As part of the diffuse interstellar medium, water molecules contribute extensively to the chemical properties of astronomical environments, both in the gaseous and adsorbed state. Along with carbon monoxide, water is expected to deposit on silicate dust grains in space, forming icy films which may provide a substrate for exotic, low-temperature surface chemistry and act as a reservoir for further deposited species. On account of the nature of its aggregation – vapour deposition over millions of years in environments as cold as 10K – the water ice is not expected to form a crystalline structure as under terrestrial conditions, but rather a metastable glass: ‘amorphous solid water’, ASW. In this work we used molecular dynamics to model the deposition of water onto cold substrates of both silica (representing a bare grain surface) and cubic crystalline ice (representing an ice-covered grain migrating back from warmer regions), with the intention of understanding film growth and properties in circumstellar environments. The water was injected at different energies, onto substrates of different temperatures, to determine what effect the thermal conditions of the water / substrate interaction had on the ice films formed.Significant differences were observed between those molecules deposited under the coldest (10K deposition on 10K substrate) and warmest (300K deposition on 130K substrate) regimes. Cold aggregation, as would be expected in the core of a dark molecular cloud, produced tall, filamentous structures with cavities. Meanwhile, warmer conditions produced dense, comparatively flat films.
|Date of Award||17 Mar 2017|
- University Of Strathclyde
|Supervisor||Neil Hunt (Supervisor) & Ben Hourahine (Supervisor)|