Continuing the investigation to tilting Uranus with a secular spin-orbit resonance
Abstract
The most accepted hypothesis for the origin of Uranus’ 98° obliquity is one or more giant collisions during the late stages of planetary formation. Morbidelli et al. (2012) argue that two collisions are needed to explain Uranus' satellites' prograde orbits. While this model is plausible, it does require impactors of at least 0.3-1 Earth masses, depending on the quantity, to strike close to Uranus’ pole and induce a tilting. Even larger mass impactors would be required for more equatorial collisions. Here we explore an alternative non-collision model to titling Uranus by using a secular spin-orbit resonance between Uranus and Neptune during the earlier stages of planetary formation. The inspiration for this model comes from a similar explanation of Saturn’s non-negligible tilt, where a secular resonance between Saturn’s spin axis and Neptune’s orbital pole is responsible (Ward and Hamilton (2004) & Hamilton and Ward (2004)). Thommes et al. (1999, 2002, 2003) argue that at least the cores of Uranus and Neptune were formed in between Jupiter and Saturn, as the density of the protoplanetary disk was greater there.If Neptune was scattered outward before Uranus, then a secular spin-orbit resonance between the two planets is possible. The magnitude of this effect, however, may not be sufficient to drive Uranus all the way to its current obliquity. A resonant kick, where the speed of the migrating planet is near the adiabatic limit, can tilt Uranus up to 40° in a reasonable timespan, and so could replace one of the impactors required in the collisional scenario of Morbidelli et al. (2012). In most situations, however, the effect of such a resonant kick is only of order 10°. We are now considering various hybrid models that involve resonant captures and kicks as in Hamilton and Ward (2004), collisions and captures, and collisions and kicks, and will report on our findings.
- Publication:
-
AAS/Division for Planetary Sciences Meeting Abstracts #49
- Pub Date:
- October 2017
- Bibcode:
- 2017DPS....4950801R