Abstract
Water, methane, and ammonia are commonly considered to be the key components of the interiors of Uranus and Neptune. Modelling the planets’ internal structure, evolution, and dynamo heavily 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 Uranus and Neptune profiles (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 GEKKO XII and LULI2000 laser facilities using standard optical diagnostics (Doppler velocimetry and optical pyrometry) to measure the thermodynamic state and 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.
Highlights
IntroductionWhile it is customarily assumed that their peculiar structure is due to the fact that they originate in a thin outer shell, it is not clear if the main contribution to the planetary dynamo is given by hydrogen, which is present in the atmosphere and becomes metallic at around 1 Mbar[3,14], or by the icy mixture in the deep interiors[15,16]
The complex behaviour of these mixtures at planetary interior conditions – pressures of several megabar (1 Mbar = 100 GPa) and temperatures of a few thousand Kelvin – is at the basis of several gaps in our understanding
We have measured the thermodynamic state of the shocked sample and the optical reflectivity of the shock front up to a pressure of 2.6 Mbar using standard rear-side optical diagnostics: two Doppler velocity interferometers (VISAR), a VISAR-independent reflectivity measure, and a streak optical pyrometer (SOP)
Summary
While it is customarily assumed that their peculiar structure is due to the fact that they originate in a thin outer shell, it is not clear if the main contribution to the planetary dynamo is given by hydrogen, which is present in the atmosphere and becomes metallic at around 1 Mbar[3,14], or by the icy mixture in the deep interiors[15,16] Resolving this situation is even more urgent today, given that the number of known exoplanets keeps growing steadily with lots of them similar in size and radius to Uranus and Neptune. We have measured the thermodynamic state of the shocked sample and the optical reflectivity of the shock front up to a pressure of 2.6 Mbar using standard rear-side optical diagnostics: two Doppler velocity interferometers (VISAR), a VISAR-independent reflectivity measure, and a streak optical pyrometer (SOP)
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