Naor Movshovitz

Comparing Jupiter interior structure models to Juno gravity measurements and the role of a dilute core

The Juno spacecraft has measured Jupiters low-order, even gravitational moments, J2–J8, to an unprecedented precision, providing important constraints on the density profile and core mass of the planet. Here we report on a selection of interior models based on ab initio computer simulations of hydrogen-helium mixtures. We demonstrate that a dilute core, expanded to a significant fraction of the planets radius, is helpful in reconciling the calculated Jn with Junos observations. Although model predictions are strongly affected by the chosen equation of state, the prediction of an enrichment of Z in the deep, metallic envelope over that in the shallow, molecular envelope holds. We estimate Jupiters core to contain a 7–25 Earth mass of heavy elements. We discuss the current difficulties in reconciling measured Jn with the equations of state and with theory for formation and evolution of the planet.

NUMERICAL MODELING OF THE DISRUPTION OF COMET D/1993 F2 SHOEMAKER-LEVY 9 REPRESENTING THE PROGENITOR BY A GRAVITATIONALLY BOUND ASSEMBLAGE OF RANDOMLY SHAPED POLYHEDRA

We advance the modeling of rubble-pile solid bodies by re-examining the tidal breakup of comet Shoemaker-Levy 9, an event that occurred during a encounter with Jupiter in 1992 July. Tidal disruption of the comet nucleus led to a chain of sub-nuclei ~100-1000 m diameter; these went on to collide with the planet two years later. They were intensively studied prior to and during the collisions, making SL9 the best natural benchmark for physical models of small-body disruption. For the first time in the study of this event, we use numerical codes treating rubble piles as collections of polyhedra. This introduces forces of dilatation and friction, and inelastic response. As in our previous studies we conclude that the progenitor must have been a rubble pile, and we obtain approximately the same pre-breakup diameter (~1.5 km) in our best fits to the data. We find that the inclusion of realistic fragment shapes leads to grain locking and dilatancy, so that even in the absence of friction or other dissipation we find that disruption is overall more difficult than in our spheres-based simulations. We constrain the comets bulk density at ρbulk ~ 300-400 kg m–3, half that of our spheres-based predictions and consistent with recent estimates derived from spacecraft observations.

Disruption and reaccretion of midsized moons during an outer solar system Late Heavy Bombardment

We investigate the problem of satellite survival during a hypothetical late heavy bombardment in the outer solar system, as predicted by the Nice Model (Tsiganis, Gomes, Morbidelli, & Levison 2005, Nature 435). Using a Monte-Carlo approach we calculate, for satellites of Jupiter, Saturn, and Uranus, the probability of experiencing a catastrophic collision during the LHB. We find that Mimas, Enceladus, Tethys, and Miranda experience at least one catastrophic impact in every simulation. Because re-accretion is expected to be rapid, these bodies will have emerged as scrambled mixtures of rock and ice. Tidal heating may have subsequently modified the latter three, but in the nominal LHB model Mimas should be a largely undifferentiated, homo geneous body. A differentiated Mimas would imply either that this body formed late, or that the Nice model requires significant modification.