The first spacecraft to record sounds on another planet was supposed to be NASA’s Mars Polar Lander, which launched in January 1999. But NASA lost communication during the landing, so the atmosphere’s acoustic properties—including information on turbulence and the speed of sound within it—remained unknown.
The second attempt at obtaining audio from Mars, organized jointly by several space programs, universities, and companies, was to have come in May 2008 with the landing of the Phoenix spacecraft. But although the mission was mostly successful, it didn’t make acoustic measurements. Prelaunch testing revealed that powering the microphone would have posed a serious risk to the electronics of another instrument, so the microphone was never turned on.
The third attempt, though, was successful. Almost immediately after NASA’s Perseverance rover landed on the Martian surface in February 2021, its two microphones began collecting in situ acoustic data in the audible range of 20 Hz to 20 kHz. Sylvestre Maurice of the University of Toulouse, Baptiste Chide of Los Alamos National Laboratory, and their collaborators have now published an analysis of the observations. They found that in the low-pressure Martian atmosphere, sound waves travel at two different speeds, and most sounds attenuate at one-eighth the distance they do on Earth.
Earth and Mars each have a planetary boundary layer—the thin skin of turbulent air at the lowest level of the atmosphere that extends vertically a few kilometers from the planet’s surface. That’s the layer in which the Perseverance microphones recorded pressure variations associated with the wind’s convective and turbulent mixing of the Martian atmosphere. (You can listen to a clip below. In addition, daily audio recordings of Mars’ ambient sound are available at NASA’s data archive, the Planetary Data System Geosciences Node.) On Earth, turbulent kinetic energy from wind gusts and other atmospheric activity is dissipated as heat by molecular viscosity. That mechanism helps control the transport and mixing of aerosols and other chemical species in the atmosphere.
The dissipation of kinetic energy to heat on Earth is relevant for atmospheric motion of up to millimeter scales. In the Martian atmosphere, a similar dissipation regime was observed. In the graph below, the sharp decline in the power spectral density with increasing frequency is indicative of viscous molecular friction converting the energy of small eddies in the atmosphere into heat. On Mars that transition was visible at up to centimeter scales.
Chide says, “We never observed this transition before on Mars. In the models that forecast the Mars atmosphere, this amount of energy has been empirically represented. Now with direct observations, it will be a way to refine our models with real data and then provide better atmospheric models.”
Although Earth and Mars have some similarities, their differences critically affect the propagation of sound. The lower-pressure Martian atmosphere receives less sunlight and is thus colder (the average is −60 °C). In addition, the atmosphere of Mars has a higher concentration of carbon dioxide. As a sound wave travels through the atmosphere, it provides energy that moves gas molecules around. In the cold, low-pressure atmosphere of Mars, the rotational motion of the CO2 molecule relaxes nearly instantaneously after the wave passes, but the vibrational mode takes more time to relax.
The different time scales of the rotational and vibrational motion separate a sound wave into its component frequencies as it passes through the Martian atmosphere, and the wave’s phase velocity is consequently affected. At high frequencies the vibrational modes are nearly frozen, and that results in a higher value of the heat-capacity ratio of the gas. The speed of sound, in turn, depends on the square root of the ratio; at higher frequencies, therefore, sound travels faster.
The microphone recorded daytime sound speeds of 246–257 m/s. (In Earth’s more balmy atmosphere, sound travels at about 340 m/s at sea level with an air temperature of 15 °C.) The spinning blades of the mission’s Ingenuity robotic helicopter were used as a sound source for low-frequency tones. While it was in operation, the microphone also recorded a second speed of sound about 10 m/s slower. The researchers identified the transition point for the two sound speeds at 240 Hz.
In their paper, the researchers write that “the most remarkable property of sound propagation on Mars is the magnitude of the attenuation at all frequencies.” The collection of data on sound attenuation over distance was possible because of well-localized sound sources produced by Perseverance’s activity. The first source came from Ingenuity flying overhead; the second from the laser vaporization of rock by the SuperCam instrument.
The graph shows a strong reduction in acoustic power above 1000 Hz. A wave with a frequency of 8000 Hz, for example, attenuated 40 dB after traveling 8 m. (Increasingly dark blue lines represent rising levels of turbulence; increasingly dark red lines indicate laser targets at growing distance.) To reach a similar attenuation on Earth, the sound wave would have to travel 65 m.
As was the case for the two values for the speed of sound, the high CO2 concentration in the Martian atmosphere is also responsible for the sound attenuation. At those frequencies, the vibrational motion of CO2 molecules relaxes rather slowly, so a sound wave rapidly loses energy to excitation of those vibrational modes and to heat generated from molecular viscosity.
Additional acoustic measurements should help to better resolve the turbulence in the Martian atmosphere and better constrain the simulation of CO2-rich atmospheres such as Venus’s. What’s more, acoustic data will likely be used in the future with other information to perform diagnostics on the status of scientific instrumentation on spacecraft and rovers not only on Mars but also on Venus, Titan, and other bodies in the solar system.
