Speed of Sound on Mars

Robotic exploration of the red planet has taught us a great deal about our nearest planetary neighbor, and NASA’s Curiosity rover had an outsized role in accomplishing that. Its successor, a very similar rover called Perseverance, arrived on Mars last year. Perseverance sports an impressive array of instruments for studying Mars, including a microphone. Recently a team from Los Alamos National Laboratory was able to use the microphone and a laser to calculate the speed of sound on Mars (PDF), and it’s surprisingly slow. 
NASA did not build Perseverance with a microphone for this particular task, reports Phys.org. The mic sits at the top of the SuperCam assembly — that’s the part of the rover you probably think of as its “head.” The microphone is there to record pressure fluctuations from the use of the rover’s powerful infrared laser, which it uses to blast rocks for spectroscopic analysis. The audio recordings are available on NASA’s rover page, and a team of researchers from Los Alamos National Laboratory (LANL) figured they could tease some interesting details out of the data. 
According to LANL’s Baptiste Chide, it was possible to calculate the speed of sound using the propagation time of the laser-induced acoustic signals. We know that the SuperCam laser fires with a cadence of 3Hz, and the microphone and laser enjoy precise synchronization. Plus, the SuperCam can provide the distance to the laser’s target to within half a percent. Put it all together, and you can calculate the speed of sound to about the same level of accuracy. 

The LANL team says the speed of sound on Mars is 240 meters per second (about 536 mph), substantially slower than its speed in Earth’s atmosphere of 340 meters per second (761 mph). Mars has a very thin atmosphere, but it’s still enough to transmit sound. It just might not sound the way you expect. The team says different frequencies travel at noticeably different speeds on Mars. Anything above 400Hz will propagate about 10 meters per second faster, so different parts of speech could arrive at different times. That would make conversations much harder to understand, but that’s all theoretical — no one is on Mars to chat, and if they were, they’d be wearing protective gear operating at Earth-like pressures. 
Still, this is a clever way to make use of the microphone on Perseverance. The team also says they used the mic to calculate surface temperature, which is possible because sound travels at different speeds depending on temperature. Firing the ChemCam laser caused temperature spikes that were detectable with the microphone. LANL researchers plan to continue monitoring the sounds of Mars as Perseverance continues its mission.
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