Carrying on a conversation on Mars would be difficult. This is partly because Mars can be really cold and your teeth can chatter. But that’s also because the Red Planet’s thin atmosphere, made up mostly of carbon dioxide, doesn’t transmit sound well. In fact, someone talking next to you on Mars will sound as quiet as if they were talking 60 meters (200 feet) away.
“It’s a pretty drastic difference from Earth,” says Baptiste Chid. “You don’t want to do that.” Better to use microphones and headphones, he says, even at close range. Chid is a planetary scientist at Los Alamos National Laboratory in New Mexico. He and his colleagues shared these new findings about the sound of Mars in the May 26 issue of Nature.
Chide’s team analyzed some of the first sound recordings ever made on the Red Planet. The recordings were captured by a microphone on NASA’s Perseverance rover. This space robot has been exploring Mars since February 2021.
What Perseverance recorded were not the sounds of events on Mars. These were noises as the rover fired a laser at small rocks nearby. This impact created a sound wave – similar to thunder, but on a much smaller scale. Chid and his team studied about five hours of sounds collected in this way.
This data allowed researchers to measure the speed of sound on Mars and revealed a surprise. There is more than one on this planet. Within the range of human hearing, high-pitched sounds travel at about 250 meters per second (559 miles per hour). Low sounds travel more slowly – about 240 meters per second (537 miles per hour). These low waves will only travel a few meters before they become inaudible. Louder sounds dissipate at even shorter intervals.
“To an earthling, this may be surprising. But it makes sense,” says Andy Piasek. He is a physicist at Central Washington University in Ellensburg. He was not involved in the new research, but studies how sound waves travel through different materials.
When a sound wave travels through air or liquid, it adds energy to the molecules around it. The air will gradually move this energy. This is called the relaxation effect.
For sound waves traveling through air, the relaxation depends on the frequency of the sound and the type of molecules in the air. On Mars, relaxation after a loud sound occurs faster than after a low sound. This is because the atmosphere has low pressure and consists mostly of carbon dioxide.
“This doesn’t happen on Earth because the pressure of our atmosphere is much higher than on Mars,” says Piasek. Also, Earth’s atmosphere is mostly nitrogen. Under these conditions, the relaxation effect is approximately the same for high and low tones. So on Earth, all sound normally travels at about 343 meters per second (767 miles per hour). (To hear how sounds differ between Earth and Mars, visit NASA’s Sounds of Mars site.)
If a song is played from a speaker on Mars, the higher sounds will reach the listener before the lower sounds. “Let’s say you somehow had a city on Mars with birds,” Chid says. “Bird frequency is too high. You wouldn’t hear them. You will only hear the sounds of the city. The high carbon dioxide content of the Martian air is to blame, Cheed says.
Sure, there are no birds on Mars, but that’s not why scientists study sound on alien worlds. Measuring the speed of sound could give scientists a precise way to study the Martian atmosphere, Cheed says. Air pressure, temperature and humidity affect the speed of sound. So by measuring changes in the speed of sound over time, Cheed says, researchers can learn more about the weather on Mars. “We can measure temperature for small fractions of time,” he says—even day to day.
With Perseverance beaming more sounds back to Earth, scientists will be able to study how its soundscape changes over the course of the Martian seasons, Cheed says. “We’re very excited to see how the sound behaves in the winter and fall—every season on Mars.”