Some years back at a Princeton conference I had the pleasure of hearing Richard Gott discussing the age of Saturn’s rings. Gott is the author of, in addition to much else, Time Travel in Einstein’s Universe (Houghton Mifflin, 2001). I admit the question of Saturn’s rings had never occurred to me, my assumption being that the rings formed not long after the formation of the planet. But of course there is no reason why this should be, and a number of reasons why it should not. How long, for instance, does it take moons to collide with each other, contributing debris to a growing ring system? And are such collisions the only way a ring system can form?
With all this in mind, I was interested in a new paper that a number of readers referenced in emails. Lead author Matija Ćuk (SETI Institute), working with Luke Dones and David Nesvorný (both at SwRI), offers up the possibility that the inner moons of Saturn and possibly the rings were actually formed much later than we would expect. In fact, they may be positively recent in astronomical terms, having formed during or not so long after the era of the dinosaurs.
Image: The new paper finds that Saturn’s moon Rhea and all other moons and rings closer to Saturn may be only 100 million years old. Outer satellites (not pictured here), including Saturn’s largest moon Titan, are probably as old as the planet itself. Credit: NASA/JPL.
The work involves the moon Rhea and all the other moons and rings closer in to Saturn. The outer satellites, including Titan, are still thought to be as old as the planet itself. But using numerical simulations, the trio explored the tidal effects that should be causing the inner moons of Saturn to spiral out to larger orbital radii. Each of the moons would experience different growth in its orbit, which would occasionally produce orbital resonances. Such effects, in a system crowded with moons, can cause orbits to diverge from their original plane.
The team’s simulations homed in on a hypothetical 3:2 resonance in the past between the moons Tethys and Dione, along with a 5:3 resonance crossing between Dione and Rhea. Remember what happens in such a resonance: A moon’s orbital period becomes a fraction — one-half, or two-thirds, for example — of another moon’s orbital period. The paper notes that the current Tethys/Dione and Dione/Rhea orbital period ratios are just above 2/3 and 3/5. Does this mean these resonances were crossed at some point in the past?
Perhaps not, for interestingly, the 3:2 resonance crossing should have led to an excitation in the orbital inclinations of both Tethys and Dione, something that is not observed in their current orbits. The 5:3 resonance between Dione and Rhea, according to the authors, probably did happen, to be followed by a previously unknown Tethys-Dione resonance. The combination can explain the current inclinations of both Tethys and Rhea. Quoting from the paper:
We can therefore state that Tethys and Dione evolved tidally by only a modest amount over their lifetimes, which is only about a quarter of the tidal evolution envisaged in Murray & Dermott (1999). There are two possible interpretations: either tidal evolution of Saturn’s moons has been very slow, or Saturn’s mid-sized moons are significantly younger than the Solar System. While both interpretations are consistent with the lack of the past Tethys-Dione resonance, we favor the idea that the moons are young, possibly as young as 100 Myr… The Trojan moons of Tethys and Dione that share their inclinations must have formed even more recently, after their passage through the secular resonance.
The inclination of the orbits of the moons in question, in other words, should have been altered more than they have been by gravitational interactions, an indication that orbital resonances have been few. And that, the authors conclude, is evidence they must have formed recently. That leads directly to the question of how the inner moons formed. Says Ćuk:
“Our best guess is that Saturn had a similar collection of moons before, but their orbits were disturbed by a special kind of orbital resonance involving Saturn’s motion around the Sun. Eventually, the orbits of neighboring moons crossed, and these objects collided. From this rubble, the present set of moons and rings formed.”
All this has implications for our view of Enceladus, which experiences intense tidal heating that is incompatible with a slowly evolving system. The presence of an internal ocean gives high astrobiological interest to this moon, but according to these researchers, Enceladus, Mimas and the rings could have formed at the same epoch as Dione and Rhea or be even younger (the authors intend to explore the tidal evolution of Mimas and Enceladus in future work). Would an Enceladus as young as the Cretaceous Period on Earth have had time to develop life? It’s a question we can clarify with future missions designed to fly through the Enceladus plume.