First snapshots of F ring simulation (and a comment on moonlets)

I described in an earlier post the mother of all F ring simulations. This has now gone far enough that I feel I can say it is working properly and I can make a plot of what it happening in the early going. I have put this up on my normal research site. I have a fair bit of discussion along with two plots. The plots don't show all that much beyond the initial configuration because there simply hasn't been that much time for the system to evolve yet. Still, I think they will give anyone with a feel for the F ring a feel for what this simulation might be able to show us.

It also hit me this evening that while I feel like this simulation isn't running all that fast, the reality is that it is running in close to real time. It finishes an orbit in about 7 hours. The real material in the F ring takes almost that long to get around Saturn. So on the whole I'd say that is pretty good. Maybe after this one has gone far enough to wipe out the transients I can consider scaling it up a bit more. I'd be really interested in seeing what happens if I make it so that Prometheus really does run into the apron of material at the edge of the ring. I expect the computers will be less than happy about such an experiment though.

As a side note, on the recommendation of Matt Tiscareno I did some simulations on moonlet stability that didn't include the background. Unlike previous work by Porco, et al., I am not putting a large central core into these moonlets. I am assuming all particles of nearly the same size. This is significant because the Hill sphere of the large body can enclose nearby smaller bodies when two bodies of the same size sit largely outside of one another's Hill spheres. I was able to find a configuration where a moonlet was stable without background material and got broken up when background material was present. The moonlet was made as a lattice roughly filling a triaxial ellipsoid at 130,000 km from Saturn. The lowest density I could get to be stable i this configuration with a 2:1:1 ratio was a bulk density of 0.7 g/cm^3 or 1.0 g/cm^3 for the constituent bodies. Even with that high a density, it got knocked apart when I put it in a background. I'm not going to be doing too much more work on this right now because Crosby found it interesting and will likely work on it as part of his senior research project in Physics.

Moonlet Surprise

So Crosby has been working on doing data analysis on the higher resolution moonlet simulations that I had done over the last two years or so. These simulations use smaller particles in the background and were intended to help answer the question of exactly what we are seeing with small propellers in the rings. He has made some movies that I will put up on the rings research page under the second moonlets paper. We have seen huge differences in what appears based on the size of the moonlet. Yesterday we got another surprise as well. I had started a new simulation that had a size distribution in the hopes that we could get a better feel for how that impacts things. I made some other changes to this particular simulation as well. I upped the surface density and lowered the internal particle density to try to better match what other people have been using. The result, the moonlet fell apart.

That last bit needs some explaining. For numerical reasons I stopped using a single large particle for my moonlet a while back. Instead, I use a lattice of spheres with an enhanced density so the total mass matches that of a single sphere. In my other simulations this has worked great and the lattice holds together. In this one, it got knocked to pieces in just one orbit. We haven't worked out all the implications of this, but here are a few possibilities.

• The internal density of constituent parts of moonlets has to be high enough so this doesn't happen.
• The moonlets can't be loose rubble piles. They have to have physical strength.
• The constituent pieces of a moonlet have to be significantly larger than the top end of the surrounding size distribution.

Granted, it could be some combination of these. In this simulation, the moonlet was fairly small too so I need to do something with a larger moonlet without varying anything else and see if it gets broken up.