Planetary ice

 
 

With a focus on the cryominerals likely found as secondary phases on icy moons of the outer solar system, my PhD project at Brown focused on various physical properties of ice and two-phase ice/hydrate aggregates. I examined eutectic solidification in binaries of H2O-MgSO4, H2O-Na2SO4, H2O-H2SO4, and H2O-Na2CO3 and found that the eutectic morphology of these systems could be predicted and understood from thermodynamic and kinetic analyses and that the physics controlling crystallization of well-understood metal-alloy systems holds true for water/salt binary systems.


Using two separate instruments (a high-pressure gas deformation apparatus and an ambient pressure dead weight apparatus), I measured the creep strength of the binaries and found that the viscosity of ice-I/magnesium sulfate hydrate is approximately two orders of magnitude greater at low stress than that of pure polycrystalline ice at the same grain/colony size and for the same thermodynamic potentials. At high stress, however, the binaries showed significant weakening due to semi-brittle behavior. The presence of the dispersed hydrate in these specimens significantly suppresses the glide of dislocations; the result is a material both stronger and more brittle than pure ice.


Additionally I designed and implemented a high-strain-resolution load train and cryostage for a servo-mechanical (INSTRON) testing apparatus that I used to measure the attenuation and modulus dispersion in polycrystalline ice and of ice/hydrate aggregates. Energy dissipation in pure polycrystalline ice was found to be due to diffusion on low-angle (subgrain) boundaries augmented by non-linear losses wrought by glide of lattice dislocations. The attenuation of pure polycrystalline ice, as well as that of the eutectic aggregates, was found to be significantly greater than that predicted by inversion of a Maxwell solid model (i.e., one informed by the steady-state viscosities of the materials). The reason for this behavior is that the transient aspects of the mechanical response, which are ignored in the simple Maxwell model, are actually responsible for a significant portion of the dissipation. The results from my research should provide more realistic parameters for models of multi-phase flow and attenuation, ones based upon a broader understanding of the underlying physics.


The anomalous heat fluxes that have been observed on icy moons have been attributed to both energy dissipation (above) and to frictional heating from sliding on faults. For my postdoctoral project at Lamont-Doherty, I will investigate dynamic friction using a custom-designed cryogenic friction apparatus. Using a double-direct shear configuration, I will slide ice samples past one another to obtain frictional properties as functions of things like normal stress and temperature. We will apply a sinusoidal driving force to simulate the tidal loading (and resulting frictional heating) of pre-existing cracks in an icy satellite.



 

Photos (clockwise from top left):

  1. 1.Jupiter’s moon Europa

  2. 2.Eutectic microstructure of magnesium sulfate hydrate and ice

  3. 3.in front of the low-temperature bath used to fabricate samples

  4. 4.Anomalous heat flux observed on Saturn’s moon Enceladus

Experimental studies on planetary ice