Constraints to Uranusʹ Great Collision—II

It is generally assumed that the Jovian planets share common features as a consequence of similar formation and evolutionary processes. Nevertheless, Uranus is the only one without observed outer satellites and with a large spin axis inclination. This obliquity is usually attributed to a great tangential collision with another protoplanet during the stage of planetary formation. This collision could also justify the lack of outer satellites of the planet. Dynamical constraints to Uranusʹ Great Collision are settled in connection with the observational evidence. The transfer of angular momentum at collision is considered, which allows the two free parameters of the problem (the impactor incident velocity υi and its mass mi) to be reduced to one, giving a relation υi(mi). By means of simplified energy considerations, it is also possible to set an upper bound on the impactor velocity as a function of its mass. In addition, the most probable situation is considered in which the impactor would have been bound to the Solar System. In this case the maximum allowed value for υi is ∼ 31.54 km s−1, being the minimum possible mass to satisfy the required angular momentum to tilt Uranus ∼ 1–1.1 m⊕. The lower allowed value for υi corresponds to a body undergoing a temporary satellite capture. This value is ∼ 22 km s−1, being the minimum mass for this body ∼ 1.5 m⊕. On the other hand how the orbits of hypothetical, preexisting satellites of Uranus are unbound as a consequence of the energy injected by the collision is considered. The relevant result is that any massless particle orbiting Uranus beyond ∼ 70–90 planetary radii was probably swept out from the system. A further intensive search for faint objects orbiting Uranus beyond 70 planetary radii would provide a constraint to the Great Collision Hypothesis.