If a rover falls on Mars and there's no one around to hear it, does it still make a sound? Yes. But no one will know unless that probe carries a microphone, so that the sound can be detected back on Earth.
When NASA sends its Mars 2020 rover to the Red Planet, the bot may include an instrument to detect sound waves. The main scientific purpose of the instrument would be to study the composition of Martian rocks, but scientists with the mission said listening to the sounds of Mars could garner great interest from the public.
"There's a lot of good science that can be done by having a microphone on Mars," Sylvestre Maurice, a planetary scientist at the Research Institute in Astrophysics and Planetology in France, told Space.com. He and his colleagues investigated the possibility of pairing a microphone with a laser that will be used to vaporize rocks on the Martian surface. Maurice presented the results last month at the Lunar and Planetary Sciences Conference in The Woodlands, Texas. [NASA's Mars Rover 2020 Mission in Pictures (Gallery)]
In addition to gathering science, a microphone would represent a public relations coup, Maurice said.
"It will be the first time we can listen to a sound on Mars," he said.
This wouldn't, however, be the first time humans have put a microphone on Mars. NASA's Mars Polar Lander carried a microphone when it unsuccessfully crashed into the planet in 1999. The robotic Phoenix Lander successfully set down with a microphone at the Martian northern pole in 2008, but a glitch kept controllers from turning the mic on due to concern over shorting out the lander's system. The European Space Agency's ExoMars mission, which launched earlier this year (2016), also carried a microphone.
The new probe will aim to break that poor track record, and will listen for wind on the Red Planet, Maurice said.
"We know there's noise [on Mars]," he said. "There's a lot of wind on the planet."
In addition to the fast-moving winds of Mars, there will be the sounds that any rover traveling across the surface produces on its own, Maurice said.
The microphone could do even more science with the help of a laser that is scheduled to go aboard the Mars 2020 rover. Maurice serves as deputy principle investigator for the mission's SuperCam instrument, which contains a laser similar to the one used by NASA's Curiosity rover, which is currently exploring Mars. When the laser fires at a rock, it vaporizes some of the material, and that vapor can then be examined from a distance by the rover's instruments.
When the laser heats the material, that action produces a shock wave in the surrounding air, much like an airplane breaking the sound barrier. By studying the shock wave, scientists can discover even more insights into the vaporized material, Maurice said.
Although the sound made by the vaporizing rock is brief — lasting about 5 nanoseconds — it is easily detectable. Tests of the laser create a boom that can be heard across the lab, Maurice said. On Earth, the brief sound would be about as loud as a power saw or an incredibly brief rock concert. The atmosphere of Mars is thinner than Earth's, so sound travels slower, about 530 mph (240 meters/second) compared to 760 mph (340 m/s) on Earth.
So, under Martian conditions, the noise of the vaporization is reduced by about 10 percent, roughly the level of a lawn mower or jackhammer.
"It's a very powerful system. That's why it creates a very noisy sound," Maurice said.
The sound wave produced by the experiment is related to the amount of material vaporized by the laser, a property related to the material's composition.
And of course, in addition to those scientific results, the mission presents the awesome opportunity to hear Martian sounds back on Earth.
"It's science, but it's a little bit different," Maurice said. "It's cool, not obscure."