Frozen Worlds: How High-Pressure Ice Phases can Influence Their Structure.

Abstract:

Exploration by spacecraft of the icy bodies of the outer solar system has revealed that the moons of the giant planets are not just dead, frozen amalgamates of ice and rock but are active worlds, with surface reforming processes taking place. Some could support subsurface oceans. Ice and water hence play a major role in the physical processes that take place on those frozen worlds and on other cold exoplanets across the galaxy.

The geological features of such frozen worlds can be emulated by models constructed from distinct layers of ice within their interiors. Increasing pressures en route for their cores, cause phase changes take place. These transform water-ice into a variety of high-pressure polymorphs. Phases, such as ices II, III and V all have contact with the liquidus and occur in the pressure, temperature regime likely to be found in the mantle of an icy moon.

Each phase of ice has a different density and its own set of elastic moduli. Consequently, an accurate evaluation of these parameters will be highly useful to those scientists engaged in understanding the planetary geology of such icy worlds or building computer models of the likely mantle processes taking place inside them.

Our work involves manufacturing samples of the ice phases existing at pressures below ~2GPa. Their high-pressure structures are then preserved at ambient pressure in the laboratory by means of recovery and storage under liquid nitrogen. Samples of ices II, III, V and VI were subsequently investigated using the High-Resolution Powder Diffractometer (HRPD) at the ISIS facility in the UK.

These experiments were carried out at ambient pressure over a temperature range of 10-150K, using neutron powder diffraction techniques to accurately determine their thermal expansion coefficients. Structural refinements were carried out at 10K and these, together with the extrapolated zero temperature volumes, will provide reliable parameters for future computer simulations.