Investigating tidal stripping of a pre-existing moon as the origin of Saturn's young icy rings

Abstract

The origin of Saturn's rings has been debated for decades. Measurements from Voyager and Cassini have suggested that the rings could be as young as ~100 Myr and composed of nearly pure water ice. Several scenarios have been proposed to explain these properties. One hypothesis (Wisdom et al 2022) is that the rings formed through the recent tidal disruption of a pre-existing moon, Chrysalis, which experienced a close encounter with Saturn following its highly eccentric orbit. However, the mechanism by which this hypothesis would have formed the rings remains largely unexplored, in particular, whether Chrysalis could supply ring material of the desired mass and composition. To address these questions, we perform smoothed particle hydrodynamics simulations to investigate the tidal response of Chrysalis during close encounters with Saturn. Our results demonstrate that preferential tidal stripping of the ice mantle from a differentiated Chrysalis can produce rings with both mass and composition resembling the present rings -- provided that the closest encounter occurs between the parabolic Roche limits for ice ~1.53Rs and rock ~1.07Rs -- consistent with Wisdom et al 2022. Moreover, multiple close encounters can extend the effective disruption limit by spinning up the body, enhancing the tidal stripping efficiency. Following close encounters, the rocky remnant of Chrysalis would have been removed in less than few kyr, either by collision with Saturn or ejection onto a hyperbolic orbit. These findings support the hypothesis that Saturn's rings could originate from a recent lost moon, and imply a highly dynamical evolution of the Saturnian system over the past few hundred million years.

Figures (12)

• Figure 1: Mass constraint of Chrysalis according to wisdom2022loss. Each point represents a simulation of the resonance model backward in time over 1.4 Gyr, for given Chrysalis mass. The final obliquity should be below about $10^{\circ}$, i.e., the shadow area, requiring the mass of Chrysalis from about $0.6M_{\rm Iapetus}$ to $2.0M_{\rm Iapetus}$.
• Figure 2: Saturn-grazing cases from wisdom2022loss with closest distance $<2R_{\rm S}$. (a) Orbit elements of Chrysalis at each closest approach, where the solid-line cases end with impacting Saturn (cross marked), and the dotted-line cases ultimately become hyperbolic. The parabolic Roche limits sridhar1992tidal for ice and rock are provided for reference. (b) Orbit evolution of case 23-75 from panel (a), with the shadow region enlarged to show the close encounter details. This case ends in a hyperbolic orbit with a negative semi-major axis. (c) Same as panel (b) but for case 23-18, which ends with impact on Saturn. Note that a close approach (dot points) requires a small transient periapsis $q$, but the reverse does not hold: a small $q$ does not guarantee a close approach.
• Figure 3: SPH simulation snapshots with varying ice-to-rock ratios and periapses (d- and i- cases in Table \ref{['tab:simu_setup']}). Red and blue particles represent the rock core and ice mantle, respectively. In all cases, the snapshot corresponds to 6 h after periapsis, not the simulation end time. The dashed lines indicate the final fates of particles, with yellow for hyperbolic and cyan for bound (excluding the largest remnant).
• Figure 4: SPH simulation of tidal stripping of Chrysalis during a close encounter with Saturn (case d5 in Table \ref{['tab:simu_setup']}). The initial body is assumed to be differentiated with 50 wt.% ice and 50 wt.% rock. The pre-encounter semi-major axis is $200R_{\rm S}$, with a periapsis of $1.3R_{\rm S}$. The snapshot shows a cross section 6 h after periapsis. In the left panel, red particles indicate the rock core and blue the ice mantle. The full simulation spans 50 h. The right panel shows the specific orbit energy of each particle at the end, where a positive value means a hyperbolic orbit and negative for bound orbit. For reference, the pre-encounter orbital energy is about $-1.5\times10^6$ m$^2$s$^{-2}$. A supplementary video of this case is available at https://github.com/jiaoyf-thu/tidal-stripping.
• Figure 5: Saturn-centric orbit distribution of the post-encounter Chrysalis and its debris at the end of SPH simulation (case d5 in Table \ref{['tab:simu_setup']}). Each dot represents a cluster of particles, with its size representing the cluster mass and color for the ice fraction. The large light-red dot is the largest remnant of Chrysalis retaining the rock core, which experiences minor changes to the pre-encounter orbit (marked plus). The magenta dotted line indicates the specific angular momentum of the pre-encounter orbit of Chrysalis.