Exploration of Joint Geophysical Inversion of Gravity and Magnetic data for Ocean World Habitability using PlanetProfile

2025-07-09

Introduction: Combined gravity and magnetic induction measurements can characterize subsurface oceans in icy worlds [1, 2]. Joint inversion based on forward modeled geophysical data is a key tool to constrain the presence, thickness, and salinity of oceans at Jupiter’s large icy moons [3, 4]. Large inversion analyses explored the measurement sensitivity to ice and ocean thickness, and the presence/size of a metallic core. But the ocean salinity/density analysis was based on binary solution compositions of NaCl/seawater and MgSO4 owing to limited electrical conductivity and thermodynamic data. But natural waters host numerous ions from the water-rock interactions with metamorphic and metasomatic minerals. Minor dissolved species such as carbonates and ammonia may have outsized contributions to properties sensed by upcoming missions [5].To address geochemical questions in the context of geophysical measurements we implemented speciated thermodynamic and electrical conductivity data into the open source PlanetProfile package and their application to modeling oceans in Europa and Ganymede. This more comprehensive approach reveals limitations in available geochemical datasets. Using such models in more comprehensive inversion analyses is essential to meeting the main objectives of the Europa Clipper and Juice missions, which seek to understand the evolution and habitability of Jupiter’s large icy moons [2,6].Speciated Aqueous Chemistry: Reaktoro is an efficient open source framework that implements thermodynamic property calculations using existing equilibrium databases supcrt16 and phreeqc [7]. Both databases describe Pitzer activity model parameters for the thermodynamics of aqueous solutions. Reaktoro transforms these predictions into plausible self-consistent Gibbs energy-based thermodynamics. This innovation is key for PlanetProfile, which models ocean and ice layers by querying thermodynamic properties such as chemical potential, specific heat capacity, thermal expansivity, density, sound speed, etc. Reaktoro and Phreeqc reproduce the capability of frezchem [8] using the frezchem.dat database to predict the formation of ice Ih eutectic conditions, up to 100 MPa pressure [9]. We implemented Reaktoro as an option in PlanetProfile, using its Phreeqc capability to compute the freezing temperature for a specified ice thickness/bottom pressure. Ammonia and other relevant volatiles are included as an updated frezchem database [10]. Supcrt16 is used at higher temperatures, owing to better coverage of high-pressure behavior up to 500 MPa.High-Pressure Chemistry: We evaluated a simple correction to align Reaktoro predictions with the established SeaFreeze equation of state (EOS) for pure water as Reaktoro adopts by default the IAPWS EOS for water [11]. The resulting adiabatic ocean profiles and formation of high-pressure ice match well with the predictions of an in-preparation EOS representation of laboratory thermodynamics for NaCl(aq). For a given thermodynamic property, we retain the apparent molal contribution of the Reaktoro prediction and substitute the H2O contribution with a SeaFreeze H2O EOS (valid 0.1-5000 MPa and 240-400 K), as . We linearly extrapolate the adjusted supcrt16 predictions above 500 MPa and below 273 K. The adjustments results in plausible adiabatic profiles for Ganymede that reasonably predict the formation of high-pressure ice as compared with the use of an in-development NaCl(aq) EOS [12]. Electrical Conductivity: Ocean electrical conductivity (EC) vs depth [13] uses a recent representation for ionic conductivity pertainsingto 0.1MPa and concentrations up to ~1 molal (ion dependent). It reproduces UNESCO seawater EC values used by the Gibbs Seawater framework also available in PlanetProfile [14].Simulating Aqueous Properties for Icy Ocean Worlds: Fig. 1 depicts the predicted complex magnetic induction responses for fully speciated models for Europa (differentiated by color: Table 1) based on published and new geochemical models [15-19]. Results are shown for representative 5- and 30-km ice shell thicknesses (filled vs. unfilled shapes, respectively). The different ocean compositions and ice thicknesses show a range of amplitude phase shifts (y-axis), with separation larger than the expected 1.5 nT resolution of the Europa Clipper magnetic induction investigation [20]. Fig. 2 depicts how he synodic (Jupiter rotation) signal depends mainly on ice thickness (y-axis) and the orbital signal has characteristics depending on the oxidation state of the ocean (x-axis). Joint Inversion of Magnetic Induction and Gravity Data: A full inversion in the parameter space of Europa’s ocean requires a large statistical data set, likely comprising millions of models or more. This work is part of a larger effort toward joint inversion making use of established methods [21-23] and using the present PlanetProfile simulations to direct new experimental equation of state and electrical conductivity measurements [e.g., 23]. We describe joint predictions of magnetic induction, compared with gravity properties computed using PyALMA3 [24]. References: [1] Roberts, J. H. et al. (2023) Space Sci. Rev., 219(6), 46. [2] Vance, S. D. et al. (2023) Space Sci. Rev., 219(8), 81. [3] Vance, S. D. et al. (2018) JGR: Planets, 10.1002/2017JE005341. [4] Vance, S. D. et al. (2021) JGR: Planets, 10.1029/2020je006418. [5] Castillo-Rogez, J. C. et al. (2022) GRL, 49, e2021GL097256. [6] Van Hoolst, T. et al. (2024) Space Sci. Rev., 220(5), 54. [7] Leal, A. M. M. (2015) https://reaktoro.org [8] Marion, G. M. et al. (2005) GCA, 69(2), 259–274. [9] Toner, J. D. and D. C. Catling (2017) J. Chem. Eng. Data, 62(3), 995–1010. [10] Brown et al. in prep. [11] Wagner, W. and A. Pruss (2002) J. Phys. Chem. Ref. Data, 31(2), 387–535. [12] Klenner, F. et al. (2025) Planet. Sci. J., 6(3), 65. [13] Vance, S. D. et al. (2021) JGR: Planets, 126(2), e2020JE006418. [14] McDougall, T. J. and Barker, P. M. (2011) SCOR/IAPSO WG, 127:1–28. [15] Journaux, B. et al. (2020) JGR: Planets, 125, e2019JE006176. [16] Melwani Daswani, M. et al. (2021) GRL, 48(18), e2021GL094143. [17] Millero, F. J. (2008) Deep Sea Res. I: Oceanographic Res. Papers, 55(1), 50–72. [18] Zolotov, M. and Shock, E. (2001) JGR, 106(E12), 32815–32828. [19] Zolotov, M. Y. and Kargel, J. (2009), Europa, U. AZ Press, Tucson, AZ, 431–458. [20] Kivelson, M. G. et al. (2023) Space Sci. Rev., 219(6), 48. [21] Petricca, F. et al. (2023) GRL, 50(17), e2023GL104016. [22] Cochrane, C. J. et al. (2022) Phil. Trans. Royal Soc. A: Math., Phys. Eng. Sci., 382(2286), 20240086. [23] Psarakis, C. A. et al. (2024) ACS Earth Space Chem., 8(6), 1146–1153. [24] Petricca, F. (2023) Icarus, 417, 116120. Figure 1. Table 1. Figure 2.

Understanding Failure Strain and Hardness in Solid Solutions and High-Entropy Superhard Metal Dodecaborides

Chemistry of Materials · 2025-09-08

A range of intrinsically hard metal dodecaboride binary, ternary, and quinary solid solutions containing Y, Zr, Gd, Hf, and/or Ho were investigated using radial X-ray diffraction under nonhydrostatic compression up to 60 GPa to understand how metal composition influences boron cage structures and therefore controls hardness. Differential strain was measured to study the deformation mechanisms of these materials. Y0.23Zr0.53Gd0.24B12, which has the highest Vickers hardness (HV = 46.9 ± 2.4 GPa at 0.49 N load), was also found to show the highest differential strain in all lattice planes studied, and hardness was found to correlate with differential strain across a range of materials. Differential strain did not correlate in any simple way with material composition, but it did correlate with the density of atom packing within the unit cell if samples were refined using a noncubic unit cell. Specifically, materials with the highest hardness showed anomalously compact atomic packing, with metal atoms of complementary sizes contained in the smallest volume in the rigid boron cage network. This atomic packing hypothesis was then tested by applying it to a series of quinary high-entropy borides (HEBs). The HEBs were found to have a wide range of plateau differential strain values, and all values correlated very well with the proposed atomic packing hypothesis, where higher differential strain/hardness correlates with more compact structures. Finally, the bulk modulus was calculated for all dodecaborides. All dodecaborides have high bulk moduli (K0 > 170 GPa), but the first derivative with respect to pressure (K0′) of most samples is well below 4, indicating that significant compression is possible in these materials before repulsive interactions start to raise the modulus. This structural flexibility is consistent with the compactness hypothesis.

High-pressure studies of size dependent yield strength in rhenium diboride nanocrystals

Nanoscale Horizons · 2024-01-01 · 3 citations

samples, in agreement with its high yield strength. This behavior, likely arises from an increased grain boundary concentration in the smaller nanoparticles. Overall, these results highlight that even superhard materials can be made more mechanically robust using nanoscale grain size effects.

Electrical Conductivity of Subsurface Ocean Analogue Solutions from Molecular Dynamics Simulations

ACS Earth and Space Chemistry · 2024-06-08 · 3 citations

Investigating the habitability of ocean worlds is a priority of current and future NASA missions. The Europa Clipper mission will conduct approximately 50 flybys of Jupiter’s moon Europa, returning a detailed portrait of its interior from the synthesis of data from its instrument suite. The magnetometer on board has the capability of decoupling Europa’s induced magnetic field to high precision, and when these data are inverted, the electrical conductivity profile from the electrically conducting subsurface salty ocean may be constrained. To optimize the interpretation of magnetic induction data near ocean worlds and constrain salinity from electrical conductivity, accurate laboratory electrical conductivity data are needed under the conditions expected in their subsurface oceans. At the high-pressure, low-temperature (HPLT) conditions of icy worlds, comprehensive conductivity data sets are sparse or absent from either laboratory data or simulations. We conducted molecular dynamics simulations of candidate ocean compositions of aqueous NaCl under HPLT conditions at multiple concentrations. Our results predict electrical conductivity as a function of temperature, pressure, and composition, showing a decrease in conductivity as the pressure increases deeper into the interior of an icy moon. These data can guide laboratory experiments at conditions relevant to icy moons and can be used in tandem to forward-model the magnetic induction signals at ocean worlds and compare with future spacecraft data. We discuss implications for the Europa Clipper mission.

Kinetic Isotope Effects During Reduction of Fe(III) to Fe(II): Large Normal and Inverse Isotope Effects for Abiotic Reduction and Smaller Fractionations by Phytoplankton in Culture

Geochemistry Geophysics Geosystems · 2024-06-01 · 2 citations

Abstract Iron stable isotopes (δ 56 Fe) are a useful tool for studying Earth processes, many of which involve redox transformations between Fe(III) and Fe(II). Here, we present two related experimental efforts, a study of the kinetic isotope effects (KIEs) associated with the reduction of Fe(III)‐ethylenediaminetetraacetic acid (EDTA) to Fe(II), and measurements of the biological fractionation of Fe isotopes by phytoplankton in culture. Reductants tested were ascorbate, hydroxylamine, Mn(II), dithionite, and photoreduction at pH between 5 and 9 and temperatures from 0 to 100°C. Isotope fractionations were very large, and included both normal and inverse KIEs, ranging from −4‰ to +5‰. Experiments were reproducible, yielding similar results for triplicate experiments run concurrently and for experiments run weeks apart. However, fractionations were not predictable, without a clear relationship to reaction rate, temperature, pH, or the reductant used. Acquisition of Fe by eukaryotic phytoplankton also often involves the reduction of Fe(III) to Fe(II). Several species of diatoms and a coccolithophore were tested for Fe isotope fractionation in culture using EDTA, NTA, and DFB as Fe(III) chelating ligands, yielding fractionations from −1.3‰ to +0.6‰. Biological isotope effects were also unpredictable, showing no clear relationship to species, growth rate, or Fe concentration. Variability in Fe isotope fractionation observed in culture may be explained in part by the sensitivity of KIEs. This work has implications for the industrial purification of isotopes, interpretation of natural δ 56 Fe, and the use of Fe isotopes as a tracer Fe source and biological processes in the ocean and other natural systems.

Hardening in Tungsten Tetraboride with the Addition of Carbon, Zirconium, and Silicon: Intrinsic <i>vs</i> Extrinsic Effects

Chemistry of Materials · 2024-03-20 · 3 citations

Alloys of tungsten tetraboride (WB4) with the addition of C and Si were prepared by arc-melting of the constituent elements. The phase purity was established by powder X-ray diffraction (PXRD) and surface morphology by scanning electron microscopy (SEM) analysis. Vickers hardness measurements showed hardness enhancement for alloys with a nominal composition of (W0.98Si0.02):11.6B and (W0.95C0.05):11.6B of 52.2 ± 3.0 and 50.5 ± 2.5 GPa, respectively, compared to 41.2 ± 1.4 GPa for pure WB4. (W0.92Zr0.08):11.6B was determined in previous work to have a hardness of 55.9 ± 2.8 GPa. Bulk moduli were calculated following analysis of high-pressure radial diffraction data and were determined to be 329 ± 4 (K0′ = 2) and 390 ± 9 (K0′ = 0.6) GPa for 8 atom % Zr and 5 atom % C-doping, respectively, compared to 326–339 GPa for pure WB4. Computational analysis was used to determine the dopant positions in the crystal structure, and it was found that Zr primarily substitutes W in the 2c position, Si substitutes for the entire B3 trimers, and C inserts in the Bhex-layer. The hardness enhancement in the case of Zr-doping is attributed primarily to extrinsic hardness effects (nanograin morphology), in the case of C─to intrinsic effects (interlayer bond strengthening), and in the intermediate case of Si─to both intrinsic and extrinsic effects (bond strengthening and fine surface morphology).

Electrochemical Kinetics of Stable Isotopes (Final Report)

2023-07-18

Stable isotopes are sensitive indicators to the mass transport and chemical reactions that occur during potentiostat-controlled electroplating of metals from aqueous (and some nonaqueous) metal-salt solutions.The goal of this research program is to measure the stable isotope fractionation during electrochemical processes, how the isotope-dependent rates are sensitive to chemistry, temperature, and electrochemical kinetics, and to use these measurements to develop a theoretical framework for a kinetic isotope effect that is able to predict kinetic isotope fractionation factors. Executive Summary:The main objective of this grant was to understand the kinetic isotope effect associated with electrochemical reactions.The specific aims of this project were to: (1) continue measurements of the kinetic isotope effect associated with potentiostatic-electroplating transition metals from their salt solutions, as a function of temperature, electrochemical kinetics, and mass transport to the electrode; and (2) use this data set to help build and test theoretical frameworks that are able to predict trace element separation (including isotopes) as a function of kinetics of their chemical reactions.The outcome of this proposed work is an enhanced understanding of kinetic isotope and trace-element effects, which is useful for interpreting observations in natural and anthropomorphic environments, as well as for tailoring chemical reactions that can either promote or hinder chosen separations of isotopes and/or trace elements.

Exploring the hardness and high-pressure behavior of osmium and ruthenium-doped rhenium diboride solid solutions

APL Materials · 2023-03-01 · 2 citations

Rhenium diboride (ReB2) exhibits high differential strain due to its puckered boron sheets that impede shear deformation. Here, we demonstrate the use of solid solution formation to enhance the Vickers hardness and differential strain of ReB2. ReB2-structured solid solutions (Re0.98Os0.02B2 and Re0.98Ru0.02B2, noted as “ReOsB2” and “ReRuB2”) were synthesized via arc-melting from the pure elements. In-situ high-pressure radial x-ray diffraction was performed in the diamond anvil cell to study the incompressibility and lattice strain of ReOsB2 and ReRuB2 up to ∼56 GPa. Both solid solutions exhibit higher incompressibility and differential strain than pure ReB2. However, while all lattice planes are strengthened by doping osmium (Os) into the ReB2 structure, only the weakest ReB2 lattice plane is enhanced with ruthenium (Ru). These results are in agreement with the Vickers hardness measurements of the two systems, where higher hardness was observed in ReOsB2. The combination of high-pressure studies with experimentally observed hardness data provides lattice specific information about the strengthening mechanisms behind the intrinsic hardness enhancement of the ReB2 system.

Anomalous thermal transport under high pressure in boron arsenide

Nature · 2022 · 89 citations

• Condensed matter physics
• Materials science
• Physics

Enhanced Hardening Effects on Molybdenum-Doped WB₂ and WB₂–SiC/B₄C Composites

Chemistry of Materials · 2022 · 5 citations

• Materials science
• Composite material
• Metallurgy

Tungsten diboride (WB2) solid solutions with increasing molybdenum (Mo) substitution were synthesized by resistive arc-melting from the pure elements and characterized for their mechanical properties. The WB2-type structure is maintained up to 30 atomic percent (at%) Mo substitution. W0.70Mo0.30B2 achieved a maximum Vickers hardness of 45.7 ± 2.5 GPa at 0.49 N, resulting in the hardest WB2 solid solution to date. In agreement with this fact, high-pressure radial diffraction studies indicate that substitution of Mo into WB2 strengthens metal–boron bonding, as the solid solution supports high differential stress and has a bulk modulus of 355 ± 2 GPa. WB2 and W0.70Mo0.30B2 composites were then synthesized with increasing additive content (0–30 wt%) of B4C or SiC to study extrinsic hardening effects through multiphase formation. These composites show extrinsic effects on the Vickers hardness because of secondary-phase precipitation. While WB2–30 wt% B4C exhibited the highest hardness (53.8 ± 6.0 GPa at 0.49 N), WB2–30 wt% SiC demonstrated the slowest oxidation rate. This work offers new insights for tailoring transition-metal boride systems with optimized hardness, grain morphology, and thermal stability.