CT Scan of Saturn’s Rings
JPL issued a press release today describing the results of our occultation analysis of Saturn’s rings. I’ve written about the basic idea and preliminary results before here and here. Press releases mean conversations with reporters, so I’ve had a few and it made me think of ways to describe what we’ve done that is easy to relate to. Larry Esposito, Principal Investigator of the Cassini Ultraviolet Imaging Spectrograph (the instrument team I work on), was the first to come up with the CAT scan analogy. The only problem with this is that most people (including me when this first came up) don’t know how a CAT scan works either (or even what “CAT” stands for).
CAT stands for Computed Axial Tomography, and these days they are typically known as CT scans, dropping the “axial” part because the same general technique can be done without the requirement of axial symmetry. That’s good, because that’s what we do with the occultations of Saturn’s rings, so in essence we’re doing CT (Computed Tomography) scans of the rings. CT scans take linear x-ray images of the body from multiple vantage points to construct a model of a slice of the body. By doing these scans at multiple “levels” or locations along the length of the body a full three-dimensional model can be constructed. In CT imaging the number of views through the body is large and in regularly-spaced viewing geometries so that an accurate model of the interior of the body can derived. We do not have the full range of viewing geometries available in our stellar occultations of the rings, however, and I only used about a dozen occultations to arrive at the model of clumps in the rings reported today.
So I thought of another analogy for the technique we used to see how ring particles clump together. Imagine that you are looking at a forest from a great distance, too far away to see individual trees. Now imagine that someone on the other side of the forest shines a bright light at you through the forest while you walk by. As you move you’ll see the light flicker as it is blocked by individual trees and occasionally passes through a gap between the trees. This single observation would tell you that there are things in the forest that completely block the light and it would tell you something about how big they are, but not their shape. But if you repeat the experiment with your friend on different sides of the forest you can begin to deduce that trees have cylindrical trunks.
Our paper in the journal Icarus describes our results for clumps known as self-gravity wakes in the B ring of Saturn. These clumps exist throughout the A ring (the outermost main ring) where they have long been known to produce a very visible “azimuthal brightness asymmetry” in images of the rings. The effect in the B ring is far more subtle. Observations from the Hubble Space Telescope also show the presence of self-gravity wakes in the B ring as described in a paper by Richard French and co-authors, also in Icarus. The advantage of the occultations as observed by Cassini is that they are able to probe most of the B ring even where the particles are tightly packed, allowing very little light to pass through. By using a model of self-gravity wakes as, essentially, parallel tree trunks lying flat in the rings, we were able to make the observations fit the model if the wakes are very broad and flat, more than ten times wider than they are thick. We are not yet able to measure the length of the clumps directly. The image this gives of the B ring is not of a sea of particles, but of a sea of broad sheets or clumps of particles, with relatively narrow and mostly empty space between them. Further observations will enable us to use more sophisticated models than the parallel tree trunk model and learn more about what Saturn’s rings would look like if we could get right on top of them.