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Project Progress

Since INV 3 began, team members have presented and published new ideas pertaining to the habitability of Europa, Ganymede, and Pluto, and we have made crucial progress in developing the needed underpinnings for investigating deep ocean world geobiochemistry.

ICY WORLD MODELING STUDIES
Vance is leading a model for the evolution of the Europa system that estimates the coupled fluxes of hydrogen and oxygen in its ocean, and compares this simple global redox model with the Earth system. The model, a project with collaborators Pappalardo and Kevin Hand (Vance et al., under revision)—presented at AbSciCon (Vance et al. 2015d), and more recently at the ELSI Symposium and Georgia Tech Earth and Atmospheric Science colloquium—is being revised for publication. The high fluxes of oxidants and reductants mean that Europa might have produced complex multicellular life.

Vance’s 2015 summer student, Garrett Levine a sophomore at Caltech, has continued their work during the 2015-2016 school year. Levine will attend the LPSC meeting to present simulations of Europa’s ocean composition (Levine et al. 2016) building on work by prior-year student Marika Leitner, now a graduate student at Cornell working with Jonathan Lunine.

To constrain how habitable environments may develop and evolve in ocean worlds, investigators Bills, Choukroun, Goodman, Kimura, Sotin, and Vance are developing models to constrain the interior structure, orbital evolution, and solid state ice convection in Ganymede. The models can be adapted to other worlds using information from spacecraft missions, which constrain their bulk density, density structure, and composition. A long term plan of the team is to develop a common model that incorporates features of our work, and to make that model publicly available.

Vance has modified the team’s self-consistent interior model for Ganymede (Vance et al. 2014), the first such model to include affects of salinity on ice dynamics. Recent summer student Rodolfo Batisti Negri helped add the ability to track Ganymede’s thermal evolution through time. Vance has applied the model to interior structures of other ocean worlds, Europa, Callisto, Titan, and Triton. Publication of new model results awaits an equation of state for salts that is optimized to the high pressure conditions identified as being beyond the reach of the team’s equation of state for MgSO₄ (Vance and Brown 2013). In the coming year, the team will begin moving beyond MgSO₄ as the sole model ocean composition, as described below.

Investigator Kimura has modeled the evolution of Ganymede through time, focusing on running over a longer time base of 4 billion years, with necessarily lower spatial precision and simpler thermodynamics (Kimura et al. 2015a). Kimura recently modified his model to add ocean chemistry for both MgSO₄ and ammonia using information provided by Vance (Kimura et al. 2015b). The model also includes effects of the latent heat of melting and freezing of different ices present in Ganymede’s interior. A major result and insight he latent heat from freezing of hundreds of km of ice is required to explain the present-day existence of Ganymede’s ocean. Future work by the team will consider how tides from Jupiter and Europa may have influenced Ganymede’s evolution, with implications for convection in its rocky mantle, maintenance of its intrinsic magnetic field, and the introduction of chemical energy into the ocean. Concurrent with this work, Kimura is working with other science team members for the JUpiter ICy moon Explorer (JUICE) to determine what signatures of oceanic and tidal motion could be observed (Kimura and Kuramoto 2015, Kamata et al. 2015)

Collins is developing a model for the tidal and thermal evolution of Pluto, which will help to understand whether it presently has a liquid water ocean in its interior. He presented early results from this model at the American Geophysical Union meeting in San Francisco (Collins and Barr 2015). He has also been working with a student and a diverse team of geology experts from around the country to critically examine the nature of putative plate tectonics on Europa (Cutler et al. 2015).

Sotin is working with Caltech postdoc Klara Kalousova to determine how melting in high pressure ices influences the composition and thermal evolution of Ganymede (Kalousova et al. 2015). The team finds that melting is inevitable in high pressure ices, which can also convect like Earth’s mantle. The results, shown schematically in Figure 1, imply that high pressure ices are warm and that fluids move freely from the lower icy layer to the upper icy layer over millions of years (as assumed by Vance et al. 2014). Co-I Goodman has developed his own thinking on fluids in high pressure ices, constructing a model for porous flow with in the ice (Goodman 2016). The model is a complement to the work by Sotin and others in that it predicts what happens in the ice when less melting occurs. Ganymede and other large oceanic moons are clearly very complex worlds. Salinity and temperature are probably heterogeneous, and gradients in composition and temperature may provide niches for life.

NEW THERMODYNAMICS FOR ICY WORLDS
Our team is working to provide new thermodynamic data to consider different environments in ocean worlds, many of which will have conditions of pressure and temperature overlapping with those at subduction zones on Earth. We seek to accurately model the chemistry and production of possible pre-biotic materials in hydrothermal systems that cannot exist on Earth, but which may be common in ocean worlds and watery exoplanets. The strategy involves both new laboratory experiments and new analytical capabilities.

New laboratory facilities for ocean world chemistry are coming online at the University of Washington, built by investigators Brown and Vance, and postdoc Olivier Bollengier (Figure 2). We can measure sound speeds, the most sensitive with precision of a part in 10,000, in materials at pressures exceeding those at the bottom of Ganymede’s H₂O layer, as high as 1.2 GPa or 12,000 atmospheres, and temperatures from 180 K to 420 K. We are leveraging the new data with previously published values at higher pressures to construct broadly applicable thermodynamic frameworks applicable to broader pressures and temperatures in oceanic exoplanets.

Investigator Brown has developed a powerful new methodology that will allow us to process large the large parameter space of equation of state data needed for understanding the deep interiors of ocean worlds. The new methodology (Brown, in prep) uses modern computing power and geophysical inverse methods to create localized fits of sparse data sets. The methodology is analogous to modern tools for digitially viewing and processing photos. The toolbox can replace traditional polynomial based fitting methods, which are notoriously difficult to construct, requiring hand fitting of individual parameters, and nonlineear in extrapolation. We are thus promoting an era of “do-it-yourself” thermodynamics. Brown presented the new methodology as part of colloquium for the Earth and Space Sciences department at the University of Washington, and subsequently at the American Geophysical Union (Brown et al. 2015). Brown is writing up the model for publication, and packaging the toolbox for common use.

We are establishing the new thermodynamic capabilities as a foundation for further modeling of the habitability of ocean worlds by revising common equations of state for water, MgSO₄ (aq), and ammonia over a broad range of pressures. Brown has analyzed all prior thermodynamic data for liquid water, and has developed a new equation of state for water appropriate for chemical simulations in Ganymede and exoplanets. Bollengier has obtained new measurements for water that revise the equation of state in the imporant region of the freezing point at pressures in Ganymede’s deep ocean (Bollengier et al., in prep). Vance has completed the first part of a revised ammonia-water equation of state for application to Titan and analogous exoplanets. The new equation of state for pure ammonia (Vance and Brown, in prep) incorporates sound speed measurements for the first time. The resulting heat capacities are substantially different from prior predictions, with implications for the deep interiors of gas giants and the tidal heat generated in their oceanic satellites.

In related work, Co-I’s Kargel and Marion visited JPL recently to discuss modifications to FREZCHEM, and possibly enlisting as a postdoc recent U. Arkansas PhD and former Vance summer student, Amira Elsenousy.

References:
Bollengier, O., J. M. Brown, G. Shaw, S. Vance The speed of sound and thermodynamics of aqueous solutions to 700 MPa. I. Pure water. In prep.

Brown, J. M. Local Basis Function Representations of Thermodynamic Surfaces: Water at High Pressure and Temperature as an Example, In prep.

Collins, G. and A. Barr. 2015. Melting and Tectonics from Coupled Orbital and Thermal Evolution of the Pluto-Charon System. AGU Fall Meeting, P51A-2045.

Cutler, B., G. Collins, L. Prockter, G. Patterson, S. Kattenhorn, A. Rhoden, C. Cooper. 2015, Reconstructing Plate Motions on Europa with GPlates. AGU Fall Meeting, P31B-2059.

Goodman, J.C. 2016, Snow, Slush, or Solid? Latent Heat Transfer Through Porous High-Pressure Ice Layers in Icy Satellites and Other Water Worlds. LPSC, 2836.

Kalousova, K., S. Sotin, G. Tobie, G. Choblet, and O. Grasset. 2015, Two-phase convection in the high-pressure ice layer of the large icy moons: geodynamical implications. Fall AGU, P31C-2078.

Kimura J. Vance S. Hussmann H. Kurita K. (2015), Stability of an Internal Ocean in Ganymede, AbSciCon contribution #7773.

Kimura, J., and K. Kuramoto, 2015, “Interior evolution of Ganymede and its surface manifestation: toward JUICE measurements”. JpGU Meeting, Chiba.

Kamata, S., Kimura, J., Matsumoto, K., Nimmo, F., Kuramoto, K., 2015 “Tidal deformation of Ganymede and effects of a subsurface ocean: a model calculation in preparation for JUICE-GALA measurements”. AOGS Meeting, Singapore.

Levine, W.G., M.A. Leitner, and S.D. Vance. 2016, Geochemical Constraints on Europa’s Ocean Composition and Possible Signatures of Hydrothermal Activity. 47th LPSC, 2500.

Vance, S., and J. Brown. (2013), Thermodynamic properties of aqueous MgSO₄ to 800 MPa at temperatures from -20 to 100 °C and concentrations to 2.5 mol kg−1 from sound speeds, with applications to icy world oceans. Geochimica et Cosmochimica Acta, 110:176–189.

Vance, S.D. and J. M. Brown, A Local Basis Function Equation of State for Liquid Ammonia, in prep.

Vance, S., M. Bouffard, M. Choukroun, and C. Sotin. (2014) Ganymede’s internal structure including thermodynamics of magnesium sulfate oceans in contact with ice. Planetary and Space Science, 96:62–70.

Vance S. D. Barnes R. Brown J. M. Bollengier O. Journaux B. Sotin C. Choukroun M. (2015c) On the Role of Oceans in the Thermal Evolution and Habitability of Super-Europas and Super-Ganymedes. AbSciCon contribution #7760.

Vance, S., Barnes, R., Brown, J. M., Bollengier, O., Sotin, C., & Choukroun, M. (2015b, March). Interior Structure and Habitability of Super-Europas and Super-Ganymedes. In Lunar and Planetary Science Conference (Vol. 46, p. 2717).

Vance, S., Brown, J. M., Choukroun, M., Bollengier, O., Journaux, B., Sotin, C., & Barnes, R. (2015a, February). Thermodynamic Equations of State for Aqueous Ammonia and Sodium Chloride Applied to Exoplanet Oceans and Interiors. In Physics of Exoplanets: From Earth-sized to Mini-Neptunes, proceedings of a conference held Feb. 23-27, 2015 at the Kavli Institute for Theoretical Physics/Edoted by Eric Ford, Louise Kellogg, Geoff Marcy, Burkhard Militzer. (Vol. 1, p. 6).

Vance S. D., Hand K. P. Pappalardo R. T. (2015d). The Potential for High Fluxes of both Oxygen and Hydrogen in the Europa System. AbSciCon Invited Contribution #7222.

Vance, S., D., K. P. Hand, and R. T. Pappalardo. Geophysical controls of chemical disequilibria in Europa and other wet, rocky worlds, under revision.