Evidence for a primordial origin of Saturn’s rings

Press Release for 2007 DPS Meeting

Saturn’s rings may be more massive than previously thought. Both Cassini observations and theoretical simulations of Saturn’s rings point towards extensive particle clumping in Saturn’s rings. Our simulations of the rings show that the surface density of particles can be substantially larger than one would infer from a uniform distribution of particles. The gravitational attraction between ring particles causes the particles to bunch up into clusters that organize themselves into connected strands resembling a giant spider web. As the particles orbit Saturn, strands of the spider web are slowly sheared apart and new strands are formed. Adding more particles to the simulation simply piles more particles onto the spider web strands while leaving relatively clear gaps in between the strands of the spider web.

As a result, when the Cassini spacecraft observes the light of a star passing through the densest part of the rings, it is actually measuring the fractional area between these opaque strands of particles rather than the density of material within the spider web itself. When we calculate the amount of light that can pass through our simulation of the rings, we see strong variations with the viewing geometry similar to the variations observed by the Cassini Ultraviolet Imaging Spectrometer (UVIS) instrument when starlight passes through the rings.

We estimate that the densest part of the Saturn’s ring, located in the core of the B ring, contains more than three times the mass previously estimated. The inferred total mass of Saturn’s ring is therefore likely to be at least three times more massive than the satellite, Mimas. Such a massive ring system is unlikely to have formed by the tidal disruption of a comet or by the collisional disruption of a former satellite during the last 4 billion years. Instead, we argue that the massive B ring must have a primordial origin, dating back from the period of late heavy bombardment when the collisional disruption of a massive satellite was more probable. Placing a massive satellite inside Saturn’s Roche zone by orbital migration due to resonant interactions with a proto-satellite disk may even be a natural outcome in the context of Canup and Ward’s model of satellite formation in a “gas-starved” accretion disk around Saturn.

So how do we reconcile this scenario with youthful features in Saturn’s rings? The massive B ring could be quite ancient while the A and C rings are still relatively young. Our simulations suggest that the spreading rate of the B ring is substantially slower than in the A ring because the extensive particle clumping in the B ring results in a slow taffy pull of long strings of particles rather than the vigorous stirring seen in the A ring simulations. In this scenario, the massive B ring evolves slowly while the lower mass A ring and C ring evolve faster and show the most evidence for youthful features.