  It is widely accepted that Titan and the mid-sized regular satellites around
Saturn were formed in the circum-Saturn disk. Thus, if these mid-sized
satellites were simply accreted by collisions of similar ice-rock
satellitesimals in the disk, the observed wide diversity in density (i.e., the
rock fraction) of the Saturnian mid-sized satellites is enigmatic. A recent
circumplanetary disk model suggests satellite growth in an actively supplied
circumplanetary disk, in which Titan-sized satellites migrate inward by
interaction with the gas and are eventually lost to the gas planet. Here we
report numerical simulations of giant impacts between Titan-sized migrating
satellites and smaller satellites in the inner region of the Saturnian disk.
Our results suggest that in a giant impact with impact velocity > 1.4 times the
escape velocity and impact angle of ~45 degree, a smaller satellite is
destroyed, forming multiple mid-sized satellites with a very wide diversity in
satellite density (the rock fraction = 0-92 wt%). Our results of the
relationship between the mass and rock fraction of the satellites resulting
from giant impacts reproduce the observations of the Saturnian mid-sized
satellites. Giant impacts also lead to internal melting of the formed mid-sized
satellites, which would initiate strong tidal dissipation and geological
activity, such as those observed on Enceladus today and Tethys in the past. Our
findings also imply that giant impacts might have affected the fundamental
physical property of the Saturnian mid-sized satellites as well as those of the
terrestrial planets in the solar system and beyond.
