no reflected light to smack the CCD's on Cassini, so the area around Saturn appears dark.

The reason for the gradual fall-off as you get further from Saturn is really quite simple. Go driving in fog sometime. Actually, don't, just look at this:

Light Painting (flashlights in fog)

You'll notice that these totally bangin' photos all show a flashlight in fog - as you get further away from the beam's center line, the angle between the light beam's center line and the particles starts to get quite oblique, until the light's basically being reflected back at the flashlight. Also, since it's a flashlight, there's a reflector which gives you an actual beam, but it works the same way in reverse when we think of our eye, or a camera, looking towards the Sun - all that dust beyond a few dozen degrees away from right smack in front of us just isn't reflecting a ton of light towards us - most of it's going off at a slightly glancing angle and passing us by, just a few degrees away.

So, there's your quick primer on Zodiacal Light, all stimulated by a picture of Earth from just beyond Saturn.

Have a nice weekend!