Saturn’s Rings, Rainbows, and Spokes!

A “spoke” in Saturn’s rings. Taken by Cassini Sept 22, 2009.

Okay, a lot of you already know I really like processing images, a lot. So I’ve been going through some of Cassini’s earlier images, and looking for images that put Saturn’s rings in the spotlight! The one above is one such image. I shared it on Twitter, thinking it was refraction of sunlight in the rings. But it turns out that what you see in the above image is actually a “spoke”, as pointed out by my friend and fellow image processor Ian Regan on Twitter. So I figured, what better than my first blog post being on something I’m not really familiar with?! So, let’s talk about these weird things called spokes!

Turns out that spokes occur in in Saturn’s B-ring (like in the above image), and were first seen in Saturn’s rings by the Voyagers about 30 years ago. In fact, there’s some incredibly neat footage showing these spokes move along Saturn’s rings, both from Cassini as well as Voyager 1, which you can find (and should check out) in this article by planetary scientist Emily Lakdawalla. Anyway, they’re called spokes because they look like spokes on a wheel. The weird thing about spokes is that they move at constant speeds. If they were caused by the particles in Saturn’s rings, this would not be the case because the particles in Saturn’s rings obey Kepler’s laws of motion, such that the force of gravity decreases with the square of distance from the source (in this case, Saturn), hence those further away would move less quickly than those closer in (just like the planets about the Sun). So these spokes should kinda fade out due to the Keplerian orbits of the particles in Saturn’s rings, if they were caused by the particles in Saturn’s rings.  So it can’t be from particles in the ring; instead, it’s due to particles that are electrically charged, and as such, are affected by Saturn’s magnetic field!

Turns out that for some reason, dust particles become electrically charged and elevated from the ring disk, and then follow Saturn’s magnetic field at nearly constant speeds. And recent data suggests these dust particles may actually be made entirely of ice, as stated by science journalist Nancy Atkinson. If light is backscattered by these particles, the spokes appear dark; if light is forward scattered, they appear light. What’s more is that this phenomenon seems to occur seasonally during the equinoxes throughout Saturn’s nearly 30-year-orbit about the Sun. And it looks like the spokes tend to appear in areas where the rings are rotating outward from Saturn’s shadow, as described in this article from NASA JPL (then, in the above image, the rings must be rotating from right to left). As much as we’ve learned about spokes and how they form, the mechanism for the phenomenon still remains a mystery.

So, it turned out the image I processed wasn’t exhibiting refraction of sunlight after all. But then, does refraction in the rings ever happen? Yes, all the time! They are ice particles, after all. But the effect that I’m about to discuss with you isn’t refraction of sunlight, but rather, the opposition effect. Here’s an image taken by Cassini on June 12, 2007, that I processed:

Rainbow-like feature due in Saturn’s rings. Taken by Cassini on June 12, 2007.

See that rainbow-like feature, right out at the edge of the A-ring near the Encke Gap? That’s due to the opposition effect. You can read a quick summary about it here, but let me explain it further. What this means is that the Cassini Spacecraft had the Sun directly behind it with respect to that bright point in the rings; because of this, that area looks brighter because Cassini can’t see the shadows created by those ring particles—they are directly sunlit, and therefore the shadows are behind those particles, hidden from Cassini’s view. Those further out from that brightened spot, however, do start showing shadows, which is why this effect decreases circularly from that bright spot. But wait! Why does it look like a rainbow? Well, the image I processed is put together from images taken by Cassini in the red, green, and blue filters. Since Cassini is moving (and so are the rings), even though it takes images pretty fast, the spot will have moved slightly in each frame (so will the entire planet, making alignment one of the many difficulties of image processing!) Here’s a GIF I put together of the frames I used (cropped a bit to make it easier to see the spot) so you can watch as the spot moves from left to right in the images taken in the blue filter, in the green filter, and finally, in the red filter:

This is the reason we have a rainbow-like effect in the image! The blue data, green data, and red data in the bright spot are not aligned, so you get a rainbow effect. And notice that the blue was to the left, green center, and red to the right? Now look at my processed image, and you’ll see that indeed, the blue part of the rainbow is to the left, while the red is to the right!

Another reason for the opposition effect is owed to something known as coherent backscatter. This happens when light (or electromagnetic radiation) scatters through a medium in which it can scatter many times—like Saturn’s rings, because ice particles galore! So what ends up happening is that the light scatters back towards Cassini, but coherently—that is, the light waves amplify each other as opposed to canceling each other out, making the spot appear brighter.

In summary, we have a bunch of cool physics going on in every single image you see of Saturn, or any other planet, for that matter. In everything! And that’s what makes it all so fascinating!