The mystery of the formation of the satellites of Uranus revealed

The ice-giant planet Uranus, the 8th planet of the solar system, has its axis of rotation and its satellites inclined by 98°. Therefore, Uranus “rolls” on its orbit unlike the other planets. Shigeru Ida and his team suggest that uranian satellites formation is due to the evolution of a water vapor disk generated by a giant impact.

Abstract: The ice-giant planet Uranus probably underwent a giant impact, given that its spin axis is tilted by 98 degrees. That its satellite system is equally inclined and prograde suggests that it was formed as a consequence of the impact. However, the disks predicted by the impact simulations generally have sizes one order smaller and masses two orders larger than those of the observed system at present. Here we show, by means of a theoretical model, that the Uranian satellite formation is regulated by the evolution of the impact-generated disk. Because the vaporization temperature of water ice is low and both Uranus and the impactor are assumed to be ice-dominated, we can conclude that the impact-generated disk has mostly vaporized. We predict that the disk lost a substantial amount of water vapour mass and spread to the levels of the current system until the disk cooled down enough for ice condensation and accretion of icy particles to begin. From the predicted distribution of condensed ices, our N-body simulation is able to reproduce the observed mass–orbit configuration of Uranian satellites. This scenario contrasts with the giant-impact model for the Earth’s Moon, in which about half of the compact, impact-generated, solid or liquid disk is immediately incorporated into the Moon on impact.