Recording under natural conditions of the acoustic environment on Mars

Before the Perseverance rover landed on Mars, the planet’s acoustic environment was unknown. Various modeling processes have predicted: (1) that atmospheric turbulence on the planet changes over centimeters or less at the points where molecular viscosity converts kinetic energy into heat, (2) that the speed of sound varies with its frequency at the planet’s surface, (3) and that high-frequency waves are greatly attenuated with distance in the medium permeated by carbon dioxide. However, theoretical predictions for these modeling processes were uncertain due to the lack of experimental data in low pressure conditions, and the difficulty in characterizing atmospheric turbulence or weak frequencies in closed environments. In this published paper, using audio recordings made through the microphones of the Perseverance rover, the research team in this study provided the first description of the acoustic environment of Mars and the pressure fluctuations in the audible and other sound bands, from frequencies of 20 Hz to frequencies of 50 kHz. It was also found that the observed atmospheric sounds widen the range of variation of the measured pressure values ​​to values ​​1,000 times smaller than those previously observed, which proves the existence of an energy dispersal system spanning a range of five orders of magnitude. By using sound sources at specific points (the vehicle’s Ingenuity robotic helicopter and laser-induced sparks), the research team in this study sheds light on two different values ​​of sound speed, with a difference of approximately 10 meters/sec in frequency bands below or greater than 240 Hz, a unique characteristic of CO2-dominated low pressure atmospheric environments. The research team also reviews in this paper the issue of sound attenuation with increasing distance in frequency bands above two kilohertz, which allows explaining the significant role played by the vibrational relaxation process of carbon dioxide in the audible range. These results establish facts from direct observations that are needed to model acoustic phenomena and are critical for atmospheric studies such as those of Mars and Venus.