  The ice-giant planet Uranus likely 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 significant 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.
