Tilting Uranus: Collisions versus Spin–Orbit Resonance

Abstract

In this paper, we investigate whether Uranus’s 98° obliquity was a by-product of a secular spin–orbit resonance assuming that the planet originated closer to the Sun. In this position, Uranus’s spin precession frequency is fast enough to resonate with another planet located beyond Saturn. Using numerical integration, we show that resonance capture is possible in a variety of past solar system configurations but that the timescale required to tilt the planet to 90° is of the order ∼108 yr—a time span that is uncomfortably long. A resonance kick could tilt the planet to a significant 40° in ∼107 yr only if conditions were ideal. We also revisit the collisional hypothesis for the origin of Uranus’s large obliquity. We consider multiple impacts with a new collisional code that builds up a planet by summing the angular momentum imparted from impactors. Because gas accretion imparts an unknown but likely large part of the planet’s spin angular momentum, we compare different collisional models for tilted, untilted, spinning, and nonspinning planets. We find that a 1 M ⊕ strike is sufficient to explain the planet’s current spin state, but that two 0.5 M ⊕ collisions produce better likelihoods. Finally, we investigate hybrid models and show that resonances must produce a tilt of at least ∼40° for any noticeable improvements to the collision model. Because it is difficult for spin–orbit resonances to drive Uranus’s obliquity to 98° even under these ideal conditions, giant impacts seem inescapable.

Searching for Subsurface Oceans on the Moons of Uranus Using Magnetic Induction

“…First, they may have accreted out of material that condensed in a circumuranian nebula (Szulágyi et al, 2018). However, it is uncertain if this is consistent with Uranus's 98° obliquity, which is thought to be the product of giant impacts near the end of Uranus' formation (Kegerreis et al, 2018;Morbidelli et al, 2012;Rogoszinski & Hamilton, 2021;Safronov, 1966). An alternative scenario is that the moons accreted subsequently out of the ejecta disk (Ida et al, 2020;Rogoszinski & Hamilton, 2021;Slattery et al, 1991).…”

“…However, it is uncertain if this is consistent with Uranus's 98° obliquity, which is thought to be the product of giant impacts near the end of Uranus' formation (Kegerreis et al, 2018;Morbidelli et al, 2012;Rogoszinski & Hamilton, 2021;Safronov, 1966). An alternative scenario is that the moons accreted subsequently out of the ejecta disk (Ida et al, 2020;Rogoszinski & Hamilton, 2021;Slattery et al, 1991). A third possibility is that the moons formed from tidal interactions between Uranus and rings created from the disruption of cometary material [e.g., Crida & Charnoz (2012)].…”

Tilting Uranus via Spin–Orbit Resonance with Planet Nine

“…In reality, however, the solar system's gas giants span a range of obliquities, with Uranus being most extreme with θ = 98°.7. The prevailing and best-studied theory posits that some type of giant impact was responsible, likely a 1-3 M ⊕ body impacting the primordial Uranus, simultaneously generating the large axial tilt and spurring the formation of its satellite system (Harris & Ward 1982;Benz et al 1989;Slattery et al 1992;Izidoro et al 2015;Kegerreis et al 2018Kegerreis et al , 2019Ida et al 2020;Reinhardt et al 2020;Rogoszinski & Hamilton 2021). The theory has significant merit, and giant impacts are indeed both feasible and capable of explaining much of Uranus's presentday configuration.…”

“…In a few aspects, however, it encounters difficulties. Rogoszinski & Hamilton (2021) presented an indepth discussion of both the merits and drawbacks of the giant impact hypothesis. An immediate issue lies in the near match between the spin rates of Uranus and Neptune.…”

Raining Rocks: An analytical formulation for collision timescales in planetary systems

“…Furthermore, collisions with these remnants may have a dramatic effect on a planet's orbit. For example, repeated collisions may have resulted in the tilt of Uranus (e..g, Brunini 1995;Parisi & Brunini 1997;Rogoszinski & Hamilton 2021). Lastly, the Chicxulub impact on Earth is suspected to be the main cause of the extinction of the dinosaurs (e.g., Alvarez et al 1980;Schulte et al 2010).…”