The earliest recordings of the sounds of Mars showed us that the planet is a quiet place, with wind gusts being its main sound sources. However, after scientists carried out a thorough analysis of five hours of audio recordings made by the Perseverance rover's sensitive microphones, they uncovered some oddities about the speed of sound propagation in the Martian atmosphere. It turns out that sounds with different frequencies propagate at different speeds in the Martian atmosphere, and this leads to some very strange acoustic effects.
Aside from sounds of natural origin, microphones mounted on a SuperCam mounted on the rover's mast made recordings as the tiny Ingenuity helicopter flew. They also made audio recordings as the laser system, which analyses the chemical composition of the Martian rocks, made distinctive, loud, high-frequency clicking noises as it operated. "We have a very localised sound source located two to five metres away, and we know the exact moment in time when the laser 'shot' was made," the researchers wrote.
The available data has enabled scientists to calculate that the speed of sound on Mars is 240 metres per second. For comparison, the speed of sound in the Earth's atmosphere is about 340 meters per second. The resulting sound velocity was expected because Mars' atmosphere is 95 percent carbon dioxide and 100 times less dense than that of Earth. In addition to the difference in sound speed, the parameters of the Martian atmosphere attenuate sound by about 20 decibels.
But the surprise began when scientists calculated that the sound from the laser "shots" propagated at 250 metres per second, 10 metres per second faster than the lower-frequency sound from the blades of the Ingenuity helicopter. "We even panicked a little at first," the scientists write, "At first glance, there was an error in one of the measurements, because we're all used to the fact that on Earth the speed of sound propagation is the same for all frequencies."
However, repeated measurements and calculations confirmed the fact that sounds of different frequencies propagate at different speeds in the Martian atmosphere. "When you listen to a symphony orchestra on Earth, because of the same speed of sound, you hear the whole composition at once," the scientists wrote, "On Mars, a performance by such an orchestra would turn into a cacophony because the higher frequency sounds would reach your ears a little earlier."
Moreover, such exotic acoustic parameters of the Martian atmosphere, according to the scientists, would make it impossible for a direct conversation between two people if the distance between them exceeds five meters.
Discovered acoustic oddities of Martian atmosphere may also explain failures of previous attempts to record sounds in 1999 and 2008. Back then, microphones recorded numerous unexplained "squeals" and "clangs" as the rover navigated a difficult surface and its metal wheels came into contact with sharp edges of Martian rocks. Scientists initially hoped that audio recordings of the rover's movement could alert them to problems, much like how motorists recognise emerging faults when their car starts making uncharacteristic sounds and noises.
However, the Perseverance rover is still far from completing its mission, and it is hoped that its microphones will be able to record something interesting and extraordinary. In the meantime, scientists have already adjusted some plans so that the spacecraft that will go to Venus and Titan, Saturn's satellite, will now also have their own microphones.