Title: Micrometer-Sized Water Ice Particles for Planetary Science Experiments: Influence of Surface Structure on Collisional Properties,
Abstract: Models and observations suggest that ice-particle aggregation at and beyond
the snowline dominates the earliest stages of planet-formation, which therefore
is subject to many laboratory studies. However, the pressure-temperature
gradients in proto-planetary disks mean that the ices are constantly processed,
undergoing phase changes between different solid phases and the gas phase. Open
questions remain as to whether the properties of the icy particles themselves
dictate collision outcomes and therefore how effectively collision experiments
reproduce conditions in pro- toplanetary environments. Previous experiments
often yielded apparently contradictory results on collision outcomes, only
agreeing in a temperature dependence setting in above $\approx$ 210 K. By
exploiting the unique capabilities of the NIMROD neutron scattering instrument,
we characterized the bulk and surface structure of icy particles used in
collision experiments, and studied how these structures alter as a function of
temperature at a constant pressure of around 30 mbar. Our icy grains, formed
under liquid nitrogen, undergo changes in the crystalline ice-phase,
sublimation, sintering and surface pre-melting as they are heated from 103 to
247 K. An increase in the thickness of the diffuse surface layer from $\approx$
10 to $\approx$ 30 {\AA} ($\approx$ 2.5 to 12 bilayers) proves increased
molecular mobility at temperatures above $\approx$ 210 K. As none of the other
changes tie-in with the temperature trends in collisional outcomes, we conclude
that the surface pre-melting phenomenon plays a key role in collision
experiments at these temperatures. Consequently, the pressure-temperature
environment, may have a larger influence on collision outcomes than previously
thought.