Thursday, July 15, 2010

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.