A molecular dynamics study of the effect of annealing temperature on the structure of ASW

Amorphous solid water (ASW) is a disordered form of ice created by low-temperature and low-pressure vapour deposition. The ASW deposited under these conditions are usually very porous - allowing for a large amount of other molecular species to be stored within these pores. However, metastable interstellar ASW seems to lose porosity as a function of time or temperature. The chemical physics understanding of ASW pore evolution remains unresolved. This paper utilises molecular dynamics simulations to track the structural evolution of vapour deposited ASW upon annealing, using the TIP4P/2005 water potential. Our results exhibit good quantitative agreement with laboratory experiments, despite the time and size limitations of MD simulations. Upon annealing, our ice structures undergo significant compaction and pore collapse. These changes are found to be governed by a very subtle mechanism in the ice, wherein the water molecules continuously undergo small rearrangements until the highest temperatures above 160 K.