A molecular simulation study of amorphous solid water under extraterrestrial conditions

  • John MCCANN

Student thesis: Doctoral Thesis

Abstract

This thesis studies Amorphous Solid Water (ASW) using Molecular Dynamics for the purpose of investigating what governs asteroid, planet and comet formation, which in turn provides the foundations for life to exist in the universe. This thesis begins by presenting the astronomical background to this study, focusing on our current understanding of how the universe evolved from the Big Bang to its current state, and the role which interstellar dust grains have played in stellar evolution. It is currently believed that the icy mantles which reside on interstellar dust grains in cold dense regions of the interstellar medium (ISM) act as a catalyst allowing the formation of prebiotic molecules. The catalytic properties of icy mantles responsible for synthesis of these molecules depend on the concentration of unsatisfied, or 'dangling', ASW hydrogen bonds within these icy mantles, Observational studies do not detect dangling O-H bonds but experimental studies do. This study suggests that while ASW dangling O-H bonds do exist in these icy mantles, most water molecules exist in a hydrogen bonded network with about 3.78 hydrogen bonds per water molecule. This should be compared with crystalline ice for which all four hydrogen bonds per molecule. Moreover, this work also indicates that there are more dangling bonds in the interfacial region than the bulk region.Work in this thesis also investigates the role that 'shadowing' and 'electrostatic steering' play in ASW growth on extraterrestrial dust surfaces. Currently, it is believed that the coral-like structure of ASW is caused solely by shadowing, a geometric phenomenon. However, this work finds that although shadowing has an influence, ASW growth and its subsequent structure are primarily determined by electrostatic steering, i.e. a process where depositing water molecules are steered towards dangling O-H bonds on the ASW surface. This thesis concludes by contrasting these new findings with the properties and structure of ASW, and describing the implications for our understanding of stellar evolution.
Date of Award1 Oct 2008
LanguageEnglish
Awarding Institution
  • University Of Strathclyde

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