Constraining the flow and fracture of planetary ices through laboratory experiment
Abstract
Like snowflakes, no two self-gravitating icy bodies in the outer solar system are the same. Essentially all of those visited by interplanetary spacecraft show unique forms of surface geology. Internal dynamics analogous to Earth's are indicated on the larger bodies. The icy fraction of these dirty snowballs is based on H2O, but important amounts of other phases consisting of CO2, CH4, N2, and other molecules exist. NH3 has been implicated cosmochemically but is curiously absent from spacecraft observations. Imagery and remote sensing of the icy moons has given planetologists plenty of fodder for evolutionary models. Our job in the laboratory has been to provide some constraint to these models in the form of physical properties measurements of the relevant icy phases. We specialize in rheological measurements, that is, the relationship between stress and rate of plastic deformation, at in situ conditions of pressure and temperature. In this talk we will review some of these relationships to give a feel for the relative strength of ices and for importance of the various environmental variables. To date we have provided some measure of constraint, quite detailed in some instances and very limited in others, for the flow of pure water ice I and its phases II, III, V, and VI; clathrate hydrates of CH4 and CO2; hydrates of NH3 and of some sulfate salts, pure CO2, and mixtures of several of these phases with each other and with silicate dust and other hard particulates. Huge gaps in our knowledge base still remain. Our lab experiments focus on pure phase polycrystalline materials with carefully controlled grain texture and fabric. Given the geology of our own planet, we should expect petrologic and structural complexity in the real setting. The current focus of our research is in the important area of rheology of mixtures, where our studies include (1) the role of hard particulates in the flow of ordinary ice I at planetary conditions, (2) the rheology of mixtures of two deformable icy phases, and (3) the role of grain-size-sensitive (GSS) vs. grain-size-insensitive (GSI) deformation mechanisms in water ices I and II. The last topic combines the physics of multiphase flow (fine-grained and coarse-grained ice being viewed as rheologically distinct phases) with aspects of the effects of particulates on grain growth. Since ice I predominates on the surface and upper layers on nearly all bodies, the playoff between GSS and GSI creep in the ices is particularly important to the interpretation of surface geology of all the icy moons, and to the thermal structure of the larger moons.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.P42B..02D
- Keywords:
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- 0738 Ice (1863);
- 3902 Creep and deformation;
- 5104 Fracture and flow;
- 5422 Ices;
- 6200 PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS