Excitation of the Orbital Inclination of Iapetus during Planetary Encounters

TL;DR

This study investigates whether early planetary encounters could have excited Iapetus's orbital inclination without significantly increasing its eccentricity, suggesting a rare but plausible dynamical process in the early solar system.

Contribution

The paper demonstrates that planetary encounters can excite Iapetus's inclination under specific conditions, providing a potential explanation for its current orbital tilt.

Findings

In 20% of cases, planetary encounters could explain Iapetus's inclination.

Most encounters either over-perturbed the orbit or failed to excite inclination sufficiently.

The eccentricity was often increased, raising questions about damping mechanisms.

Abstract

Saturn's moon Iapetus has an orbit in a transition region where the Laplace surface is bending from the equator to the orbital plane of Saturn. The orbital inclination of Iapetus to the local Laplace plane is ~8 deg, which is unexpected, because the inclination should be ~0 if Iapetus formed from a circumplanetary disk on the Laplace surface. It thus appears that some process has pumped up Iapetus's inclination while leaving its eccentricity near zero (e=0.03 at present). Here we examined the possibility that Iapetus's inclination was excited during the early solar system instability when encounters between Saturn and ice giants occurred. We found that the dynamical effects of planetary encounters on Iapetus's orbit sensitively depend on the distance of the few closest encounters. In four out of ten instability cases studied here, the orbital perturbations were too large to be plausible. In one case, Iapetus's orbit was practically unneffected. In the remaining five cases, the perturbations of Iapetus's inclination were adequate to explain its present value. In three of these cases, however, Iapetus's eccentricity was excited to 0.1-0.25, and it is not clear whether it could have been damped to its present value (0.03) by some subsequent process (e.g., tides and dynamical friction from captured irregular satellites do not seem to be strong enough). Our results therefore imply that only 2 out of 10 instability cases (~20%) can excite Iapetus's inclination to its present value (~30% of trials lead to >5 deg) while leaving its orbital eccentricity low.