Speed of sound in methane under conditions of planetary interiors

We present direct observations of acoustic waves in warm dense matter. We analyze wave -number- and energyresolved 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 +/- 0 . 1 eV and 0 . 8 +/- 0 . 1 g / cm - 3 , corresponding to a pressure of similar to 13 GPa. Inelastic x-ray scattering was used to observe the collective oscillations of the ions. With a highly improved energy resolution of similar to 50 meV, we could clearly distinguish the Brillouin peaks from the quasielastic Rayleigh feature. Data at different wave numbers were utilized to derive a sound speed of 5 . 9 +/- 0 . 5 km / s, marking a high -temperature data point for methane and demonstrating consistency with Birch's law in this parameter regime.