Dr T Mitchell
Prof P Sammonds
Applications accepted all year round
Funded PhD Project (European/UK Students Only)
Water ice is an important planetary forming rock in the outer solar system, where it is
the majority phase of the exteriors of many of the jovian and saturnian satellites, and on the terrestrial planets Earth and Mars, where it is found co-existing with silicates. The evolution and processes on the icy planetary bodies of the outer solar system are dependent on the properties of these ice and ice-rock mixtures. The evolution and processes occurring on icy bodies are of particular interest, as current and future missions to icy planetary bodies return new information on the surfaces, compositions and interiors of these bodies. The continuing NASA-ESA Cassini-Huygens mission to the Saturnian system supplements previous datasets such as that returned from the NASA Galileo mission to the Jovian system allowing comparison between the observations of the satellites of Jupiter, and those of Saturn. New missions which are arriving at icy planetary bodies include the NASA New Horizons mission, which will arrive in the Kuiper Belt in 2015, to study the Pluto-Charon system and other Kuiper Belt Objects, and the Dawn mission, which returned data from the asteroid Vesta, and now enters orbit around the potentially ice-rich dwarf planet Ceres in 2015.
Understanding the rheology of ice is important for understanding surface deformation processes in planetary bodies other than the Earth (e.g. the satellites of gas giants), where very little information about the internal structures or evolution is known. As the strength and flow stress of ice are strongly anisotropic, the deformation processes of the ice bodies of planets and satellites depends strongly on how ice texture evolves as a function of deformation, thermal history and time. This is particularly important on planets that are subject to strong tidal forces, where cyclic loading of ice could result in strongly anisotropic structures. Whether or not ice structure and strength can ‘reset’ in one cycle or evolves to a weak state is not fully understood, and if ice has a memory of these multiple cycles, or evolves dynamically. Understanding whether the ice evolution rate is as fast as the tidal forces, will effectively indicate the controls on the strength of the ice shell of these bodies.
In addition, within outer solar system bodies, ice is often combined with other materials, such as rock or salt hydrates. For example, mixed ice-rock rheology is important for understanding sub-surface processes on Mars where geomorphological studies have indicated the presence of non-submarine soft sediment slumps. One interpretation of these features is they are mixed ice-rock features, possibly related to melting of the ice and subsequent mobilization of the rock. As such, the rheological properties of ice-rock mixtures are also important.
The question this PhD proposal will address is: how does evolving fabric development during deformation affect the physical properties (e.g. strength and flow stress) of ice and ice- rock mixtures? In order to address this question, low to high pressure triaxial deformation experiments will be conducted in order to constrain the effect of strain rate, stress state, temperature and time on the development of ice fabric/microstructure, that will be quantified using state of the art imagining techniques (e.g. Electron Back Scatter Diffraction), and related to evolving 3D wave velocity structure during and after deformation. Acoustic emissions and wave velocities will be used to identify the relative roles of brittle and ductile processes, and be related back to quantitative microstructure observations. Deformation experiments on thin specimens of ice/rock mixtures carried out in a variable-pressure SEM equipped with a cryogenic chamber will elucidate deformation mechanisms operating.
The student will extrapolate the behavior of candidate materials from the laboratory scale to the planetary scale. The results of simple preliminary models of planetary differentiation indicate differentiation is dependent on the rock fraction in the rock-ice mixture. Ice-rock rheology in convection modelling plays an important role in inhibiting convection (due to the increased viscosity), promoting heating and making subsurface oceans viable. As icy planetary bodies are not composed of pure water ice, it is important to include the effects of other phases in more advanced models of the planetary processes, in order to create more accurate models. The student will introduce realistic rheological behaviour of rock/ice mixtures into planetary models in order to determine these rheological controls.
The student will gain experience in high pressure deformation experiments in both ice and rock, the characterization of microstructures using state of the art imagining techniques, elastic wave velocity measurements and the scanning electron microscope.
Deadline is March 31st and stipend is £16,057
Middleton, C. A., Sammonds, P. R., Grindrod, P. M., Fortes, A. D., & Vocadlo, L. (2008). The rheology of ice-rock mixtures – application to the satellites of the outer solar system. The Science of Solar System Ices, Houston, USA (2008)
Middleton, C. M., Wood, I. G., Grindrod, P. M., Fortes, A., Hunt, S. A., Zhang, S. Y., . . . Sammonds, P. R. (n.d.). The use of neutron diffraction in studies of planetary ice-rock analogue mixture. International Mineralogical Association 20th General Meeting, Budapest, Hungary (2010)
Prior, D. J., S. Diebold, R. Obbard, C. Daghlian, D. L. Goldsby, W. B. Durham, and I. Baker (2012), Insight into the phase transformations between ice Ih and ice II from electron backscatter diffraction data, Scripta Materialia, 66(2), 69-72.
Payne, A. and Sammonds, P. (Eds.). Evolution of the Antarctic Ice Sheet: new understanding and challenges. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, A364 (1844), 1579-1872 (2006)
Sammonds, P.R. Creep and flow on the icy moons of the outer planets. Science, 311, 1250-1251. doi:10.1126/science.1123985 (2006)
Sammonds, P. R., Boon, S. A., Hughes, N. and Rist, M. Flow of anisotropic ice from the EPICA core. Annals of Glaciology, 30, 1-7 (2000)
How good is research at University College London in Earth Systems and Environmental Sciences?
(joint submission with Birkbeck College)
FTE Category A staff submitted: 34.15
Research output data provided by the Research Excellence Framework (REF)
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