  Collisions between large, similar-sized bodies are believed to shape the
final characteristics and composition of terrestrial planets. Their inventories
of volatiles such as water, are either delivered or at least significantly
modified by such events. Besides the transition from accretion to erosion with
increasing impact velocity, similar-sized collisions can also result in
hit-and-run outcomes for sufficiently oblique impact angles and large enough
projectile-to-target mass ratios. We study volatile transfer and loss focusing
on hit-and-run encounters by means of Smooth Particle Hydrodynamics
simulations, including all main parameters: impact velocity, impact angle, mass
ratio, and also the total colliding mass. We find a broad range of overall
water losses, up to 75% in the most energetic hit-and-run events, and confirm
the much more severe consequences for the smaller body also for stripping of
volatile layers. Transfer of water between projectile and target inventories is
found to be mostly rather inefficient, and final water contents are dominated
by pre-collision inventories reduced by impact losses, for similar
pre-collision water mass fractions. Comparison with our numerical results shows
that current collision outcome models are not accurate enough to reliably
predict these composition changes in hit-and-run events. To also account for
non-mechanical losses we estimate the amount of collisionally vaporized water
over a broad range of masses, and find that these contributions are
particularly important in collisions of ~Mars-sized bodies, with sufficiently
high impact energies, but still relatively low gravity. Our results clearly
indicate that the cumulative effect of several (hit-and-run) collisions can
efficiently strip protoplanets of their volatile layers, especially the smaller
body, as it might be common e.g. for Earth-mass planets in systems with
Super-Earths.
