In situ recording of Mars soundscape - s41586-022-04679-0.pdf

Nature | Vol 605 | 26 May 2022 |

653

Article

In situ recording of Mars soundscape

S. Maurice

✉

, B.

Chide

✉

, N.

Murdoch

, R. D. Lorenz

, D.

Mimoun

, R. C. Wiens

2,5

, A.

Stott

X. Jacob

, T.

Bertrand

, F.

Montmessin

, N. L. Lanza

, C.

Alvarez-Llamas

, S. M. Angel

M. Aung

, J.

Balaram

, O.

Beyssac

, A.

Cousin

, G.

Delory

, O.

Forni

, T.

Fouchet

O. Gasnault

, H.

Grip

, M.

Hecht

, J.

Hoffman

, J.

Laserna

, J.

Lasue

, J.

Maki

, J.

McClean

P.-Y. Meslin

, S. Le Mouélic

, A.

Munguira

, C. E. Newman

, J.

Rodríguez Manfredi

J. Moros

, A.

Ollila

, P.

Pilleri

, S.

Schröder

, M. de la Torre Juárez

, T.

Tzanetos

, K. M. Stack

K. Farley

, K.

Williford

11,21

& the SuperCam team*

Before the Perseverance rover landing, the acoustic environment of Mars was

unknown. Models predicted that: (1) atmospheric turbulence changes at centimetre

scales or smaller at the point where molecular viscosity converts kinetic energy into

heat

, (2) the speed of sound varies at the surface with frequency

and (3)

high-frequency waves are strongly attenuated with distance in CO

(refs.

–

). 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, using Perseverance microphone recordings, we present the first

characterization of the acoustic environment on Mars and pressure fluctuations in the

audible range and beyond, from 20 Hz to 50 kHz. We find that atmospheric sounds

extend measurements of pressure variations down to 1,000 times smaller scales than

ever observed before, showing a dissipative regime extending over five 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 that are

about 10 m s

−1

apart below and above 240 Hz, a unique characteristic of low-pressure

-dominated atmosphere. We also provide the acoustic attenuation with distance

above 2 kHz, allowing us to explain the large contribution of the CO

vibrational

relaxation in the audible range. These results establish a ground truth for the

modelling of acoustic processes, which is critical for studies in atmospheres such as

those of Mars and Venus.

Before the landing of Perseverance (18 February 2021), no pressure

fluctuations had ever been monitored on Mars at a frequency >20 Hz,

namely, in the acoustic domain. The recording of sounds offers the

unique opportunity to study the atmosphere as the main natural source

of sound and as the propagation medium for acoustic waves. From the

knowledge of Mars atmospheric pressure (about 0.6 kPa) and the physi

cal properties of CO

, one can predict (see Methods) that: the acoustic

impedance results in approximately 20 dB weaker sounds on Mars than

on Earth if produced by the same source, the speed of sound should

be around 240 m s

−1

near the surface and acoustic waves are heavily

damped in CO

at these atmospheric pressures and temperatures. A few

studies

proposed very detailed models of acoustic propagation on

Mars but with large discrepancies between their results because of a lack

of experimental data at low pressure and appropriate temperatures,

and the difficulty of characterizing attenuation in a closed environment.

Acoustic data are also sensitive to wind speed and direction and, to a

lesser extent, other environmental parameters

. As such, owing to the

high sampling frequency of microphones (up to 100 kHz), the acoustic

data allow us to explore the atmospheric behaviour on a microscale

that has never been accessible before on Mars.

The SuperCam instrument suite

on Perseverance carries an electret

microphone, similar to that carried by the Mars Polar Lander

, lost during

atmospheric entry, and the Phoenix spacecraft

, on which technical issues

prevented the device from being operated. SuperCam’s microphone is

able to record air pressure fluctuations from 20 Hz to 12.5 kHz or 50 kHz, at

sampling rates of 25 kHz or 100 kHz, respectively. After landing (Martian

https://doi.org/10.1038/s41586-022-04679-0

Received: 7 December 2021

Accepted: 23 March 2022

Published online: 1 April 2022

Open access

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Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse 3 Paul Sabatier, CNRS, CNES, Toulouse, France.

Space and Planetary Exploration Team, Los Alamos National

Laboratory, Los Alamos, NM, USA.

Institut Supérieur de l’Aéronautique et de l’Espace (ISAE-SUPAERO), Université de Toulouse, Toulouse, France.

Space Exploration Sector, Johns Hopkins

Applied Physics Laboratory, Laurel, MD, USA.

Present address: Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA.

Institut de Mécanique

des Fluides, Université de Toulouse 3 Paul Sabatier, INP, CNRS, Toulouse, France.

Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique, Observatoire de Paris, CNRS,

Sorbonne Université, Université Paris Diderot, Meudon, France.

Laboratoire Atmosphères, Milieux, Observations Spatiales, CNRS, Université Saint-Quentin-en-Yvelines, Sorbonne Université,

Guyancourt, France.

Universidad de Málaga, Málaga, Spain.

Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC, USA.

Jet Propulsion Laboratory,

California Institute of Technology, Pasadena, CA, USA.

Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, CNRS, Sorbonne Université, MNHN, Paris, France.

Heliospace

Corporation, Berkeley, CA, USA.

Haystack Observatory, Massachusetts Institute of Technology, Westford, MA, USA.

Department of Aeronautics and Astronautics, Massachusetts Institute

of Technology, Cambridge, MA, USA.

Laboratoire de Planétologie et Géosciences, CNRS, Nantes Université, Université Angers, Nantes, France.

Escuela de Ingeniería de Bilbao, Universidad

del País Vasco UPV/EHU, Bilbao, Spain.

Aeolis Corporation, Sierra Madre, CA, USA.

Centro de Astrobiología (INTA-CSIC), Madrid, Spain.

Deutsches Zentrum für Luft- und Raumfahrt (DLR),

Institute of Optical Sensor Systems, Berlin, Germany.

Blue Marble Space Institute of Science, Seattle, WA, USA. *A list of authors and their affiliations appears at the end of the paper.

✉

e-mail:

sylvestre.maurice@irap.omp.eu

;

bchide@lanl.gov

654

| Nature | Vol 605 | 26 May 2022

Article

solar day ‘Sol’ 0; one Sol = 88,775 s), the microphone was turned on for

the first time on Sol 1 while the mast was still stowed. Since deployment

on Sol 2, the microphone is approximately 2.1 m above the ground; it has

performed nominally up to the time of writing. SuperCam also consists

of a laser-induced breakdown spectroscopy (LIBS) capability to analyse

the chemistry of Mars at stand-off distances from 1.5 to 7 m (refs.

When the laser pulse interacts with the target, a luminous plasma emits

characteristic optical emission lines of the elements present in the tar

get

. Plasma expansion generates a shock wave that decouples from the

plasma within the first microsecond after laser interaction

and results

in a clearly detectable acoustic signal

. Moreover, Perseverance carries

a second microphone as part of the Entry, Descent, and Landing Camera

(EDLCAM

), which has a frequency response from 20 Hz to 20 kHz at a

sampling rate of 48 kHz. The EDL microphone is mounted on the port side

of the rover, 1 m above the ground. It was activated on Sol 2.

Figure

provides an overview of sounds acquired by SuperCam’s

microphone (see Methods). Sol 38a is the quietest recording in our data

set. Later on that same day (Sol 38b), the power spectral density (PSD)

increases above the quiet state at frequencies below 100 Hz. On Sol 117,

we associate this increase of power to an increase in the turbulent activ

ity, which extends up to 300 Hz; this is the situation we observe most

often. The recording of Sol 148 is the most active one shown, with the

same shape starting towards higher frequencies but with a slope break

near 200 Hz; turbulence is detected up to 600 Hz. All non-saturated

atmospheric recordings from Sol 0 to Sol 216 fit between the bounda

ries given by the Sol 38a and Sol 148 spectra. The laser-excited plasma

generates a short, roughly 300-μs acoustic pulse (see Methods), with

95% of its energy between 3 and 15 kHz. Various spectral notches are

caused by acoustic interferences owing to echoes from the base of the

microphone itself (6 kHz and 12 kHz) or from nearby rocks. The total

intensity varies as a function of target distance, as shown for recordings

at 2 m, 5 m and 8 m. During laser-induced spark recording sessions, the

atmospheric signal below 1 kHz is masked by electromagnetic interfer

ence

. The Ingenuity rotorcraft tones (see Methods) are also shown.

Atmospheric turbulence

The Martian planetary boundary layer (PBL) is the part of the atmos

phere in contact with the surface

, extending to several km. It is prone

to convective turbulence and vertical mixing during daytime, owing to

the thin atmosphere and low surface thermal inertia that induce strong

and unstable near-surface temperature gradients

–

. This turbulence

translates into high-frequency variations in atmospheric pressure,

wind speed and temperature that can be measured by in situ instru

ments. Conversely, during night-time, the strong radiative cooling of

the atmosphere induces highly stable conditions, which efficiently

inhibit most convection and turbulence

. Analysing the PBL at the

surface is therefore important to understand how the Martian atmos-

phere transports and mixes heat, momentum, aerosols and chemical

species

. The Mars Environmental Dynamics Analyzer (MEDA

) instru

ment on Perseverance and the meteorological suites of previously

landed missions

typically measure pressure, temperature and wind

fluctuations with sampling frequencies of 0.1 Hz to 10 Hz. These instru

ments study the turbulence variability

and the Martian turbulent

energy cascade

Specifically, we report here the observation of the dissipative

turbulence regime in the PBL, in which the InSight mission could

see a hint of a regime change at the limits of the instrument capabil

ity

. This regime, in which molecular viscosity dissipates the turbu

lent kinetic energy into heat, is now fully characterized by a rapid

decrease of the power spectrum with increasing frequency (Fig.

)

over roughly five orders of magnitude. The scale at which the viscous

dissipation becomes notable is characterized by the Kolmogorov

length scale

= (

)

0.25

, in which

is the kinematic viscosity and

is the turbulence energy dissipation rate per unit mass, typically

around 0.001 m

−1

and 0.005 m

−1

on Mars, respectively

. Thus

about 0.02 m and the timescale of these small eddies,

= (

)

0.5

, is

about 0.45 s. Hence the dissipation regime should be observable at

frequencies above 2 Hz on Mars, at centimetre or smaller scales only

(on Earth, this transition occurs at millimetre scales or smaller

This theoretical prediction is confirmed by the acoustic data; the

threshold moves with frequency, depending on the dissipation

rate

. The balance between energy production and molecular

dissipation controls the total amount of turbulent kinetic energy

in the boundary layer and, as such, the dissipation mechanism is

intrinsically linked to the PBL dynamics; a larger dissipation leads

to a faster turbulence decay, in turn suppressing small-scale wind

gustiness, and vice versa.

Ingenuity tones

Atmospheric turbulence

LIBS acoustic spectra

Frequency (Hz)

100

1,000

10,000

–5

–6

–7

–8

–9

–10

–11

Power spectral density (Pa

–1

)

SuperCam

micr

ophone

Sol 148

Sol 117

Sol 38b

Sol 38a

Fig. 1 | Variety of sounds recorded by SuperCam.

Atmospheric spectra

spread over the light blue area; turbulence increases in the direction of the

arrow. LIBS acoustic spectra spread over the light red area. Ingenuity tones are

recorded at 84 Hz and 168 Hz (purple). The black spectrum is the quietest

recording so far below 1 kHz. SuperCam’s microphone is located on the rover

mast ( green).

Nature | Vol 605 | 26 May 2022 |

655

The microphone records rapid deviations from ambient pres

sure (>20 Hz) that are correlated to variations in the wind flow, as

shown by Fig.

, in which a spectrogram of Sol 38b microphone data

(see Methods) is overlaid with the wind speed as measured by the MEDA

(see Methods). As expected

, there is a clear correlation between the

intensity of acoustic data and the wind speed. This can be owing to

the flow-induced turbulence from the rover/mast itself but also to the

direct sensing of the incoming flow fluctuations, seen to be the domi-

nant factor for outdoor microphones in other studies

. Moreover, the

daytime local turbulence is known to increase for larger ambient wind

speeds

. The high microphone sampling rate provides an opportunity

to observe very intense but short wind gusts, on a timescale of 10 s. In

Fig.

, the same acoustic data are plotted in the frequency domain and

combined with low-frequency measurements of pressure and wind

from the MEDA, for a 51-min time period of continuous data around

the microphone acquisition. The large difference in slope between the

MEDA and microphone data is indicative of regime change. The transi

tion from the probable shear-dominated regime

to the dissipation

regime occurs in this case between 1 and 20 Hz.

Speed of sound on Mars

In a cold CO

atmosphere, the speed of sound is expected to be lower

than on Earth. Furthermore, owing to the low pressure and the physi-

cal properties of CO

, we also expect a dispersion of this speed with

frequency

. On Earth, the adiabatic ratio

is constant up to a few MHz

at ambient pressure

and sound speed does not vary with frequency

near the surface. At low pressure on Mars, still within the framework of

small Knudsen numbers

(10

−6

at 100 Hz to 2.10

−4

at 20 kHz), the con-

tinuum theory still holds, but energy exchanges at molecular scales are

modified. Part of the energy associated with the translational motions

of molecules, which constitute the acoustic waves, is spent on the exci

tation of inner degrees of freedom (vibrational modes and rotational

motions). The relaxation of the rotational motion is almost instantane

ous, whereas relaxation of the vibrational modes occurs over a much

longer timescale, a property of small and rigid polyatomic molecules

such as CO

. If the frequency

is smaller than

= 1/

, in which

is the

relaxation time, all modes are equally excited and then relaxed. The

seven degrees of freedom that result from three translational modes,

two rotational modes and one doubly-degenerate vibrational mode (

bending) lead to an adiabatic index

= 9/7 = 1.2857. Conversely, if

there is no time to relax the vibrational mode; in that case, there are only

five active degrees of freedom and

∞

= 7/5 = 1.4. In CO

at Earth-ambient

pressure,

is about 40 kHz (ref.

). This frequency depends on the rate

at which molecules can collide, hence

is proportional to the pressure.

As a result, at 0.6 kPa, the relaxation frequency is about 240 Hz on Mars.

The recording of pulsed waves generated in LIBS mode provides

a unique opportunity to measure directly and repetitively the local

speed of sound for acoustic waves above 2 kHz, that is, for

(see Methods). From the daytime measurements, sound speeds

between 246 m s

−1

and 257 m s

−1

are obtained (Fig.

), with maxi

mum values between 11:00 to 14:00 Local True Solar Time (LTST) and

minimum values around 18:00. The 1σ-dispersion of the sound speed

during the approximately 20 min of a target analysis with LIBS is at

its maximum at noon (1.5%) and is reduced to 0.5% at 18:00, which

highlights the vanishing of the atmospheric turbulence at dusk. These

measurements are compared with temperature-derived speeds of

sound obtained from: (1) the MEDA temperature datasets at the sur-

face, at the heights of 0.85 m and 1.45 m, and (2) the temperature at

the surface and at a height of 2 m given by the Mars Climate Database

(MCD

) (see Methods), using

∞

= 1.4 (because

). The agreement

between the MEDA and MCD predictions is excellent. SuperCam sound

speeds are comparable with temperature-derived values at the height

of the MEDA’s 0.85-m temperature sensor or higher. This is consistent

with the fact that the speed is integrated between a height of 2.1 m

and the surface, possibly biased towards the surface when the tem

perature gradient is larger.

Ingenuity’s blade passage frequency (BPF)

is close to a harmonic

source centred around 84 Hz, and — in that case — for

(see Meth-

ods). This signal recorded by SuperCam’s microphone is modulated

by the variations of the distance range between the microphone and

the helicopter. An emitted frequency at 84.43 Hz and a speed of sound

= 237.7 ± 3 m s

−1

are estimated on the basis of a fit of the Doppler effect

for Ingenuity’s fourth flight (see Methods). Accounting for the pres

ence of a wind of about 2.5 m s

−1

along the microphone-to-helicopter

line of sight towards the helicopter (MEDA data), the true sound speed

is about 240 m s

−1

at this frequency. At the time of the flight, the atmos

pheric temperatures ranged between 232 K and 240 K at a height of

1.45 m. Using

= 1.2857 (the BPF is below

), the temperature-derived

speed of sound ranges from 238.8 m s

−1

to 242.9 m s

−1

, which is con

sistent with the speed directly derived from Ingenuity’s flight plus

wind (Fig.

). As a summary, SuperCam’s microphone highlights a

sound speed dispersion of about 10 m s

−1

in the audible range at the

surface of Mars.

0.01

100

Time (s)

Frequency (Hz)

100

200

300

400

500

Wind speed (m s

–1

)

PSD (unit

–1

)

–11

–9

–7

–5

–3

–1

–10

–9

–8

–7

–6

PSD (Pa Hz

–1

)

Wind sensor 1

Wind sensor 2

Pressure (slope –1.1)

Wind (slope –0.95)

Acoustic (slope –4.9)

Shear-dominated regime

Dissipative

regime

160

140

120

100

Fig. 2 | Sound recordings and correlation with atmospheric data.

Recording

of Sol 38b.

, On top, the

axis of the time series ranges from −0.2 to 0.2 Pa. The

spectrogram (bottom) shows bursts that extend to 300 Hz. Overlaid, with the

axis on the right, are wind speeds from MEDA booms.

, The PSD calculated

for SuperCam’s microphone (in Pa

−1

for 167 s) and for MEDA pressure (in

−1

for 51 min around the microphone acquisition time) and MEDA wind

data (in (m s

−1

)

−1

). The wind PSD is artificially offset by 10

−2

in the

axis.