2010 Annual Science Report
Arizona State University Reporting | SEP 2009 – AUG 2010
Habitability of Water-Rich Environments, Task 5: Evaluate the Habitability of Small Icy Satellites and Minor Planets
A better understanding of the physical and chemical properties of putative aquatic systems on icy satellites is needed to assess their potential for life. We developed new models to assess the origins of carbon and nitrogen species on Titan, the composition and salinity of an ocean on Enceladus, the chemical energy available for metabolism on Enceladus, and the formation of crystalline ice on the surfaces of icy moon.
The origin of atmospheric methane and N2 on Titan was studied by Graduate Student Christopher Glein and colleagues using equilibrium thermodynamic models. This is an important topic in astrobiology, because Titan’s atmosphere may provide insights into the chemistries of carbon and nitrogen on icy moons. Glein et al. (2009) tested the hypothesis that Titan’s atmospheric methane formed in hydrothermal systems by predicting the deuterium/hydrogen (D/H) ratio of hydrothermal methane, and comparing that value to the measured D/H ratio of atmospheric methane. It was found that the two values are inconsistent, so it was concluded that atmospheric methane did not form in hydrothermal systems. Instead, Glein et al. (2009) suggested that Titan obtained methane directly as clathrate hydrate during the satellite’s accretion from cold icy planetesimals. The apparent absence of hydrothermal methane on Titan most likely means that hydrothermal systems have not existed on Titan, or the geochemistry of hydrothermal systems on Titan has not been conducive to the formation of methane. It was found that hydrothermal conditions that prevent the production of methane promote the production of N2. This implies that Titan’s N2 could have an aqueous origin.
Glein and Everett Shock completed a project on the geochemistry of a subsurface ocean on Saturn’s moon Enceladus. They related recent measurements of sodium chloride in Saturn’s E ring to chemical-physical properties of Enceladus’ putative ocean, using geochemical and geophysical modeling. Geochemical modeling shows that Enceladus may have a large and dilute ocean. They suggest that this ocean underlies terrains significantly northward of the geologically active south polar region. While these inferences are perfectly consistent with present data, Glein and Shock (2010) also explored alternative ocean models for completeness, and predicted some of their characteristics, such as size and salinity. A hypothesis for the anomalously large energy output of Enceladus was also developed using geophysical modeling, which showed that partial freezing of an ocean could be a major contributor to the observed heat flux of Enceladus.
Mikhail Zolotov presented arguments for dry sources of plume gases emitted from Enceladus. He also used thermodynamic calculations to assess amounts of chemical energy available for metabolism in Enceladus’ subsurface aqueous systems. It is shown that the comet-like abundances of major plume gases and apparent redox disequilibria in putative aquatic systems are consistent with a minimal influence of aqueous processes on endogenic chemical reactions and may indicate abiotic interior.
Zolotov also evaluated physical-chemical conditions in the interior of largest asteroid Ceres and argued for habitable interior. Some insights about habitability of icy moons have been obtained from these models. Finally he reviewed chemical processes of formation and evolution of oceans in the outer solar system.
Graduate Student Simon Porter and Steve Desch investigated whether or not spectroscopic observations revealing the presence of crystalline water ice on icy bodies is a signature of cryovolcanism. They demonstrated that sufficient kinetic energy is delivered by micrometeorites to anneal this ice, showing that cryovolcanism is not necessarily implied.