Assessing diurnal and seasonal variations of near-surface turbulence through acoustic wave scintillation on Mars

Turbulence within the Mars Planetary Boundary layer is a crucial surface-atmosphere interaction as it drives dust lifting, heat exchange and transport, all of which having a global impact on the overall dynamics of the atmosphere. Detailed knowledge of the turbulent processes on all spatial and time scales is, therefore, key to understand the whole climate system. Yet direct measurements of the small-scall turbulent eddies remain scarce.

One of the particular advantage of the SuperCam microphone onboard the NASA’s Perseverance rover, is that it is coupled with its own reproducible sound source, the spherical acoustic waves generated by the SuperCam laser-induced plasma expansion. Fluctuations of sound travel time and amplitude develop as it propagates through the daytime buoyancy-driven turbulence or the wind-shear induced nighttime turbulence. The variances of the travel times and the scintillation index (normalized variance of the sound intensity) were studied within the formalism of the propagation of acoustic wave propagation throughout random media to infer statistical properties of the turbulence field. After two Martian Years operating on Mars (i.e. 1335 Sols, Martian Solar days), the SuperCam microphone has recorded more than 300,000 laser-induced shock waves.

This presentation will report on the diurnal and seasonal evolution of the turbulence properties, as inferred from the scintillation index of these acoustic waves. The scintillation index, which is indicative of the turbulence strength, exhibits an increase from shortly after sunrise, reaching a plateau at 10:00 Local Time. Subsequently, it begins to decrease at 19:00. The presence of low turbulence levels is typically observed in the post-sunset period. However, certain higher-level points may suggest shear-induced turbulence during nocturnal hours. A variety of variations in the seasonal progression of the scintillation index have been identified. Specifically, the low levels observed around Sol 1100 and, to a lesser extent, around Sol 600, are likely attributable to a weakening of turbulence convection resulting from an increase in global dust opacity within the atmosphere. Subsequent comparisons will be made with relevant meteorological and ground properties.

Fig. 1. Diurnal variation of the scintillation index for laser-induced shock-waves recorded with the SuperCam microphone.

Fig. 2. Seasonal evolution of the scintillation index recorded during the daytime, between 11:00 and 14:00 Local time.