All very well, but there’s something else, not quite as familiar, about the speed of sound: it varies with the temperature of the medium through which it is travelling with the density of the medium. This 1,235 kmph figure is really for sound travelling through air at about 20°C. It is slower in colder air and faster in liquids and solids. For example, sound slows to 1200 kmph in air at 0°C. It speeds up to 5,300 kmph in water and 18,500 kmph in iron.
All of which came to mind because of what happened on Mars about a year ago. Nasa’s Perseverance rover landed on the planet in February 2021, carrying an ingenious little helicopter called, you guessed it, Ingenuity. Ingenuity made its first flight on 19 April 2021 and has since made 25 more. There’s history written right there.
Then on 7 May last year, Ingenuity and Perseverance jointly made history of another kind. Ingenuity flew for the fourth time that day, and Perseverance recorded the sound of Ingenuity’s rotors as it flew. That was the first time a spacecraft on another planet recorded another spacecraft nearby. (On this planet, you can hear and see this flight: bit.ly/3kn6sAF)
That might not strike you as much of an achievement. Still, consider what was involved. For one thing, these are intricate, complex pieces of machinery on a planet that’s hundreds of millions of km away. So they are operated by commands that take several minutes to travel back and forth from our earth. But for another, the two craft were positioned 80m apart before the flight. So when Ingenuity’s rotors fired up, the sound they generated had to travel that distance through the Martian atmosphere to be recorded by Perseverance. At -60°C, the surface of Mars is much colder than earth. Not only that, its atmosphere is much thinner as well, about a hundredth as dense as ours. Sound travels slower through the air there, such as it is, than it does here. There’s more too. The atmosphere on Mars is largely made up of carbon dioxide, which strongly dampens sound—especially high-pitched sounds.
For all those reasons, scientists were uncertain that Perseverance would hear Ingenuity’s rotors at all; or, perhaps more correctly, how long the sound would take to reach Perseverance’s sensors and how attenuated it would become in that time. For an analogy, think of standing 80m away from a friend on the streets of Kalkaji and trying to strike up a conversation. That’s a fair distance for voices to travel. You would certainly have to speak more loudly than you would in a room, especially given the everyday hubbub in Kalkaji. Move now to the top of Mt Everest and repeat this effort. No hubbub, but the much colder and thinner air will have its effect on your voices. Minimal hubbub on Mars, too—traffic and street vendors tend to be quiet there, though there’s a persistent wind that can whip up enormous dust storms.
Put it this way—on Mars as on earth, the surroundings in which sounds are generated affect how those sounds are heard.
Still, Perseverance heard Ingenuity. The measurements revealed that the sound of the little chopper’s rotors had travelled those 80m at about 240 m/s, significantly slower than the 343 m/s that would have been the measure on our planet. So far, so good.
But Perseverance also listens for other sounds. In particular, to study the chemical composition of rocks in the neighbourhood, it fires lasers at them. The lasers set off sparks and make sounds as they strike, which offer clues about the composition. These sparking sounds are much softer than the rotors, but they also happen much closer than 80m away. So sensing them offers another set of data for analysing the characteristics of sound on Mars.
A team of French scientists did just such an analysis (“In situ recording of Mars soundscape", Sylvestre Maurice and others, Nature, 1 April 2022, go.nature.com/3Mz1FbB). What they found surprised them—one even used the word “panicked". The laser-generated sounds travelled to the sensor not at 240 m/s—like Ingenuity’s rotors—but at 250 m/s. That may not seem like much of a difference. The scientists wondered at first whether “one of the two measurements was wrong because on earth you only have one speed of sound". After all, there had been two previous attempts to record sounds on Mars —in 1999 and 2008—which had failed. Were Perseverance’s sensors malfunctioning? Were the measurements going haywire this time too?
But no, the numbers were correct. Their conclusion: on Mars, there are “two distinct values for the speed of sound that are [about] 10 m/s apart below and above 240 Hz, a unique characteristic of low-pressure [carbon dioxide]-dominated atmosphere". Higher-pitched sounds like laser strikes travel faster on that planet than lower-pitched ones like Ingenuity’s rotors. The study also showed the significant “acoustic attenuation with distance" of higher-pitched sounds by the carbon dioxide in the atmosphere. That is, the laser strike sounds travel faster but get more quickly dampened as they do. What’s the implication of all this? The French scientists write that their findings “establish a ground truth for modelling of acoustic processes, which is critical for studies in atmospheres like Mars". In other words, these differences in how sound behaves “would make it difficult for two [future Mars residents] to have a conversation only five metres apart". That’s because their voices might be at different pitches. Exactly how difficult it would be is hard to pin down until two real people get to Mars and have a real conversation. But this possible difficulty is something to be aware of and ready for as we prepare to send humans to Mars over the next decades. It’s just one more aspect—to go with low temperatures and pressures, with the paucity of oxygen and water, with much more—of the challenges that the alien environment of Mars presents to human visitors. Learning all we can about them gives us the chance to find ways to overcome them.
I’ll leave you with a reminder of the old philosophical question that might now find new expression and resonance—pun intended —on Mars. If a helicopter rotor fires up and there’s nobody to listen or no machinery to sense, or if the sound dies out before reaching a sensor or an ear—did it really make a sound at all? In fact, whether on earth or Mars, what defines sound? Being made? Or being sensed?
Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun.
