The fragility of the Uranian moons during the giant planet instability

Abstract

It is thought that, sometime after their formation, the solar system's giant planets experienced a dynamical instability that caused their orbits to excite, diverge, and ejected one or more objects with masses comparable to the ice giants. A key feature of this model is that the planets experience encounters with other planetary bodies, and these encounters facilitate the capture of nearby small bodies as irregular satellites. Instability simulations indicate that planet-planet encounter distances can typically fall below 0.1 au, which is only roughly an order of magnitude larger than the radial extent of the modern planets' regular satellite systems. In this paper we model the effects of these encounters on the dynamical stability of the regular moons of Jupiter and Uranus. We tested encounter histories from 122 plausible outer solar system dynamical histories. We find that the survival probability for the Jovian and Uranian moon systems are both less than 15%. Moreover, we only identify one case where both Uranus and Jupiter's large satellites consistently survive the same instability. Interestingly, Jupiter's moons are most likely to survive in instabilities initialized with two smaller extra ice giants, and cases with one larger additional planet provide more favorable conditions for Uranian system survival. In either case, if Uranus encounters another ice giant at D<0.02 au, or one of the gas giants at D<0.1 au, satellite system destruction is effectively guaranteed. Wider encounters can also affect the system, particularly when they occur successively. Since the Laplace resonance likely would not be in place today if Jupiter's moons experienced an instability that led to collisions, our results indicate that Uranus' moons were likely perturbed to the point of collisions at least twice: as a result of both the impact that tilted the planet and the giant planet instability.

Figures (8)

• Figure 1: Cumulative distribution of the deepest encounter between one of the giant planets (red, gold, cyan and blue curves) and an Ice Giant (left panel) or Gas Giant (drawn from the combined distributions of encounters with Jupiter and Saturn, right panel) in our sample of 122 instability evolutionary tracks from clement21_instbclement21_instb2 that satisfied all important outer solar system dynamical constraints. The dashed line plots the semi-major axis of Oberon, and the right vertical line plots the result of deienno14 that encounters within 0.02 au regularly destroyed the Galilean moon system.
• Figure 2: Cumulative distribution of the total number of successive encounters less than 0.1 au (solid lines) or 0.03 au (shaded lines) experienced by each giant planets (red, gold, cyan and blue curves) with an Ice Giant (left panel) or Gas Giant (right panel) in our sample of 122 instability evolutionary tracks from clement21_instbclement21_instb2 that satisfied all important outer solar system dynamical constraints. Note that the x-axis in the left panel is log-scaled, while the right panel is not.
• Figure 3: Cumulative distribution of times between successive encounters ($dt_{enc}$) less than various distance thresholds ($D_{enc}$) at Uranus in our 122 instability evolutionary tracks.
• Figure 4: Cumulative distributions of various properties of instability evolutionary tracks where the regular satellites remained stable (light blue for Uranus and red for Jupiter) or were destabilized (dark blue for Uranus). For comparison, we also plot the distributions for all 5GP (solid grey line) and 6GP (dashed grey line). Panel F shows the total number of Ice Giant encounters deeper than 0.02 and 0.05 au, as well as Gas Giant encounters deeper than 0.05 au for these same collections of simulations.
• Figure 5: Cumulative distribution of duration of the times between the first ice giant encounter at Uranus with $D_{enc}<$ 0.1 au, and the last such encounter. The different shades of blue lines show instabilities where the Uranian moons survived, and those where they did not. The different styles of black lines show instabilities where Neptune finished inside of, and outside of its modern semi-major axis. Here, we plot systems with only a single encounter with $D_{enc}<$ 0.1 au at 0, and those with no such events at infinity.