Rheological behavior of icy mixtures with application to the outer planets

(proposal summary)

The diverse icy worlds of the outer solar system invite speculation about a spectrum of dynamic internal and surface processes. One piece of the dynamics puzzle that we are well positioned to address is the mechanical response of icy “rocks” to geologic forces. Understanding the flow behavior of ices, that is, the relationship between applied stress and the resulting rate of deformation, helps planetologists model and understand the evolution and current state of icy moons. Our laboratory program takes a systematic approach to establishing flow relationships for relevant materials and combinations of materials, at relevant conditions of low temperatures and (often) high pressures. We conduct mechanical tests followed by careful microanalysis of deformed samples in order to understand the physics of deformation sufficiently well to intelligently extrapolate to planetary settings, where deformation rates are many orders of magnitude slower than in the lab. Our workhorse instrument is a custom designed and built deformation high-pressure cryogenic creep apparatus, constructed years ago under NASA support.

There are two tasks. Task 1 involves systematic measurement and analysis of the rheological behavior of water ice I in the regime of composite flow by grain size sensitive and grain size insensitive mechanisms of deformation. Existing rheological laws (some developed by us) suggest that viscosity of an ice-I-rich outer layer on a large icy moon, including a moon as small as Enceladus, may be strongly grain size dependent; yet in the current description of convective flow of ice I on the planets, there is a conspicuous gap in knowledge of the grain size of ice I. Here we propose to test the hypothesis that grain size is not a free variable when ice I deforms over large strains for long periods of time, but is instead controlled by environmental conditions such as temperature and the stress state. We also aim to establish constitutive laws for calculating what the grain size distribution of ice I will be in the planetary setting. As an example, current models allow the ice I shell on Jupiter’s Europa to be anywhere from 20 to 120 km thick. A priori knowledge of grain size resulting from our work will narrow this range.

Task 2 is a study of several mixtures of ice I and planetary materials. We will measure the flow behavior ice I + clathrate hydrates of methane and certain other species to develop rheological constraints in several specific planetary settings, most notably Saturn’s moons Titan and Enceladus. Mixtures of icy materials are implicated in a wide range of planetary settings, and the work proposed here will not only help explain the existence of geysers on Enceladus and methane in Titan’s atmosphere, but more generally will help us develop systematics for two-phase flow that might be applicable to polyphase materials not yet explored in the lab. We will also measure the effect on grain size sensitive flow in fine-grained ice I of small concentrations (less than 0.05 by weight) of ammonia dihydrate, which is known to weaken ice.

Together, these two tasks are relevant to deformation processes throughout the solar system, including, but not limited to: solid-ice convection and subsurface ocean stability within icy moons; viscous crater relaxation in ice-rich terrain; cryovolcanism in the outer solar system; and terrestrial and non-terrestrial glaciation. Our work plan is focused on aiding the interpretation of new mission data by taking full advantage of our one-of-a-kind testing facilities, as well as the expertise and techniques that we have developed over the past 30 years of continuous involvement in this field.