Alice Quillen (University of Rochester, USA)

Close tidal encounters among large planetesimals and moons were more common than impacts. Using a mass spring model within an N-body simulation, we simulate the small deformation of a surface that is caused by a close tidal encounter.  We find tidal encounters can induce sufficient stress on the surface to cause large scale brittle failure of an icy crust.  Strong tidal encounters may be responsible for the formation of long graben complexes and chasmata in ancient terrain of icy moons such as Dione, Tethys, Ariel and Charon.

Mass spring models, originally developed for graphics and gaming applications, can measure remarkably small deformations and so can show tidal spin down of viscoelastic objects, directly tying simulated rheology to orbital drift and internal heat generation.  Inspired by the rapidly spinning Kuiper belt object Haumea, we go past analytical methods to measure how the tidal spin down rate depends on body shape and internal composition.

The new Horizons mission discovered that not only are Pluto and Charon’s minor satellites Styx, Nix, Kerberos, and Hydra, rapidly spinning, but surprisingly they have spin axis tilted into the orbital planet (they have high obliquities).   Long simulations of the minor satellites in a drifting Pluto-Charon binary system exhibit rich resonant spin dynamics, including spin-orbit resonance capture, tumbling resonance and spin-binary resonances.  With Charon drifting away from Pluto, Styx and Nix experience large swings in obliquity.  We propose that Styx and Nix’s currently high obliquities need not be primordial.  We attribute the tilting to a resonance between Styx or Nix’s spin precession rate, their orbit period and Charon’s orbit period.  The resonance differs from those explored previously accounting for Mars chaotic obliquity or Saturn’s current spin axis tilt.  By averaging the torque from a perturbing nearby mass (such as Charon), we construct a low dimensional dynamical model for the resonance that explains why it works on Styx and Nix.  We discuss possible application to Uranus and spin resonant processes that could take place in multiple exoplanet systems, with an eye toward being able to predict exoplanet spin and obliquity so as to inform exoplanet climate models.