The Speed of Sound in Methane under Conditions of the Thermal Boundary Layer of Uranus

Kavli Affiliate: Peter Graham

| First 5 Authors: Thomas G. White, Hannah Poole, Emma E. McBride, Matthew Oliver, Adrien Descamps

| Summary:

We present the first direct observations of acoustic waves in warm dense
matter. We analyze wavenumber- and energy-resolved X-ray spectra taken from
warm dense methane created by laser-heating a cryogenic liquid jet. X-ray
diffraction and inelastic free electron scattering yield sample conditions of
0.3$pm$0.1 eV and 0.8$pm$0.1 g/cm$^3$, corresponding to a pressure of
$sim$13 GPa and matching the conditions predicted in the thermal boundary
layer between the inner and outer envelope of Uranus. Inelastic X-ray
scattering was used to observe the collective oscillations of the ions. With a
highly improved energy resolution of $sim$50 meV, we could clearly distinguish
the Brillouin peaks from the quasi-elastic Rayleigh feature. Data at different
wavenumbers were used to obtain a sound speed of 5.9$pm$0.5 km/s, which
enabled us to validate the use of Birch’s law in this new parameter regime.

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