  The formation of Uranus' regular moons has been suggested to be linked to the
origin of its enormous spin axial tilt (~98^o). A giant impact between
proto-Uranus and a 2-3 M_Earth impactor could lead to a large tilt and to the
formation of an impact generated disc, where prograde and circular satellites
are accreted. The most intriguing features of the current regular Uranian
satellite system is that it possesses a positive trend in the mass-distance
distribution and likely also in the bulk density, implying that viscous
spreading of the disc after the giant impact plays a crucial role in shaping
the architecture of the final system. In this paper, we investigate the
formation of Uranus' satellites by combining results of SPH simulations for the
giant impact, a 1D semi-analytic disc model for viscous spreading of the
post-impact disc, and N-body simulations for the assembly of satellites from a
disc of moonlets. Assuming the condensed rock (i.e., silicate) remains small
and available to stick onto the relatively rapid growing condensed water-ice,
we find that the best case in reproducing the observed mass and bulk
composition of Uranus' satellite system is a pure-rocky impactor with 3 M_Earth
colliding with the young Uranus with an impact parameter b = 0.75. Such an
oblique collision could also naturally explain Uranus' large tilt and possibly,
its low internal heat flux. The giant impact scenario can naturally explain the
key features of Uranus and its regular moons. We therefore suggest that the
Uranian satellite system formed as a result of an impact rather than from a
circumplanetary disc.
