2012 Annual Science Report
Arizona State University Reporting | SEP 2011 – AUG 2012
Habitability of Water-Rich Environments, Task 5: Evaluate the Habitability of Small Icy Satellites and Minor Planets
The goal of this project is to determine the internal structure of small icy bodies, especially the objects like Pluto and its moon Charon, which are Kuiper Belt Objects (KBOs). The possibility exists that these icy bodies may contain liquid water at great depths, despite their frigid surface temperatures and small sizes, because radioactivities heat them and their ices might contain antifreezes like ammonia. We are also evaluating the chemical composition of aqueous solutions which could have formed shortly after formation of asteroids and moons of giant planets. Another of our tasks is to estimate chemical composition of methane-rich liquids that are present at the surface of Titan at extremely low temperatures.
Co-I M. Zolotov has evaluated composition of fluids in water-rich Solar System bodies such as large asteroids, icy moons, and trans-neptunian objects. The work included modeling of aqueous alteration of CI-type chondritic materials through calculation of chemical equilibria in water-rock-gas type systems in wide ranges of water to rock ratios, temperatures, and pressures. Initial fluids were pure water, the HCl solution that signified melted ices with HCl hydrates, and a C-rich fluid formed through melting of a simulated cometary ice. The results show formation of NaCl-, NaHCO3-, Na2CO3- or NaOH-type fluids, depending on conditions of water-rock interaction. No abundant sulfates have been observed in closed low-temperature models from which hydrogen did not escape.
The models show that hypothetical melting of HCl-bearing ices leads to early acidic fluids enriched in Cl, Mg, Fe, Na, K, Mn, Al, and P, consistent with some signs of low-pH alteration in chondrites. Neutralization of these solutions leads to alkaline Na-rich fluids. Despite possibly acidic pH of early fluids on Enceladus, alkaline solutions could have dominated throughout history of the icy moon.
The work of M. Zolotov also shows that the high Na/K ratio in these grains indicates low-temperature (< 0 oC) aqueous processes in history of Enceladus, while the low Na/K ratio in the exosphere of Europa is consistent with hydrothermal processes. Our work shows that Na-bearing clays (Na-saponite) are likely secondary minerals in interior of Enceladus. However, other types of fluids predicted in our work may not exist on Enceladus.
Recent ASU graduate student Chris Glein used his thermodynamic model to explore some geochemical processes on Titan’s surface. It was found that the equilibrium composition of surface liquids depends on the mixing ratio of CH4 in the local atmosphere. This means that we may be able to infer the composition of Titan’s lakes and seas if the abundance of gaseous CH4 can be determined at the base of the atmosphere in Titan’s polar regions. Modeling also revealed that CH4-rich and C2H6-rich liquids behave (unexpectedly) differently in a geochemical sense. Atmospheric N2 is more soluble in CH4-rich liquids, while organic minerals, such as C2H2, are dramatically more soluble in C2H6-rich liquids. Ongoing research is focused on elucidating the full range of chemical weathering possibilities on Titan.
During the past year, SESE graduate student Melissa Bunte has continued to make significant progress on an updated and enhanced geologic map of Europa. This work combines Galileo and Voyager imaging data sets to provide the most detailed assessment yet of the occurrences and physical characteristics of plains, chaos, bands, ridges, and crater materials on the icy satellite’s surface. Of particular importance is the Bunte et al. (2012) mapping and identification of “framework lineaments” that provide key stratigraphic marker boundaries for important geologic/resurfacing events in Europan history. Once completed (it will be submitted for USGS map review in early 2013), this map will serve as an excellent foundation for future landing site studies as well as other orbital- and flyby-based studies of the geology and astrobiology of Europa.
Work has continued on the issue of subsurface liquid water in small icy bodies (Kuiper Belt Objects, or KBOs). Steve Desch has worked with two students, Mark Rubin and Marc Neveu, to develop models of crustal overturn by Rayleigh-Taylor instability in partially differentiated KBOs. This work concludes that overturn is greater than had been predicted by Desch et al. (2009), but KBOs are still predicted to retain crusts 50-100 km thick, so that the models of Desch et al. (2009) are largely validated. This work was presented by Desch at a science team meeting of New Horizons in Flagstaff in August/September 2011, and again by Desch and Neveu to the Dawn Science Team at the Jet Propulsion Laboratory in August 2012. We have expanded our collaboration with the Dawn Science Team, with project scientist Julie Castillo-Rogez in particular. Models of Ceres geochemistry will be developed by Marc Neveu as the bulk of his doctoral thesis, and will be done in collaboration with Julie Castillo-Rogez.
Steve Desch has also strengthened his collaboration with Karen Meech and Sarah Sonnett at the University of Hawaii, who are also NAI members. This collaboration has resulted in two observing proposals submitted to Steward Observatories, proposing to observe a quite of KBOs in the Haumea collisional family to seek spectral signatures of incomplete partial differentiation of KBOs.