Last giant impact on Uranus

Abstract

Context. Modern models of the formation of ice giants attempt to account for the formation of Uranus and Neptune within the protoplanetary disk lifetime. These models assume a higher initial surface density well above that of the minimum mass solar nebula model and/or the formation of all giant planets in an inner compact configuration. Other effects include planetesimals migration due to gas drag and the small size of the accreted planetesimals, which accelerates the accretion rate. However, at present, none of these models account for the spin properties of the ice giants. Aims. Stochastic impacts by large bodies are, at present, the usually accepted mechanisms able to account for the obliquity of the ice giants. We attempt to set constraints on giant impacts as the cause of Uranus's current obliquity of 98 • and on the impactor masses. Methods. Since stochastic collisions among embryos are assumed to occur beyond oligarchy, we model the angular momentum transfer to proto-Uranus by the last stochastic collision (GC) between the protoplanet and an oligarchic mass at the end of Uranus's formation. We take a minimum impactor mass m i of 1 m ⊕ . Results. We find that an oligarchic mass m i ∼ 1 m ⊕ ≤ m i ≤ 4.5 m ⊕ would be required at the GC to reproduce the present rotational properties of Uranus. An impact with m i > 4.5 m ⊕ is not possible, unless the impact parameter of the collision is very small and/or the angle between the spin axis of Uranus prior and after the GC is higher than 130 • . This result is valid if Uranus formed in situ or between 10−20 AU and does not depend on the occurrence of the GC after or during the possible migration of the planet. This result is very similar to one obtained for Neptune from its rotational properties. Conclusions. If the stage of stochastic impacts among oligarchs has occurred and if the present rotational status of Uranus is the result of such processes, the 4.5 m ⊕ mass limit must be understood as an upper constraint on the oligarchic masses in the trans-Saturnian region at the end of ice giants' formation. This result may be used to set constraints on planetary formation scenarios.

Limits on the number of primordial Scattered disc objects at Pluto mass and higher from the absence of their dynamical signatures on the present-day trans-Neptunian Populations

“…It is unclear whether a substantial number of objects larger than Pluto were present. If the obliquities of Uranus and Neptune are the result of their last big impact, those impactors would have been Earths (Parisi & Del Valle 2011;Parisi 2011), but this idea is speculative, and alternative origins for their obliquities have been proposed (Kubo-Oka & Nakazawa 1995;Boué & Laskar 2010;Morbidelli et al 2012). Izidoro et al (2015) demonstrated that Uranus and Neptune could be produced from ∼ 2 − −6 Earth mass protoplanets.…”

Dynamical Interactions and Mass Loss within the Uranian System

“…Such material may be sourced from minor bodies or moons that move within the Roche limit of the planet (Canup & Esposito 1995;, impactors on the moons within the system (Plescia & Boyce 1985;Zahnle et al 2003;Kirchoff & Schenk 2010;Hueso et al 2018;Ferguson et al 2020), or even moon collisions (Barbara & Esposito 2002;French & Showalter 2012). As described in Section 1, impacts have played a profound role in the evolution of Uranus, both intrinsically and to its dynamical environment (Parisi 2011;Reinhardt et al 2020), and may have been a source for building the major moons of the Uranus system (Salmon & Canup 2022).…”

Fragmentación de planetesimales y formación planetaria

“…En el primer caso (e = 0), v 2 sto es el cuadrado del módulo del vector de velocidad relativa entre dos vectores con módulo v 0 y cualquier dirección promediada en todos los ángulos posibles entre los dos vectores. En el segundo caso (e = 1), v 2 sto representa el cuadrado del módulo del vector de velocidad relativa cuando la órbita parabólica del planetesimal cruza la órbita circular del protoplaneta, promediada en todas las posibles configuraciones geométricas (Parisi & Brunini, 1997;Brunini et al, 2002;Parisi et al, 2008;Parisi & Del Valle, 2011;Parisi, 2011).…”