Multiphase reactive transport in planetary ices
Buffo, Jacob J.
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As an ice-ocean world itself, Earth provides a number of analog environments that can be used to better understand the dynamics of ice-ocean processes occurring on bodies like Europa [Eicken, 2002; Gleeson et al., 2012; Marion et al., 2003]. Natural and laboratory grown sea ice provides an accessible sample of ocean-derived ice, where the effects of the local thermochemical environment on ice formation rate, microstructure, and biogeochemistry can be studied in detail [Wettlaufer, 2010]. The remote environment of sub-ice shelf cavities provides an additional analog to the subsurface ocean of Europa. Devoid of sunlight, trapped beneath kilometers of overlying ice, and with limited contact to the open ocean these regions can aid in our understanding of the circulatory, biogeochemical, thermodynamic, and accretion/ablation processes that may occur beneath Europa’s ice shell [Lawrence et al., 2016]. Together, these possibilities motivate Chapter 2, which focuses on building a comprehensive model of the combined influence of temperature gradients, salinity, and ice nucleation within the water column on the properties of terrestrial ices. Quantifying how environmental factors impact the dynamics and properties of terrestrial ices can then be extended to improve estimates of the characteristics and behavior of planetary ices subject to diverse thermochemical regimes [Buffo et al., in review; Buffo et al., 2019]. This ability to predict physicochemical properties of planetary ices informs numerical simulations of ice-ocean world geophysics, chemical cycling, and habitability and provides context for the synthesis and interpretation of spacecraft data. Our foundational and relatively extensive understanding of the terrestrial cryosphere provides immense leverage when attempting to decipher the complex innerworkings of much less fully understood ice-ocean worlds and provides a benchmark for validating numerical models. In Chapter 3, the foundation provided by work in Chapter 2 is extended to accommodate the composition and dynamics of Europa’s ice-ocean environment. The impacts of ocean composition and thermal regime on the state of the ice shell are explored. Fundamentally, this connects ocean properties to observable surface features via accretion and evolution of the ice shell that become accessible to future missions. The physics of multiphase materials are applicable across a wide range of Earth and planetary problems; however, the dynamics of reactive transport depend on the environmental and material properties of the system. The physical, thermal, and chemical properties of ice-ocean/brine systems in the solar system remain relatively unconstrained but likely span a substantial trade space. Moreover, the extent to which ice-ocean world environments can be assumed to be well described by known mushy layer physics has not been investigated as this area of study represents a relatively new element of planetary science. To that end, Chapter 4 explores the limits of the physics contained within this work, and comments on areas where new work and additional physics may be needed to realize a fully comprehensive systems understanding of Europa’s and other worlds’ ice shell(s). There are certainly limitations to the applicability of Earth as an analog to other ice-ocean worlds and these must be identified and accounted for to ensure an appropriate use of the transitive strategies comparative planetology offers. For example, the chemistries of oceans and ices throughout the solar system may be quite diverse [Kargel et al., 2000; Neveu et al., 2017; Zolotov, 2007; Zolotov and Shock, 2001]. The addition of exotic salts or ammonia to an ice-ocean system may alter its solidification dynamics and resultant ice properties [Fortes, 2000; Hammond et al., 2018]. While laboratory experiments have begun to investigate the effects of these additives on ice properties [Lorenz and Shandera, 2001; McCarthy et al., 2007], determining their impact on processes occurring at geophysical scales relies on theoretical predictions as no terrestrial analogue exits. The age and scale of many planetary ices also exceeds that of the oldest and thickest ice on Earth [Fretwell et al., 2013]. Thus, while Earth provides an excellent, well-studied endmember for the dynamics and properties of planetary ices there may be as yet unseen deviations in the physical behavior and characteristics of ice-ocean systems under thermochemical pressures found only on other bodies in the solar system. In the concluding Chapter 5, I highlight accomplishments that have allowed this work to make significant steps towards best reconciling Earth and planetary ices, and comment on future directions and ongoing work that will enable me to continue to make progress. In particular, I discuss the future of incorporating biological elements into these models and benchmarking this relatively new work through laboratory and field programs. This multidisciplinary approach provides the greatest foundation by which the work described here can make a future impact on how we measure and understand ice-ocean worlds.