Todd J. Barber, Cassini lead propulsion engineer
I recently took a late-night phone call from Dr. Carl Murray, a Cassini scientist from the United Kingdom. We had a fascinating chat about one of his areas of expertise, and one of my favorite Saturnian enigmas—the exquisite F-ring. Recently, Dr. Murray and his colleagues had a press release highlighting their latest results regarding Saturn’s F ring, but I thought it might be interesting to get some background on Dr. Murray and the F ring itself. Previously unknown to Earthbound observers, Saturn’s unusual F ring was discovered by the Pioneer 11 spacecraft in 1979. A crude image of the F ring appeared on the cover of “Science” magazine at this time, with its “beaded” appearance quite a mystery. Voyager 1 discovered the F ring shepherding moons Prometheus and Pandora, along with sublime images of the kinks, braids, and knots of the F ring. Within a Voyager mission punctuated by a multitude of jaw-dropping results, this one was among the most surprising. Dr. Murray was at Cornell at this time doing postdoctoral work, starting his stint there on the very first day of the 1980s. He said he remembers “watching stuff come back” from Voyager as it was happening out at Saturn, presumably leaving an indelible impression.
Dr. Murray obtained his B.S. in Applied Mathematics with Astrophysics from Queen Mary University of London in 1977, followed by a Ph.D. in 1979 from the same school. His doctoral work concerned the evolution of small particles in the solar system, with some work on planetary rings (recall at this time the Jovian rings had just been discovered). His postdoc work at Cornell included studying Kirkwood gaps in the asteroid belt, orbital locations with a paucity of asteroids due to orbital resonances with Jupiter. I think I was most shocked to learn that Kirkwood himself was inspired to look at asteroid belt gaps after studying the Cassini Division at Saturn, another gap caused by orbital resonance! I would have bet heavily that this inspiration would have occurred the other way around. After leaving Cornell, he returned to England to work on chaos in the solar system, one of my favorite topics (in fact, it nearly derailed my attempts to learn more about the F ring!). During the Voyager 2 encounter at Uranus (early 1986), Dr. Murray was lecturing back at his alma mater, but for Neptune encounter in 1989, he obtained press credentials and came to JPL to cover Voyager 2’s planetary swan song for “Physics World”! Anyway, it’s not hard to see from where Dr. Murray’s passion for the F ring comes. This perplexing, beautiful Saturnian ring seems to me the perfect marriage of his vast and diverse academic and professional career.
Following Voyager, one of the biggest mysteries of the F ring was whether there were different features present within the F ring or whether it changed with time. As it turns out, the F-ring is extremely dynamic, both spatially and temporally, on time scales of hours to decades! As Dr. Murray put it so well, there is “no such thing as a useless picture of the F ring.” Cassini has helped unlock some of the secrets of the parallel strands in the F ring, helping to decide whether orbital resonance or shepherding is the primary cause. The spacecraft also helped determine that the parallel strands were spiral in nature, a result not known to Voyager. Shearing radial jets were identified, now known to be caused by collisions. Additionally, intriguing “fans” were also discovered in the F ring, now understood to be created by embedded objects. Dr. Murray’s team also observed streamer-channels caused by the F-ring shepherding moon Prometheus, and retroactively discovered these features were predicted almost exactly in modeling work he did with his graduate student in the mid 1990s! As he pointed out, it’s amazing how well one can replicate the complexity of the F ring using just simple models, essentially solving the famous “three-body problem” (as he said he has been doing all his career). He did opine, though, that it was “the most difficult problem he ever worked” and he “had more hair when he started.”
I asked Dr. Murray about recent observations, as well as plans for the future. He told me last year was historic, for there is an orbital precession with a 19-year period that “peaked” last year, when the apochrone of Prometheus’ elliptical orbit aligned with the perichrone of the F ring. Readers familiar with the terms “apogee” and “aphelion” for the most distant portions of Earth and solar orbits, respectively, can likely deduce that “apochrone” is the farthest point in a given orbit to Saturn (“Chronos” being the Greek name for Saturn). The same holds true for “perichrone” as an analogue to “perigee” and “perihelion”—the closest points in these respective orbits. Since Prometheus orbits inside the F ring, this means last year Prometheus made its deepest possible “dip” into the F ring region, causing gravitational mischief not seen in two decades! He also told me his current paper includes a discussion of bright F ring objects that were found to cast shadows during last year’s Saturnian equinox. Outside of his area of expertise, I asked about the composition and age of the F ring, but I may have to save that for another column (particularly since some results may not have been published yet).
We closed our wonderful chat by talking about his plans for studying the F ring during the Cassini Solstice Mission. He said it’s very difficult to locate objects that collide with the F ring, but he will keep looking for them. He will also try to determine the lifetime of F ring features, and he reminded me that the mass of the F-ring remains an important unknown. Finally, Dr. Murray patiently awaits Cassini’s “F-ring orbits” in November of 2016, inclined orbital passes within just a few thousand kilometers of the F ring. Dr. Murray, I along with the rest of the world await what mysteries the F-ring will reveal and how you and your team will explain them to us. Thank you very much for your time.