Dr. Linda Spilker on the Rings, Wonderful Science, and Equinox Arrives

Todd J. Barber, Cassini lead propulsion engineer

It’s been many months since I’ve interviewed a fellow Cassini team member, so the timing seemed right to talk to Deputy Project Scientist and Saturn ring expert, Linda Spilker, in this edition of the “Insider’s Cassini” column. After all, Saturn’s equinox and ring plane “flip-flop” with respect to the sun is a mere week away, an event which hasn’t occurred for nearly fifteen years!

Linda and I started our chat with her favorite Cassini highlights in ring science during the last five years at Saturn. Writing as furiously as possible, I couldn’t help but to smile when reliving a grab bag of ring science results. Even during our first day in Saturnian orbit, we were left speechless with views of the rings in ten times the detail seen previously, a visible wake caused by the moon Pan, and even fine structure in the peaks of the density waves which resembled straw (I remember this well). Linda then reminded me about the gorgeous occultation image during our 28th revolution of Saturn and that it was far more than a neat photo of Saturn with Earth and our moon in the background. For the first time, one image captured all of Saturn’s rings, since tenuous dusty rings are easy to see in forward-scattered light. In fact, two new Saturnian rings were identified from that image!

I was also, well, charmed to learn of the “charming ringlet,” a bright ring of material within the Cassini division, and a new feature since the Voyager days. Here’s one ring science observation that slipped by my radar in my effort to stay informed of Cassini’s latest results. In general, we chatted about how Voyager turned much of Saturnian ring science on its ear, while Cassini similarly has caused a lot of rethinking of the Voyager data. As an example, early Earth-based observations suggested the Saturnian rings were primordial while Voyager suggested ring ages of only a few hundred million years (toddlers in terms of the age of the solar system). Cassini has again turned this thinking around, particularly with the B-ring. Apparently, B-ring material may be ancient but prone to bouts of recycling, continually clumping but then breaking up to expose fresh (and young-looking) water ice. Evidently, everything old is new again, but perhaps old again. Isn’t science wonderful?

After a brief discussion of how Saturn’s ring material is an excellent model for planetary disks (like the one that formed our own solar system 4.5 billion years ago), we briefly talked about the F ring and ring spokes. I was gratified to learn that we are starting to see spokes form, as expected around equinox if they are enabled by low sun-angle solar illumination on the rings, but my eyes widened when I asked Linda about the latest genesis theory of the spokes. I had never heard of the possibility spoke formation might be related to Saturnian storms or even lightning—boy, the things you learn from a ring scientist! As cool as spokes are, I quickly forgot about them when Linda reminded me that the F ring’s little pickpocket, the tiny moon Prometheus, is preparing for its deepest dip into the F ring itself late this year, when the Prometheus apoapse will line line up with the F ring periapse. What gravitational mischief will this wee satellite impart during this interaction? I plan to stay tuned as Linda and her colleagues piece together the mysteries.

With all this amazing ring science in the bank, I was humbled to realize the best days for ring science may be yet to come, even as soon as next week! During Saturn’s equinox around August 11, 2009, Cassini scientists will be busy tracing the shadows of Saturnian moons on the rings, looking for ring warping. Some of the biggest ring particles themselves may actually be able to cast shadows on the rings, too! The thickest portion of the B-rings (still no more than a few tens of meters in thickness) will see sunlight for the first time in nearly fifteen years—how quickly will they heat up and what does that tell us about their size and composition? Not only does Cassini have a front-row seat for this spectacle, this equinox is more important than ever, because the show on the third rock has been preempted this year by Saturn’s solar conjunction. That is, we on Earth will not be able to observe the fireworks of equinox at Saturn, because the ringed planet is nearly behind the sun as seen from here. How fortunate we are to have an awesome orbiting asset waiting to capture this rare celestial event!

I asked Linda when she first saw the rings of Saturn, and she couldn’t quite recall. She did mention she obtained her first telescope in third grade and was soon chasing down the moons of Jupiter, excitedly yelling to her mom to come see the spectacle. I’m sure with such early proficiency with the telescope, her first view of the ringed planet was not far behind. I remember watching Linda on PBS specials about Voyager in junior high and high school, dreaming of working at JPL, but never dreaming I’d be sitting in the office of one of my heroes, being treated as a peer. Good luck with equinox, Linda! I can’t wait to catch your contagious excitement when the first images come to Earth, as I’m always able to do after hearing you speak.