First Sounds from Mars

Perseverance’s microphones provide the first characterization of Mars’ acoustic environment and pressure fluctuations in the audible range and beyond, from 20 Hz to 50 kHz. Prior to this mission, modeling predicted that: (i) atmospheric turbulence must change at centimeter scales or smaller at the point where molecular viscosity converts kinetic energy into heat, (ii) the speed of sound varies at the surface with frequency, and (iii) high frequency acoustic waves are strongly attenuated with distance in CO2. However, theoretical models were uncertain because of a lack of experimental data at low pressure, and the difficulty to characterize turbulence or attenuation in a closed environment. Here we present in situ recordings for the first 216 solar days of the Mars 2020 mission. We find that atmospheric sounds extend measurements of the dynamic pressure variations down to 1000 times smaller scales than ever observed before, revealing a dissipative regime of the Martian atmosphere extending over 5 orders of magnitude in energy. Using point sources of sound (Ingenuity rotorcraft, laser-induced sparks), we highlight two distinct values for the speed of sound in the audible range that are ~10 m/s apart below and above 240 Hz, a unique characteristic of low-pressure CO2-dominated atmosphere. We also provide the acoustic attenuation with distance above 2 kHz, allowing us to elucidate the large contribution of the CO2 vibrational relaxation in the audible range. These results establish a ground truth for modelling of acoustic processes, which is critical for small-scale atmospheric studies in atmospheres like Mars and Venus ones.