  By the time icy objects grow to about the mass of Europa and silicate bodies
grow to approximately lunar mass, a large high-speed impact can generate an
intact melt region that allows the dense material to gravitationally segregate,
forming a large density anomaly. If this anomaly generates sufficient
differential stress, it rapidly segregates to the object's center, triggering
whole body differentiation. We used an impact melting model based on the
Hugoniot equations, the linear shock-particle velocity relationship, and the
empirical relationship between shock pressure and distance coupled with a Monte
Carlo simulation of the late accretion process to determine conditions under
which large impacts trigger differentiation in icy bodies. In a gas-free
environment, impacts of projectiles in the satellite's accretion zone have a
small probability of triggering differentiation in surviving proto-satellites
as small as Triton. The probability increases to 100% by the time surviving
proto-satellites grow to the mass of Europa. The impact of heliocentric
particles captured by the central planet also effectively triggers
differentiation in the icy satellites. If the largest projectiles in the
heliocentric distribution are comparable to the mass of the growing satellites,
they trigger differentiation in all surviving proto-satellites by the time they
grow to the mass of Callisto. However, Callisto and Ganymede have different
probabilities of impact-induced differentiation if they captured a small
fraction (0.01 and 0.5%) of the particles they accreted from a distribution of
small (largest particles ~10% of the satellite's mass) heliocentric objects.
Impact induced differentiation explains the apparent difference between these
two objects in a plausible way.
