  Water-rich super-Earth exoplanets are expected to be common. We explore the
effect of late giant impacts on the final bulk abundance of water in such
planets. We present the results from smoothed particle hydrodynamics
simulations of impacts between differentiated water(ice)-rock planets with
masses between 0.5 and 5 M_Earth and projectile to target mass ratios from 1:1
to 1:4. We find that giant impacts between bodies of similar composition never
decrease the bulk density of the target planet. If the commonly assumed maximum
water fraction of 75wt% for bodies forming beyond the snow line is correct,
giant impacts between similar composition bodies cannot serve as a mechanism
for increasing the water fraction. Target planets either accrete materials in
the same proportion, leaving the water fraction unchanged, or lose material
from the water mantle, decreasing the water fraction. The criteria for
catastrophic disruption of water-rock planets are similar to those found in
previous work on super-Earths of terrestrial composition. Changes in bulk
composition for giant impacts onto differentiated bodies of any composition
(water-rock or rock-iron) are described by the same equations. These general
laws can be incorporated into future N-body calculations of planet formation to
track changes in composition from giant impacts.
