2012 Annual Science Report
Arizona State University Reporting | SEP 2011 – AUG 2012
Habitability of Water-Rich Environments, Task 2: Model the Dynamics of Icy Mantles
One of Jupiter’s moons, Europa, is one of the few places in the solar system in which the physical and chemical conditions may be suitable for sustaining life. Europa is composed on an outer H2O layer, comprised of rigid ice overlying a liquid water ocean. It is this liquid water ocean which has been hypothesized as having the ingredients necessary for life, but it is shielded from our observation by the thick ice layer. However, under certain conditions, the ice layer is expected to undergo convection, possibly transporting chemicals from the liquid ocean to the surface, where we may be able to detect them. We perform computer modeling of ice/ocean convection to investigate how ocean material is carried up through the ice layer and whether it is expected to reach Europa’s surface. This work provides guidance for future missions which may probe the chemistry of the ice surface.
Note: this work was initiated with funds from the NAI proposal, but is now currently funded by NASA Outer Planets.
In May 2011, Divya Allu Peddinti started as the primary graduate student on this project. Throughout 2011, she learned computer programming, the essential numerical methods, and finite-element modeling. Then, McNamara trained her in numerical modeling of fluid convection. Finally she performed numerical modeling to reproduce several published results on single-phase ice convection appropriate for Europa. She proved to be an excellent student and learned the pre-requisite necessities at an exceptionally fast pace.
In 2012, she began to perform 2-phase (ice and liquid water) convection calculations, toward the aims of this research. The 2-phase convection calculations provide a self-consistent boundary between Europa’s ice shell and liquid water ocean beneath, allowing for material transport across the interface and an evolving ice shell thickness. Because these calculations have a much higher sensitivity to input parameters than typical 1-phase calculations, Divya performed an extensive parameter-sensitivity study, a necessary prerequisite for this work. For example, ice shell thickness is highly sensitive to amount of heat generation (i.e., tidal heating) and the amount of heat flow from the rocky mantle below, magnitude of viscosity, and variation of viscosity with temperature. Also, one of the key technical elements of this research is to approximate the liquid water ocean with a low viscosity fluid (higher viscosity than actual water in order for the calculations to be computationally tractable while being low enough to simulate negligible viscous coupling between the ice and water). Divya performed a study to find an appropriate viscosity for the proxy fluid, and because this value controls the rate of heat transferred through the “liquid ocean”, she performed numerous calculations to understand and document the thermal properties of the system (resulting in a particular ice shell thickness) as a function of our choice for the low-viscosity proxy fluid. In short, she has performed most of the preliminary technical work necessary for this project. She has presented this work at the CIDER summer program (http://www.deep-earth.org/summer12.shtml). She also submitted an abstract to AGU, and she will present this work there as well in December.
Divya is currently working toward her first publication. Our preliminary work shows that even for an ice shell of relatively constant thickness, there is significant mass transfer across the interface (i.e., ice melting at the base of downwellings and new ice forming at the base of upwellings). With assistance from McNamara and Zolotov, Divya is currently employing the heat of fusion into our 2-phase convection code, and we will examine how this additional heat term is expected to influence ice convection. Furthermore, she is carrying out work to understand how a thickening ice shell (for a cooling Europa) undergoes the transition from conductive to convective heat transport.