Titan Acoustics: Assessing the detectability of natural sound sources with the Dragonfly microphones

The connection between Titan's atmosphere and its surface is unique: it is at the origin of a variety of surface processes, in particular of surface-atmosphere interactions – liquid methane flows, waves, rainfall, dune movements, saltation, dust devils, and rainstorms, etc.- all play an important role in surface alteration and atmospheric dynamics. Interestingly, Titan's atmosphere is dense enough to propagate the acoustic waves generated by these phenomena. Therefore, they can be studied quantitatively and remotely by recording their acoustic signatures. In the mid-2030s, the Dragonfly mission will explore Titan near an equatorial impact crater with a relocatable rotorcraft lander. Key geophysical and meteorological measurements will be provided by the DraGMet package, a suite of 12 experiments, including three microphones.

In preparation for this acoustic exploration, modeling of sound propagation under Titan's atmospheric conditions is a prerequisite for assessing the levels and detection ranges of geophysical sound sources for the DraGMet microphone. As a first step in this modeling, acoustic attenuation calculations showed that Titan's unique N₂-CH₄ atmosphere at ~90 K can sustain acoustic waves over long distances due to relatively low absorption compared to Earth. In addition, tracking sound velocity and acoustic attenuation on Titan could help constrain the minor component fraction in the atmosphere. In this presentation we report on the second step of the modeling, which consists of a parabolic equation solver that computes the sound field produced by a sound source for given atmospheric parameters (wind and temperature profiles, ground properties, see Fig. 1). Booming sand, a possible sound source at the Dragonfly landing site, is propagated into the model to assess the maximum distance at which such an event could be recorded.

Fig. 1. Sound pressure level distribution for the propagation of a 250 Hz sound source located at 35m from a perfectly reflecting ground. (Top) Terrestrial atmosphere at 293 K and with a constant wind speed of 2 m/s. (Bottom) Titan atmosphere at 90 K and with a constant wind speed of 0.5 m/s. Sound propagates in the upwind direction.