Methods of modelling dispersive sound speed profiles of Martian atmosphere and their effects on sound propagation paths

Abstract

At present, Mars acoustic detection is gradually becoming an important new tool for understanding and exploring Mars. To explore the sources of Mars sound, it is necessary to study the sound speed and the sound attenuation in the thin and low-temperature Martian atmosphere, and to model the sound propagation in the stratified atmosphere. According to the extremely low pressure of Mars and the large variation of gas composition with altitude, we propose a simulation method based on the Navier-Stokes (NS) equation and the mixed-gas model to calculate the vertical profiles of sound speed and attenuation in the Martian atmosphere at 0–250 km altitude in this work. A comparison among sound-speed profiles at different frequencies shows that there is a notable sound dispersion in the Martian atmosphere, especially at high altitudes and in the high frequency range. It is also verified through sound speed measurement experiments that significant sound dispersion does exist in low-pressure carbon dioxide, implying the need to consider sound dispersion in the modelling of Martian sound speed profiles. The scope of application of the NS equation in modelling the sound speed of the Martian atmosphere is also discussed, as the NS equation may fail in a too rarefied gas. Next, the non-dispersive ideal-gas sound speed profiles and the dispersive NS sound speed at different frequencies (0.01, 0.1, 1 Hz) are used to simulate the sound propagation paths in the multilayered Martian atmosphere. And both cases of the Martian ground-based and high-altitude sources are compared with each other. It is found that the dispersive sound speed has a significant effect on the sound propagation path on Mars. The main influence is that the first fold back height and the first return distance of the sound ray to the surface are both shortened, which directly changes the area and location of the acoustic quiet zone. The effect of dispersion on the sound propagation path becomes more notable with both the frequency and the elevation of the acoustic source increasing, confirming that consideration of dispersion has a significant effect on the calculation of the sound propagation path.