Cassini Has Not Made Saturn’s Rings Old

After 13 years in orbit, Cassini data still show conflicting interpretations for the age of Saturn’s rings.

Scientists like to hope that new spacecraft data will confirm their beliefs. Before Cassini arrived at Saturn in 2004, ring scientists fretted over Voyager data that suggested that the rings were young (much younger than Saturn, according to conventional models). Would Cassini help lengthen the age? Here are some of the latest papers and articles:

Ancient Young Rings: Saturn Still Puzzles Astronomers (Space.com). Astrophysicist Paul Sutter reviews the history of ring observations, finally getting to “The contradiction of Saturn” – i.e., evidences that go in opposite directions about the age of the rings. The purity of the rings indicates they must be young. But to keep them old, Sutter only can offer the idea that the best time for Saturn to have formed rings was when the planet formed: “Ergo, these are old rings.” Observation should trump imagination.

Surface roughness of Saturn’s rings and ring particles inferred from thermal phase curves (Icarus). While not addressing ring age specifically, this paper reminds readers that the rings are dynamic systems, subject to clumping, erosion and roughening. Could that go on for 4.5 billion years?

Saturn’s surprising rings: The fascinating mysteries Cassini has solved (Fox News Science). Reminding readers of the age conundrum after Voyager, this article reminds readers that the ring particles constantly jostle one another, clump up and disperse. The F-ring, especially, is frequently subject to bombardments by Prometheus, which “pokes” the thin F-ring and sends out streamers of particles. Also, the rings are far brighter than they should be if old. And if they are only 100 million Darwin Years old, as data suggests, Cassini has shown that they could not have been produced by a single event, such as an asteroid breakup.

Direct detection of gaps in Saturn’s A ring (Icarus). Nearly half a million gaps have been detected in the A-ring of Saturn by the ultraviolet spectrometer. While not addressing ring age, this find shows that the structure and dynamism of these gaps, caused by self-gravity wakes according to models, adds to questions how the structures could persist for billions of years.

Saturn’s rings and associated ring plasma cavity: Evidence for slow ring erosion (Icarus). Based on just four hours of data in 2004, the Radio and Plasma Wave Spectrometer (RPWS) team has extended the ages of the rings: “we conclude that A-, B-, and C- ring lifetimes in the space environment can extend back to the formative periods of planet. Losses via the space environment will not limit ring lifetimes to the short 10′s of Myrs time scales.” But this is based on only one factor, the erosion rate by micrometeoroids, which they determined in that small window of time was slower than previously thought. They also admit that the erosion rate may be episodic, and they say that much more data is needed to determine the rings’ age. Their conclusion includes other caveats:

Admittedly, the Cassini RPWS is not sensitive to submicron and nano-sized grain impacts, and the loss of these particles cannot be determined. Also, Cassini passed over the northern portion of the rings, and the loss via the instability is expected to be largest in the southern hemisphere. However, there should still be a population of unstable ions, appearing as a ‘fountain’ of ions from the B/C ring boundary and C ring, on the northern side as well (see Fig. 2b of Tseng et al., 2013). Evidence for such an ion fountain is not observed in the corresponding electron density as detected by the RPWS plasma waves.

We conclude that the lack of the ion fountain and lack of substantial loss along the B-C ring boundary suggests some other inter-ring process, like local grain cohesion and/or electrostatics, is maintaining stability, offsetting the forces that create the instability and loss.

A single measurement that could possibly extend the lifetime of the rings does not overthrow other measurements that constrain the age. For instance, collisional spreading should undo much of the fine structure of the rings, sending some particles outward and others into the planet as particles bump into each other. The “propeller” structures in the rings indicate the wearing down of large ring particles that should no longer exist after billions of years. And over such a long time period, impacts by large bodies should have removed ring particles and discolored the ones that remain. Sunlight pressure, sputtering and other processes can additionally erode the rings.

As Cassini completes its final high dives through the D-ring gap, data are being collected that should surpass that gathered by the RPWS team in 2004. It will be interesting to see papers on the new high-resolution images and data taken during this final phase of the mission that ends on September 15.