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Characterizing the Physics of Tidal Dissipation in Icy Bodies

Tess Caswell
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Abstract Text: 

Volatiles exist in three states (solid, liquid and gas) throughout the Solar System. We are performing experiments to characterize the behavior of the solid state of water ice, in particular, and its relationship to the internal evolution of tidally-forced, ice-rich bodies such as the moons of Mars and numerous satellites in the outer Solar System. When these icy worlds undergo tidal stresses, their internal evolution depends strongly upon the “Quality Factor” (Q) of the body and its parent. In geophysics, Q is generally expressed as its inverse, attenuation (or Q-1). Under cyclic loading, Q-1 describes how much heating occurs in a tidally-forced body; it quantifies the physical processes at the heart of tidal dissipation. Accurately modeling the evolution of icy bodies requires understanding the physics of attenuation before extrapolating laboratory measurements of Q-1 to planetary conditions.
In geological settings, these physics are dominated by solid-state chemical diffusion, which has a distinct length-scale dependence that is frequently cited as the grain size. The experiments of McCarthy & Cooper [2016], however, measured grain size-insensitive attenuation in polycrystalline ice that was simultaneously creeping at steady-state. These authors’ data can instead be normalized by steady-state creep stress, implying that deformation-induced microstructure affects the magnitude of attenuation. Determining the relationship between microstructure and attenuation is therefore critical to understanding tidal dissipation in icy bodies.
We are conducting two sets of experiments to characterize the role of deformation microstructure in tidal dissipation in ice: creep/attenuation and stress reduction. The stresses, grain size, and temperature of the experiments place our specimens within rheological regimes relevant to the interiors of icy bodies.
Ongoing creep/attenuation experiments simulate tidal forcing by applying a low-amplitude, sinusoidal “tidal” stress to specimens that are simultaneously creeping at steady-state. By measuring the attenuation spectrum across a range of creep stresses, we are directly quantifying the relationship between attenuation and deformation.
Stress-reduction experiments generate a deformation microstructure via creep, then explore the microstructure’s response to transient loading through stress reductions. Our results are affected by deviatoric stress, but not by grain size, confirming a response that is dictated by deformation. The microstructures of deformed samples, analyzed via cryogenic electron backscatter diffraction (EBSD) and reflected light microscopy, suggest that the dominant stress-sensitive microstructural feature may be the structure of the boundaries between ice grains.
In summary: our stress-reduction experiments confirm a characteristic length scale, founded in stress-sensitive microstructure, which dictates the response of polycrystalline ice to transient loading such as tidal stress. Our ongoing creep/attenuation experiments will parameterize this microstructural effect on attenuation, allowing more confident extrapolation of laboratory attenuation measurements to models of icy body evolution.

J. Schiemann"...
SSERVI Identifier: 

Recognizing that science and human exploration are mutually enabling, NASA created the Solar System Exploration Research Virtual Institute (SSERVI) to address basic and applied scientific questions fundamental to understanding the Moon, Near Earth Asteroids, the Martian moons Phobos and Deimos, and the near space environments of these target bodies. As a virtual institute, SSERVI funds investigators at a broad range of domestic institutions, bringing them together along with international partners via virtual technology to enable new scientific efforts."