Acoustic recording of near-surface turbulence on Mars

To study Mars chemistry, SuperCam onboard the NASA Perseverance rover fires bursts of 30 laser shots at rocks. Its microphone records the shock wave generated between 2 and 15 kHz by each laser-induced plasma for distances between 2 m and 8 m, at varying local times, from strong thermal turbulence during daytime, to weaker one at dusk or dawn. In each case, the scintillation index (the normalized variance of the intensity), and travel time are calculated. Based on extensive literature on the propagation of a spherical wave through a turbulent medium, there are enough statistics to constrain the field that causes the dispersion of acoustic data. Initial results show that data are well modeled by a Gaussian field. A negative time shift, ∼0.5% at 6 m, is observed: the resulting velocity is higher than the mean speed of sound (fast path effect). The variance of arrival times leads to a 15 cm characteristic scale during daytime. Fluctuations of the scintillation index allow the strength of the turbulence field to constrained. Other turbulence models are tested, including the Kolmogorov energy-cascade which typically prevails on Earth for the inertial regime, but has been challenged several times on Mars.