Mars has two speeds of sound

The speed of sound on Mars depends on its intensity.

All sounds travel more slowly through Martian air than Earth’s air. On April 1, researchers reported that the high-intensity beeps of laser launch rocks travel slightly faster in Mars’ thin atmosphere than the low-intensity hum of an Ingenuity helicopter. temper nature.

These sound speed measurements from NASA’s Perseverance probe are part of a broader effort to monitor minute-by-minute changes in atmospheric pressure and temperature, such as during winds blowing on the Red Planet.

“Wind is the voice of science for us,” says astrophysicist Baptiste Scheide of Los Alamos National Laboratory in New Mexico.

To listen to the wind, persevere holds two microphones. Audio was supposed to be recorded during the complex entry, descent and landing of the mission, and while it did not work as hoped, it is now played occasionally to listen to the vital elements of the rover (SN: 2/22/21; SN: 2/17/21). The other microphone is part of the rover’s SuperCam instrument, which is a combination of mast-mounted cameras and other sensors used to understand the properties of materials on the planet’s surface.

But these microphones also pick up other sounds, such as those made by the rover itself when its wheels hit the surface, and by the perseverance buddy to fly, the ingenuity of the robotic helicopter. The SuperCam instrument, for example, contains a laser, which persistently shoots at rocks of interest for further analysis (SN: 7/28/20). The microphone on the SuperCam picks up the sounds from these laser shots, helping researchers learn about the hardness of the target material, says planetary scientist Naomi Murdoch of the Higher Institute of Space in Toulouse, France.

Murdoch, Chedd and their colleagues heard the crackling of the laser when hitting the rocks. (It doesn’t really work, says Murdoch.) When the laser hits the target, that blast produces a sound wave. Because scientists know when the laser is firing and how far away the target is, they can measure the speed at which the sound wave travels through the air toward SuperCam microphone.

The team says the speed of this sound is about 250 meters per second. This is slower than on Earth, as sound travels through the air at a speed of about 340 m/s.

The slow speed is not surprising. What we hear as sound are actually pressure waves traveling through a medium such as air, and the speed of those waves depends on the density and composition of the medium (SN: 10/9/20). The density of the atmosphere of our planet is 160 times that of the atmosphere of Mars, Earth’s air consists mostly of nitrogen and oxygen, while the air of Mars consists mostly of carbon dioxide. So sound on Mars travels slower in this different air.

The team also used a SuperCam microphone to listen to the low-pitched vortex of the Ingenuity helicopter’s blades (SN: 10/12/21). From this low-pitched sound, researchers learned that there is a second velocity of sound on Mars at frequencies below 240 Hz, or slightly deeper than middle C on a piano: 240 m/s.

In contrast, sound on the Earth’s surface in air moves at only one speed, regardless of the pitch. The researchers say that the two speeds on Mars are due to its atmosphere rich in carbon dioxide. Carbon dioxide molecules behave differently with each other when sound waves with frequencies above 240 Hz move through the air compared to those below 240 Hz, which affects the speed of the waves.

“We’ve proven that we can do science with a microphone on Mars,” says Scheid. “We can do good Science.”

SuperCam microphone captures thousands of audio snippets per second. These sounds are affected by air pressures, so researchers can use that acoustic data to track detailed changes in air pressures over short periods of time, and in doing so, learn more about the Martian climate. While other spacecraft on Mars have wind, temperature and pressure sensors, they can only sense changes over longer periods.

“Hearing sounds on another planet is another way that helps us all position ourselves as if we were there,” says Melissa Trainer, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who was not part of this work. .

The team is focused on collecting acoustic data at different times of the day and different seasons on Mars.

“The pressure on Mars changes a lot throughout the year according to the seasons,” Trainer says. “I’m really excited to see how the data can change when it’s collected over the follow-up seasons.”

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