An international team of scientists is performing the first-ever analysis of acoustics on Mars.
Johns Hopkins Applied Physics Laboratory (APL) planetary scientist Ralph Lorenz is a co-author on a new study that reveals how fast sound travels through the Red Planet’s extremely thin, mostly carbon dioxide atmosphere, how Mars might sound to human ears, and how scientists can use audio recordings to probe subtle air-pressure changes on another world — and to gauge the health of the Perseverance rover.
“It’s a new sense of investigation we’ve never used before on Mars,” said Sylvestre Maurice, an astrophysicist at the University of Toulouse in France and lead author of the study, published April 1 in the journal Nature. “I expect many discoveries to come, using the atmosphere as a source of sound and the medium of propagation.”
Most of the sounds in the study were recorded using the microphone belonging to Perseverance’s SuperCam, mounted on the head of the rover’s mast. The study also refers to sounds recorded by another microphone mounted on the chassis of the rover. This microphone recently recorded the puffs and pings of the rover’s Gaseous Dust Removal Tool, or gDRT, which blows shavings off rock faces that the rover has abraded for examination.
The result: a new understanding of strange characteristics of the Martian atmosphere, where the speed of sound is slower than on Earth — and varies with pitch (or frequency). On Earth, sounds typically travel at 767 mph (about 340 meters per second). But on Mars, low-pitched sounds travel at 537 mph (about 240 meters per second), while higher-pitched sounds move at 559 mph (about 250 meters per second).
The variable sound speeds on Mars are an effect of the thin carbon dioxide atmosphere. Before the mission, scientists expected the cold carbon dioxide atmosphere would influence sound speed, but the phenomenon had never been observed until these recordings were made. Another effect of this thin atmosphere: Sounds carry only a short distance, and higher-pitched tones carry hardly at all.
On Earth, sound might drop off after about 213 feet (65 meters); on Mars, it falters at just 26 feet (8 meters), with high-pitched sounds being lost completely at that distance.
The recordings from SuperCam’s microphone also reveal previously unobserved pressure variations produced by turbulence in the Martian atmosphere as its energy changes at tiny scales. Martian wind gusts at very short timescales also were measured for the first time.
“The microphone is proving to be a new window on small-scale turbulence at Mars,” said Lorenz, a co-investigator on the SuperCam team. “Turbulent fluctuations in wind and pressure may be very important in controlling how dust and sand are moved around by the atmosphere.”
One of the most striking features of the sound recordings, said Maurice and coauthor Baptiste Chide of Los Alamos National Laboratory in New Mexico, is the silence that seems to prevail on Mars. “At some point, we thought the microphone was broken, it was so quiet,” Maurice said.
That, too, is a consequence of Mars having such a thin atmosphere.
“Mars is very quiet because of low atmospheric pressure,” Chide said. “But the pressure changes with the seasons on Mars.”
That means, in the Martian autumn months to come, Mars might get noisier — and provide even more insights into its otherworldly air and weather.
“We are entering a high-pressure season,” Chide said. “Maybe the acoustic environment on Mars will be less quiet than it was when we landed.”
SOUNDS OF THE MISSION
The acoustic team also studied what the SuperCam microphone picked up from the spinning double rotors of Ingenuity, the Mars helicopter traveling with the rover and serving as its aerial scout. Spinning at 2,500 revolutions per minute, the rotors produce “a distinctive, low-pitched sound at 84 hertz,” Maurice said, referring to the standard acoustic measure of vibrations per second, and the rotation rate for both rotors.
But SuperCam’s laser, which vaporizes bits of rock from a distance to study their composition, makes sparks when it strikes, and those sparks create a high-pitched noise above 2 kilohertz.
Studying sounds recorded by the rover’s microphones not only reveals details of the Martian atmosphere but also helps scientists and engineers assess the health and operation of the rover’s many instruments, just as one might notice a troubling noise while driving a car.
Meanwhile, the key instrument in the study, SuperCam’s microphone, continues to exceed expectations. Lorenz has been working with rest of the SuperCam team and with the Ingenuity helicopter team to match up variations in the recorded sound with the flight path and rotor settings.
“The basic fall-off of sound with distance was expected,” Lorenz said, “but there is additional modulation that seems to be related to the angle that the rotors make when they cross each other.”
THE TONES OF TITAN
Lorenz is the mission architect for NASA’s Dragonfly, a rotorcraft-lander set to explore Saturn’s largest moon, Titan. Dragonfly will carry two microphones on its meteorological experiment — which Lorenz also leads — to monitor the health of the rotors and drills on the spacecraft as it soars between and then samples the sands of Titan’s organic dunes.
The Huygens probe that landed and operated on the surface of Titan for about 70 minutes in 2005 carried some limited acoustic instrumentation, which showed that sound generally propagates well in Titan’s dense atmosphere — although, Lorenz added, there were some hints of absorbing gases sweated out of the ground by the warm probe. Dragonfly — with a baseline mission planned for about three years — will have a much longer opportunity to listen to the Titan environment, he added.
“Like Perseverance, the presence of sound sources on Dragonfly like pumps, drills and the rotors on the lander will also give us a means to interrogate the environment, rather than just waiting for something to happen.” Lorenz said.
The Mars 2020 Perseverance mission is part of NASA’s Moon to Mars exploration approach, which includes Artemis missions to the Moon that will help prepare for human exploration of the Red Planet. The Jet Propulsion Laboratory, which is managed for NASA by Caltech in Pasadena, California, built and manages operations of the Perseverance rover. For more about Perseverance, visit mars.nasa.gov/mars2020/.
APL manages Dragonfly for NASA and will build and operate the rotorcraft-lander. Led by Principal Investigator Elizabeth “Zibi” Turtle, of APL, Dragonfly is scheduled to launch in 2027 and reach Titan by the mid-2030s. For more information on this NASA New Frontiers mission, visit dragonfly.jhuapl.edu.
Based on a NASA/Jet Propulsion Laboratory news release
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