The Sound of the Wind on Mars: Preliminary Wind Speed Analysis with the SuperCam Microphone on Perseverance

Abstract

The SuperCam instrument on the NASA Perseverance rover [1,2] is equipped with a microphone which, for the first time, has recorded sounds from Mars. These sounds include shockwaves from the laser-induce breakdown spectroscopy technique, the Ingenuity rotorcraft and the ambient Martian atmosphere [3,4]. Each of these offers a unique dataset to study Martian atmospheric dynamics at high frequencies. In this abstract we present preliminary results on studying the relationship between the wind and the sounds recorded from the microphone.

1. The SuperCam microphone

The microphone is a Knowles Electret condenser microphone model EK-23132 [1,2,5]. It can record at sampling rates of 25kHz or 100kHz for up to 167s. The nominal bandwidth is 100Hz-10kHz but lower frequencies can be retrieved. It is located on the SuperCam mast unit 2.1m above the ground and is aligned with the camera. This means that the microphone can be made to point in different azimuths and elevations.

2. Wind speed estimation from the microphone

The microphone records the atmospheric pressure at high frequencies. This dynamic pressure is linked to the winds on Mars [5,6]. In a pre-landing analogue study in a wind tunnel, Chide et al. 2021 [5] found that the root mean square (RMS) of the pressure signal is correlated to the wind speed. It was shown that the RMS value from the microphone in the bandwidth of 100-500Hz is proportional to the square of the wind speed. This frequency band was seen to not be extremely sensitive to the wind incidence azimuth but there is some uncertainty, especially for higher wind speeds.

We can calculate the RMS envelope over the recording. The square root of this envelope is then proportional to wind speed and can be used as a relative estimate. Figure 1 shows initial results from an afternoon recording on Sol 38 of the mission.

Figure 1: Microphone data from Sol 38. Top panel: microphone signal, Middle panel: spectrogram of microphone data, Bottom panel: square root of RMS envelope for 100-500Hz bandwidth.

A cross-calibration will be performed with the Mars environmental dynamic analyser (MEDA) wind sensor [7]. This will enable us to obtain an absolute wind speed determination with the microphone. The mast will also be rotated during the cross-calibration to help capture recordings for a variety of wind speed incidence azimuths and thus also determine the sensitivity to wind direction including the identification of vortex shedding [5]. Moreover, a variation of the mast elevation may lead to the determination of vertical wind speeds.

The relationship could also be sensitive to other effects, such as the atmospheric stability. To this end, we aim to track diurnal and seasonal variation, along with temperature and static pressure conditions.

3. Wind speed variation - gustiness

The microphone recordings yield an estimate of the Martian wind speed. The so far obtained relative wind speed estimates can be used for an analysis of the wind gustiness, that is, the wind speed variation with respect to its mean. This has been explored for Phoenix, Viking and InSight data [6,8] with a gustiness metric defined as the wind speed standard deviation, σ_w, over the mean wind speed, μ_w, as

normalised gustiness = σ_w/μ_w.

The gustiness metric was applied to several microphone recordings around Noon and 16:00 local true solar time (LTST). These results indicate that the later afternoons are more turbulent than midday, where some acquisitions contain very little wind. However, the noon recordings can contain infrequent very large gusts, including the largest so far observed by the microphone. This information was utilised early on in the mission to aid the understanding of potential flight conditions for the rotorcraft Ingenuity.

4. Conclusions

We have introduced how the microphone data are related to the Martian wind. It enables an estimation of the Martian wind speed and the ability to examine the high frequency content of wind gusts. This is important for the understanding of the dynamic pressure spectrum on Mars [3]. The microphone will record the high frequency portion of this dynamic pressure spectrum shedding light on the dissipative regime in Martian turbulence. The variability of the relationship to wind speed and the distribution of gustiness provides a study of high frequency Martian atmospheric dynamics, valuable for studying the Martian planetary boundary layer (PBL). 

References

[1] Wiens et al., SSR, 2021;

[2] Maurice et al., SSR, 2021;

[3] Murdoch et al., EPSC, 2021

[4] Chide et al., EPSC, 2021;

[5] Chide et al., ICARUS, 2021;

[6] Murdoch et al., SSR, 2017;

[7] Rodriguez-Manfredi et al., SSR, 2021;

[8] Spiga et al., JGR, 2021;