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SYFY WIRE Bad Astronomy

Take a stunning 250,000-kilometer-long video tour of Saturn's rings

By Phil Plait
S A T U R N. Credit: NASA / JPL-Caltech / SSI / Ian Regan

It's been a while since I've posted some jaw-dropping shot of Saturn from Cassini. To make up for it, I have something special today.

First, a brief background. August 11, 2009, was the vernal equinox on Saturn: The point in its orbit when the Sun was directly above Saturn's equator. This means the Sun was shining exactly edge-on to Saturn's rings, too. Why does that matter? Well, because lighting matters!

The rings are made of countless chunks of very nearly pure water ice, some tiny, some several meters across. The way light passes through them changes their appearance — when it's not the equinox, and the Sun is shining down on one side of the rings, they look different seen from the illuminated side versus "underneath", from the unilluminated side. That's because light reflecting off the rings (when seen from "above", the sunlit side) is much stronger than the light that gets through. This dramatically changes the way they look, and a lot can be learned about their composition this way.

A familiar view of Saturn where the rings are bright on the sunlit side (left), yet seem more eerie and moody when seen from the other side (right). Credit: (both) NASA/JPL-Caltech/Space Science Institute, left image processed by Ian Regan.

Seeing them at the equinox, when the Sun shines perfectly in their plane, gives a third option. But also, at the equinox, any deviations from the rings being flat are far more readily apparent, because something sticking up from the rings will cast a long shadow on the rings themselves. Think of a small rock on the ground at noon, casting a small shadow, versus at sunset when the shadow is really long, longer than the rock itself.

Saturn's rings are eerily flat. But not perfectly so. Due to the influence of the moons orbiting the planet, some ice particles in the ring can be pulled out of plane. This makes for dramatic pictures of towers of ice seen due to the low Sun angle, which would be invisible at other times in Saturn's year. So the folks in charge of Cassini took a zillion images of the rings during that time, both before and after the exact day of the equinox, to catch those long shadows.

Enter Kevin M. Gill, a software engineer who also does image processing. He frequently reprocesses raw Cassini images into high art, and in this case he made an astonishing video. He took a pile of images taken on July 26, 2009, from just before the equinox, stitched them together in 3D, and remapped them into a cylindrical projection, creating an animation that makes it feel like you're flying above the rings at the equinox, following them around Saturn for 250,000 kilometers!


Whoaaaaaaaa. In this animation you're flying over the outer edge of the B ring. Saturn is about 175,000 kilometers to the right, the Sun about 1.5 billion kilometers to the left. More immediately, on the left side of the screen is the inner edge of the Cassini Division, a huge gap in Saturn's rings (you can even see it in small telescopes from Earth). This part of the Division is particularly empty of ring particles, and is called the Huygens Gap.

A little over a minute into the animation you start to see irregularities in the B ring, growing larger as you travel, and then around 2 minutes in they're towering over the ring up to 3 kilometers high! These are thought to be caused by small moonlets in the B ring gravitationally interacting with the ring particles, throwing them into these huge spires.

Mind you, while you travel 250,000 km around the rings here, this is still only about 1/3 of the way around! Saturn's rings are immense. Yet for all that, they are fragile and thin, made up of countless trillions of small icy grains.

How wondrous would it be to take a trip like this in reality? I sincerely and truly hope humanity eventually gets a chance to take a journey such as this.

P.S. You can follow Gill on Twitter and see all his amazing images as he does them.