Astrogeology

The linear-mixing Approximation In Silica-water Mixtures At Planetary Conditions

By Keith Cowing

Status Report

astro-ph.EP

September 24, 2024

Filed under astro-ph.EP, Linear Mixing Approximation

The linear-mixing Approximation In Silica-water Mixtures At Planetary Conditions — Pressure-Temperature phase diagram showing Uranus and Neptune adiabats, the fluid-to-superionic transition of pure water (dashed blue) reported in Ref. (Millot et al. 2018), and the melting line of pure SiO2 (dashed red) from Refs. (Millot et al. 2015; Gonz´alez-Cataldo et al. 2016). Simulation performed in this work have been conducted at 3000 K (dotted line). We can observe two phase transitions, the one of pure-water, (blue) which is compatible previous simulations, and another in the 1:4 mixture (purple). The presence of 25% SiO2 molecules delays the superionic water phase by about 70 GPa. The corresponding phase boundary line is sketched (dashed purple) in the vicinity of 3000 K, assuming a temperature dependence similar to the pure-water case (dashed blue). Error bars denote the leftmost (rightmost) point in the pressure line where we observe a fluid (superionic) MD simulation, see Figure. 2. — astro-ph.EP

The Linear Mixing Approximation (LMA) is often used in planetary models for calculating the equations of state (EoSs) of mixtures.

A commonly assumed planetary composition is a mixture of rock and water. Here we assess the accuracy of the LMA for pressure-temperature conditions relevant to the interiors of Uranus and Neptune. We perform MD simulations using ab-initio simulations and consider pure-water, pure-silica, and 1:1 and 1:4 silica-water molecular fractions at temperature of 3000 K and pressures between 30 and 600 GPa.

We find that the LMA is valid within a few percent (<~5%) between ~150-600 Gpa, where the sign of the difference in inferred density depends on the specific composition of the mixture. We also show that the presence of rocks delays the transition to superionic water by ~70 GPa for the 1:4 silica-water mixture.

Finally, we note that the choice of electronic theory (functionals) affect the EoS and introduces an uncertainty in of the order of 10% in density. Our study demonstrates the complexity of phase diagrams in planetary conditions and the need for a better understanding of rock-water mixtures and their effect on the inferred planetary composition.

Valiantsin Darafeyeu, Stephanie Rimle, Guglielmo Mazzola, Ravit Helled

Comments: Accepted for publication in ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2409.14932 [astro-ph.EP] (or arXiv:2409.14932v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2409.14932
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Submission history
From: Ravit Helled
[v1] Mon, 23 Sep 2024 11:36:49 UTC (20,449 KB)
https://arxiv.org/abs/2409.14932
Astrobiology

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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