Molecular and pore-scale structure evolution in amorphous solid water

The pore structure of vapour deposited amorphous solid water (ASW) is of both fundamental interest and astrophysical importance, where the system's properties are believed to play a major role in processes such as star and planetary body formation. Here we report a comprehensive characterisation of D₂O ASW in the temperature range of 20 to 180 K, using combined total and small-angle neutron scattering. Significant changes to the ice morphology are observed across the temperature range 100 to 150 K, whereby there is a significant loss of specific surface area and porosity; the transition of 3D to 2D dominated pore shapes and a general compaction of the ice. The initial structure of nanometer scale microporous islands with voids between them at low temperature evolves with annealing into compact islands with larger voids. Even past crystallisation, there is still porosity present until desorption due to persisting void volume.