2011 Annual Science Report
Arizona State University Reporting | SEP 2010 – AUG 2011
Habitability of Water-Rich Environments, Task 3: Evaluate the Habitability of Europa's Subsurface Ocean
Europa is of keen interest to astrobiologists and planetary geologists because of indications that it may possess a sub-surface ocean. In this task we seek to understand the global distribution and timing of Europan geologic units, to identify regions of recent activity. We also seek to map chemical signatures on its surface. Simultaneously we are numerically modeling the convection in the icy shell overlying the subsurface ocean, with a particular aim of determining whether chemical species from the ocean can be brought to the surface. Through this combined approach we seek to understand Europa’s ocean’s pH and composition, and evaluate the habitability of this icy moon.
Previous versions of the global geologic map of Europa obtained in R. Greeley’s group (Doggett et al., 2007, 2009) are being updated (Bunte, Greeley et al. 2011) to further characterize geologic units and structural features and to illustrate the surface and near sub-surface history. The global map will serve to identify areas of recent geophysical activity which may be indicative of habitable environments. Initial work has been performed in collaboration with Steve Chien, Rebecca Castano, and David Thompson at the Jet Propulsion Laboratory to correlate chemical signatures obtained from Galileo Near Infrared Mapping Spectrometer (NIMS) with geologic features (Bunte et al., 2011). Such correlations allow the identification of young surface areas and may yield insights into the composition of the subsurface. That work will be resumed after completion of geological mapping.
Published models reveal a great diversity of possible oceanic compositions of Europa. M. Zolotov used an advanced thermodynamic modeling to evaluate the composition of oceanic fluids in equilibrium with suboceanic rocks. In particular, the stability of phyllosilicates (serpentine, clay minerals), which affects concentration of Na and pH of fluids, was evaluated. It is shown that high pressure (1.5-2 kbar) at the ocean-rock interface on Europa corresponds to elevated fugacity (f) of H2 in closed systems. Increase in pressure leads to higher contents of dissolved H2 and organic solutes. The results show that high pressure (high fH2) and low temperature stabilize Fe2+-rich serpentine, and favor higher serpentine/saponite ratio, lower Na content in saponite, and higher Na concentration in solution. A low Na content in a smectite clay saponite (or complete lack of saponite) in high suboceanic rocks may lead to NaOH dominated (Na > Cl) oceanic waters with pH as high as 12-13. These numbers are higher than previous evaluations. This high pH would affect the habitability of the putative Europan Ocean.