2013 Annual Science Report
Arizona State University Reporting | SEP 2012 – AUG 2013
Habitability of Water-Rich Environments, Task 5: Evaluate the Habitability of Small Icy Satellites and Minor Planets
The first goal of this project is to determine the internal structure of small icy bodies. Co-I Steve Desch is especially considering 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 radioactive isotopes heat them and their ices might contain antifreezes like ammonia. These models are also extended to the dwarf planet Ceres. Pluto and Ceres are both targets of two NASA missions in 2015: New Horizons and DAWN.
The second goal of this project is to evaluate the chemical composition of aqueous solutions that could have formed shortly after formation of asteroids, KBOs, and moons of giant planets. Co-I Mikhail Zolotov has considered stability of aqueous minerals on the surface of dwarf planet Ceres and suggested formation of the minerals through impacts of ice-rich surface rocks. If correct, this hypothesis implies water-rock differentiation of Ceres by ~3.9 Ga. Zolotov also argued for formation of asteroidal and Europa’s sulfates through low-temperature aqueous oxidation of sulfides by strong oxidants (O2, H2O2) formed through radiolysis of water ice. Desch, in collaboration with JPL scientist Julie Castillo-Rogez, through ASU graduate student Marc Neveu, is considering the geochemistry of subsurface water on Ceres.
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. We are also helping to develop a concept for a mission to return samples from the plumes of Enceladus.
Mikhail Zolotov, Co-I, has evaluated formation conditions of aqueous minerals seen on the surface of the dwarf planet Ceres: a Fe-clay cronstedtite, Mg carbonates and Mg hydroxide (brucite). The thermodynamic analysis shows that formation of these minerals requires an open-system, low-temperature conditions characterized by elevated water/rock ratios and low fugacities of H2 and CO2. The observed mineralogy is more consistent with a near-surface origin than with a formation within Ceres or on planetesimals. The instability of aqueous solutions at the surface implies mineral deposition during transient events of fluidal activity. But a warming of near-surface rocks by thermal processes in the interior requires dehydration of rocks in the interior, which is not consistent with the low density of Ceres. Zolotov suggests that the observed minerals could have formed by impacts of ice-rich surface materials. The minerals could have formed during the Late Heavy Bombardment which affected the Moon and terrestrial planets ~3.9 billion years ago. If correct, this hypothesis implies water-rock differentiation of Ceres before LHB. The impact hypothesis also suggests that surface mineralogy would not inform about habitability of the body. In January 2014, these results will be published in the journal Icarus.
Zolotov has investigated formation conditions of sulfate minerals in parent bodies of chondrites and icy moons. Both thermodynamics and kinetic considerations show that neither elevated temperature nor H2 escape are needed to form sulfates on parent bodies of carbonaceous chondrites. At temperatures below ~150-200oC, sulfides cannot be oxidized by water even if H2 escapes. It is shown that sulfates could have formed through aqueous interaction of sulfides with products of ice radiolysis (O2, H2O2) in the solar nebula and/or on the ice-covered planetesimals. Zolotov suggests that the apparent absence of sulfates on Enceladus and other bodies beyond the Jovian system may indicate a limited irradiation of water ices in the outer solar system. This inference is also consistent with the absence of sulfates from cometary materials. However, sulfates in the putative ocean on Europa could have formed through low-temperature aqueous interaction of accreted O2-bearing water ices with sulfides (Refer to Task 3).
Desch and his students have continued to develop models of the internal structure of KBOs and small icy worlds. These models predict the following structure for such bodies: ice and rock differentiate in the body’s interior, forming a rocky core and icy mantle, but an undifferentiated crust of rock and ice that never melts is predicted to overlie the icy mantle. A major question surrounding these models has been whether such a crust, which is denser than pure ice, can stably overlie the icy mantle. Rayleigh-Taylor instabilities are expected to cause these two layers to overturn, erasing the unstable density inversion. Improved models of differentiation and overturn, including careful descriptions of the instability growth rate and complex ice rheology, were developed by ASU graduate student Mark Rubin. It was found that crust should not overturn, a result presented at the Lunar and Planetary Science Conference and also submitted to Icarus and expected to be published in early 2014.
Meanwhile, Steve Desch has continued to look for observational signatures of undifferentiated crust, especially using KBOs that have suffered mantle-stripping collisions. Cook et al. (2011) predicted that the collisional family associated with the KBO Haumea should include rocky fragments (“black sheep”) as well as the pure-ice fragments, so far exclusively identified as collisional family members. An observational campaign to search for these black sheep KBOs was begun with 3 nights of observations at Las Campanas Observatory in Chile, in June 2013. Desch also presented a model for using the density of Charon to infer the thickness of undifferentiated crust of the pre-impact Pluto, at the New Horizons Science Team meeting in Columbia, MD, in July 2013.
ASU graduate student Marc Neveu has rapidly been developing models of fluid transport and geochemistry in small icy worlds. Work was presented at the Lunar and Planetary Science Conference, and the New Horizons Science Team Meeting. He is extending such models to include Enceladus as well.
Ariel Anbar deepened collaboration with Isik Kanik of the JPL Icy Worlds team, Peter Tsou, and others, to develop the concept of a sample return mission to the plumes of Enceladus, Life Investigations For Enceladus (LIFE). A paper was published in Astrobiology on the LIFE concept (Tsou et al., 2012). In June, the ASU team sponsored a 1-day workshop on LIFE in Monrovia, CA, at which LIFE team members (including Anbar, Adam Monroe, Chris Glein, and Phil Christensen from the ASU team) discussed the mission with staff from JPL and representatives of potential collaborating institutions in Japan, ISAS and JAMSTEC.
PROJECT INVESTIGATORS:Steven Desch
Project InvestigatorMikhail Zolotov
PROJECT MEMBERS:Ariel Anbar
RELATED OBJECTIVES:Objective 2.2
Outer Solar System exploration