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 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. 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 quasielastic Rayleigh feature. Data at different wave numbers were utilized to derive a sound speed of 5.9\(\pm\)0.5 km/s, marking a high-temperature data point for methane and demonstrating consistency with Birch's law in this parameter regime.