From InSight to Perseverance, infrasound and sound recordings : investigating atmospheric science and surface-atmosphere interactions.

Introduction

The InSight mission (Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport mission; [1]) landed on Mars in November 2018 within a quasicircular depression called Homestead hollow [2] in the Elysium Planitia region. The InSight pressure sensor [3] is capable of acquiring data up to 20 Hz and operated continuously for long periods of time in order to support the interpretation of the seismological data. These InSight measurements have made infrasound observations on other planets possible for the first time [4-5].

Figure 1: The InSight lander. Credits: NASA/JPL-Caltech.

 

On February 18, 2021, the Perseverance rover landed on Mars, and two days after, the first sounds ever recorded on Mars were transmitted back to Earth [6]. The SuperCam Microphone is not the first microphone to be implemented in a space mission, but it is the first to operate successfully and record sound on Mars [7].  If the first sound recording of the Martian wind using the SuperCam microphone was a very important achievement in terms of outreach, the expected scientific return was mostly based on the assumption that the microphone would help constrain the physical properties of the rocks vaporized by the SuperCam Laser-Induced Breakdown Spectroscopy (LIBS) [8].  However, microphones also have the capacity to provide data for new and important atmospheric investigations. Here we discuss the exciting potential of infrasound and sound recordings for studying planetary atmospheres and surface-atmosphere interactions in planetary exploration.

Figure 2: The SuperCam instrument. Credits: NASA/JPL-Caltech.

Studying atmospheric dynamics

Pressure fluctuations in the atmosphere tell us about boundary layer convection, convective cells, vortices, and the inertial and dissipative regimes. Compared with Earth, Martian daytime turbulence is characterised by a stronger radiative control, a lack of latent heat forcing and a reduced inertial range [9]. Wind gustiness, convective vortex activity and the spectral slope of pressure, wind and temperature measurements can be used as indicators of turbulent motion in the atmosphere. These variables exhibit strong diurnal and seasonal variations (e.g., [10-12]). The InSight pressure measurements were at a higher frequency than any previous measurements and have shown unexpected behavior in the pressure fluctuations [4] and remarkable bursts of daytime vortices, and nighttime turbulence (including vortices) triggered by strong wind [14].  

The SuperCam microphone with its high sampling frequency (up to 100 kHz), probes the Martian atmosphere at even higher frequencies than the InSight pressure sensor.  More than one year on Jezero confirms that a microphone is indeed an extremely valuable tool for investigating key atmospheric properties, such as high frequency wind speed measurements, the wind gustiness, sound speed variations and turbulence profiles (Fig. 3). A synthesis of these findings is provided in [6].

Figure 3. SuperCam microphone (in Pa²/Hz over 167 s), MEDA pressure (in Pa²/Hz) and MEDA wind data (in (m/s)²/Hz). Mod. from [6].

Studying surface-atmosphere interactions

Several other objectives can be addressed also by planetary microphones, and more generally, through recording of pressure and acoustic waves on planetary surfaces : the recording of quake and meteoritic impacts [15], high frequency wind gust recordings [16], or the recording of saltation by the microphone [17]. Such measurements can provide an estimation of the particle flux associated with saltation, a matter of crucial importance for aeolian research [18].

Forces imposed on planetary atmospheres can generate low frequency acoustic waves that may travel long distances.  Sound-generating phenomena on planetary surfaces may include bolide airbursts and impacts [9], spacecraft entry, seismic activity, landslides, wind–mountain interactions, atmospheric turbulence, and convective vortices [5]. In the case of having a pressure sensor and high-precision seismometer, three-axes ground deformations can complement pressure records in ascribing a given pressure perturbation to an infrasound phenomenon [19]. This technique has been demonstrated with terrestrial quarry blast experiments [20] and has been applied to InSight data in the search for infrasound signals on Mars [5]. Simultaneous seismic and pressure measurements can also be exploited to obtain the shallow subsurface properties using the atmosphere as seismic source [21-23].

 

Perspectives for Solar System exploration

With their high sampling frequency, microphones can be used to characterise planetary acoustic environments and atmospheric dynamics at high frequency. Such measurements, on previously inaccessible scales complement the lower frequency pressure and wind speed measurements fand provide a window into previously unexplored regimes of atmospheric science. From an acoustic recording perspective, Mars is probably one of the most challenging planetary atmospheres. Due both to the low pressure and to the specific absorption of carbon dioxide in the acoustic range, both the predicted [24] and measured [6] attenuation are high. We can therefore expect better performance for sound propagation on Venus and Titan (Fig. 4). Special attention should, however, be paid to the instrument design. Tunable gains as implemented for the Mars microphone, and a very rugged design able to cope with the rough planetary environment, are important. For Venus, a possible option would be to fly the above the clouds, where the temperature and pressure conditions are closer to terrestrial environment [25]. For Titan, the challenge would be to cope with an external temperature of -180°C. For the Mars microphone, our design has been tested down to -120°C [26].

Figure 4. The Attenuation coefficient for planetary bodies with atmospheres [24].

References:

[1] Banerdt et al., 2020; [2] Golombek et al., 2020; [3] Banfield et al., 2019; [4] Banfield et al., 2020; [5] Garcia et al., 2021; [6] Maurice et al., 2022;  [7] Ksanfomaliti et al., 1982; [8] Maurice et al., 2020; [9] Spiga et al., SSR 2019; [10] Davy et al., 2010; [11] Ullan et al., 201; [12] Spiga et al., 2020; [13] Larsen et al., 2002; [14] Chatain et al., 2021; [15] Garcia et al., 2022 (submitted); [16] Newman et al., 2022; [17] Murdoch et al., 2022 (in prep.); [18] Kok et al, 2007 ; [19] Martire et al., 2020 ; [20] Garcia et al. 2020; [21] Garcia et al., 2020 [22] Kenda et al., 2020 ; [23] Murdoch et al., 2021 ; [24] Petculescu & Lueptow, 2007 ; [25] Krishnamoorthy et al., 2019 ; [26] Mimoun et al., 2022 (submitted)