Laser-driven shock compression of “synthetic planetary mixtures” of water, ethanol, and ammonia
2019
Water, methane, and ammonia are commonly considered to be the key components of the interiors of
Uranusand
Neptune. Modelling the planets’ internal structure, evolution, and
dynamoheavily relies on the properties of the complex mixtures with uncertain exact composition in their deep interiors. Therefore, characterising icy mixtures with varying composition at planetary conditions of several hundred gigapascal and a few thousand Kelvin is crucial to improve our understanding of the
ice giants. In this work, pure water, a water-ethanol mixture, and a water-ethanol-ammonia “synthetic planetary mixture” (SPM) have been compressed through laser-driven decaying shocks along their principal Hugoniot curves up to 270, 280, and 260 GPa, respectively. Measured temperatures spanned from 4000 to 25000 K, just above the coldest predicted adiabatic
Uranusand
Neptuneprofiles (3000–4000 K) but more similar to those predicted by more recent models including a thermal boundary layer (7000–14000 K). The experiments were performed at the
GEKKOXII and LULI2000 laser facilities using standard optical diagnostics (Doppler velocimetry and optical pyrometry) to measure the
thermodynamic stateand the shock-front reflectivity at two different wavelengths. The results show that water and the mixtures undergo a similar compression path under single shock loading in agreement with Density Functional Theory Molecular Dynamics (DFT-MD) calculations using the Linear Mixing Approximation (LMA). On the contrary, their shock-front reflectivities behave differently by what concerns both the onset pressures and the saturation values, with possible impact on planetary
dynamos.
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