The speed of sound has been measured on Mars for the first time, and it's very different to Earth's. The Perseverance Mars rover is equipped with both microphones and lasers, and the combination has been used to measure the speed of sound in its environment. Of course, some may cynically conclude this was all just an excuse for science fiction fans in control of the rover to fire lasers on another planet and produce the closest equivalent of pew pew sounds.
For 50 years cameras aboard spacecraft have been letting us see other planets, sometimes at wavelengths of light beyond the reach of the human eye. Perseverance, however, is the first to also carry microphones, allowing us a different sense to explore another world. Since famously in space no one can hear you scream, or make any other noise, Mars is the only place our probes have been where a microphone would be of any use, aside from Titan.
Perseverance also has lasers with which it can blast nearby rocks. At the 53rd Lunar and Planetary Science Conference, Dr Baptiste Chide of the Los Alamos National Laboratory described combining the two instruments to measure the delay as the sound from the laser hitting rocks was picked up by a microphone. From there, all that was required to measure the local speed of sound was to divide by the distance traveled by the delay.
Although the speed of sound in Earth's atmosphere is conventionally referred to as 343 m/s, this is only for specific conditions. Sound's speed in air varies with the temperature, and the stiffness of the conducting material so there is no one speed of sound on either Mars or Earth. Nevertheless, the study reveals the speed at Jezero Crater under a variety of conditions.
Typically, that speed was around 240 m/s (540 mph).
The results was achieved by firing bursts of 30 laser shots over a period of 10 seconds at target locations, with 5-10 of those bursts pointed at spots a few millimeters apart. The plasma produced by the lasers hitting the Martian rocks created a shock wave that produced a high-frequency sound.
One of the features of the Martian atmosphere pointed out in the conference paper is that the daytime Martian temperature drops by an astonishing 40 degrees Kelvin (72º F) between ground level and 2 meters (6.6 feet) above the surface. That makes for a 10 percent variation in the speed of sound, creating some amazing aural effects, were there anyone to hear them, a reversal of the way sound can be focused when traveling over water.
Since the microphone is at a height of 2.1 meters, and the rocks are, by definition, at ground level, the sound measured was an average of the heights in between, rather than at any one altitude.
The authors also note what they call “The Mars idiosyncrasy,” reflecting the fact that carbon dioxide under low pressure has some curious acoustic effects. Above 240 Hz (just below a piano's middle C) CO2 molecules can't relax their vibrational modes after a peak has passed in time for the next one, which means sounds above this frequency travel 10 m/s faster than those below it. “It may induce a unique listening experience on Mars with an early arrival of high-pitched sounds compared to bass,” the conference paper notes. Mars is the only planet in the Solar System where this would be a noticeable effect.
Further research will see how the speed changes over the course of a Martian year, and in one of the Red Planet's famous dust storms.