2009 Annual Science Report
Arizona State University Reporting | JUL 2008 – AUG 2009
Habitability of Water-Rich Environments, Task 5: Evaluate the Habitability of Small Icy Satellites and Minor Planets
Small bodies in the outer Solar System may harbor liquid water-ammonia oceans and many of the chemical ingredients of life. In this project, we are assessing how much liquid may be present in the Kuiper Belt and the geochemical evolution of Saturn’s volatile-rich moons Enceladus and Titan.
We have rapidly evolved a very active and diverse research effort studying the habitability of small icy bodies.
Co-I Steven Desch, working with graduate student Simon Porter, investigated the internal evolution of Kuiper Belt Objects (KBOs) and icy satellites, and the diagnostic significance of spectral features on the surfaces of these bodies. This work builds on efforts of former student Jason Cook.
A significant milestone for this group was met in summer 2009 with a publication in Icarus. This paper presents numerical simulations of the internal evolution of KBOs including ammonia, carefully re-examining significant inputs such as the viscosity and thermal conductivity of rock and ice-ammonia mixtures. These models are unique in considering a partially undifferentiated crust. The authors find that all factors are favorable for maintenance of liquid, and that if ices on KBOs similar to Charon (density 1.65 g/cc, 604 km radius) contain just a few percent ammonia, then preservation of some liquid is possible, even to the present day. Figure 1 illustrates our predictions for the interior structure of Charon. This work also considers the ability of such liquids to reach the surface via self-propagating cracks, enabling cryovolcanism. It is hoped that moments-of-inertia measurements and imaging of the surface by the New Horizons mission will test these predictions for Charon.
Cryovolcanism is one explanation for the presence of crystalline water ice on the surfaces of outer solar system bodies, but it may not be a universal explanation. For example, Desch and Porter quantified the rates at which micrometeorite impacts deposit heat and locally anneal surface ice that would otherwise be amorphous. This work was submitted as a manuscript to Icarus in August 2009 and will be presented at the October 2009 Division of Planetary Sciences Conference.
Porter is also beginning work on the evolution of KBOs that suffer large impacts. His thesis research will involve implementing sophisticated equations of state for water ice into FLASH adaptive-mesh hydrodynamics code. The plan is to consider the effects of impacts involving KBOs that have already internally evolved according to the models of Desch et al. (2009); no previous work has considered the fate of a crust in a partially undifferentiated KBO during impacts. The specific goal is to model the formation of the KBO Haumea.
In related work, co-I Mikhail Zolotov applied thermodynamic models to explain mineral assemblages in the Viragano carbonaceous chondrite. The results led to evaluation of temperature, pressure and redox conditions in a parent asteroid during the time of aqueous alteration. The results can be used to evaluate those conditions in potentially habitable large bodies (Ceres, icy satellites). The corresponding paper appeared in Earth and Planetary Science Letters.
Co-Is Desch, Zolotov and Everett Shock, together with graduate student Christopher Glein, have been investigating moons in the Saturn system.
Saturn’s icy moon Enceladus emits various organic and inorganic species and gases. Glein contributed to a Nature paper published in July, 2009, on the composition of the plumes on Enceladus (Waite et al., 2009). The paper shows that Enceladus’ interior could contain primordial cometary volatiles (e.g., HCN) and hydrothermally processed species (e.g., N2). To explain these observations, Zolotov has developed a model that explains the emission of salt-bearing icy particles together with primordial and processed species. The idea is that primordial species are preserved in never-melted primordial ice-rock crust that sunk into a primordial ocean on early Enceladus. The model also implies that eruptive plumes’ composition represents frozen primordial ocean rather than today’s aqueous fluids on Enceladus. The model will be presented at the 2009 AGU fall meting.
Glein, Shock and Desch also performed theoretical geochemistry research on the origin of the atmosphere of Saturn’s giant moon Titan. Their work culminated with a conference presentation (Glein et al., 2009a), and with a manuscript that is in press (Glein et al., 2009b). The major conclusions were that Titan’s atmospheric methane is likely to be a primordial chemical species that was accreted by the moon, whereas the molecular nitrogen in Titan’s atmosphere may have been produced from the oxidation of primordial ammonia in hydrothermal systems on Titan. These findings shed new light on the juxtaposition of primordial and endogenic chemistries that occur on icy satellites, and provide new insights into the origin and evolution of the crucial biogenic elements carbon and nitrogen in the outer solar system. They also set the stage for more detailed studies of the chemistry and habitability of icy worlds.
Collaborator Greeley participated in modeling of aeolian transport of particles in the surface of Titan.