Collisional and structural properties of water ice in planet-forming regions

Hill, Catherine (2015). Collisional and structural properties of water ice in planet-forming regions. PhD thesis The Open University.



With the number of detected exoplanets standing at close to 2000, it seems that planets are ubiquitous throughout the universe. However, the processes leading to their formation are not well understood. It is widely accepted that planets form by dust aggregation from the material of protoplanetary disks, with micron-sized particles sticking together by van der Waals forces and kilometre-sized particles sticking together due to gravity. The process of growth between millimetre and kilometre sizes is yet to be explained, despite decades of research studying the collisions of silicate dust particles which form the main component of protoplanetary dust. However, the water ice that is present in the outer regions of protoplanetary disks has so far received less consideration.

In this thesis, the collisional and structural properties of ices analogous to those in planet-forming regions were studied experimentally. Low velocity collisions of millimetre and centimetre-sized crystalline ice particles (both pure water and ice composed of pure water and water containing 5% methanol or formic acid) were investigated using a dedicated experimental set up for use on board parabolic flights. The porosity and pore collapse of amorphous solid water (ASW) grown at a variety of temperatures was investigated using neutron scattering.

The results presented in this thesis show that crystalline water ice particles do not stick at relevant collision velocities, casting doubt on their ability to enhance planet formation by particle adhesion. However, the results of the neutron scattering experiments suggest that ASW is likely to remain porous at temperatures below 121 K, which may increase the likelihood of particle sticking. Sticking may also be enhanced by ice restructuring during pore collapse. The results of the experiments growing ASW at temperatures below 77 K show that these ices have different porosities, but further work is needed to fully characterise this.

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