Abstrakt
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 similar or equal to 8 degrees, which is unexpected because the inclination should be similar or equal to 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 similar or equal to 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 4 out of 10 instability cases studied here, the orbital perturbations were too large to be plausible.
In one case, Iapetus's orbit was practically unaffected. 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 (similar or equal to 0.03) by a 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 (similar to 20%) can excite Iapetus's inclination to its current value (similar to 30% of trials lead to >5 degrees) while leaving its orbital eccentricity low.