acoustics, astronomy, Curiosity, Mars, NASA, National Aeronautics and Space Agency, organ, popular music, Public Radio International, Saturn, solar system, space, spacecraft, Timothy Leighton, Toccata and Fugue in D minor, University of Southampton, Venus, will.i.am
On 28 August 2012, NASA’s Curiosity rover broadcast a song by singer-songwriter will.i.am from Mars. The rover, you might have heard, is currently roving the surface of the planet, and the new track sent back to Earth was titled “Reach for the Stars.” Although it’s tempting to criticize NASA for using a $2.6 billion dollar scientific instrument to transmit pop music, the event was marketed as educational. “Members of the team that successfully landed the rover on Mars earlier this month,” NASA wrote in a press release, “will explain to students the mission and the technology behind the song’s interplanetary transmission.” That’s interesting. But what would the tune sound like if it were played on Mars? Would it have that same “Boom Boom Pow”?
Not quite. If “Reach for the Stars” was recorded on Earth and then played from a rover-mounted speaker, the main thing we’d notice is that, ten meters from the sound source, the song would be imperceptible. Sound is perceived when our ears detect the vibrations of molecules in the atmosphere. Earth’s atmosphere consists mostly of nitrogen and oxygen; Mars’s atmosphere is, for the most part, made up of a very thin layer of carbon dioxide. The molecular structure of carbon dioxide makes it a great sound absorber, and, more generally, sound propagates poorly in a thin atmosphere. As Timothy Leighton, a professor of ultrasonics and underwater acoustics at the University of Southampton, told me, “Mars is quite dead acoustically. You’d have to be pretty close to the sound source to actually hear anything.”
But if will.i.am were to somehow sing “Reach for the Stars” standing on the red planet, and we were close enough to hear him, he’d actually sound a lot taller than his five-foot-nine frame. As Leighton explained, when vocal cords beat during speech, subtle echoes are produced in the windpipe, the tube that connects the larynx to the lungs. The rate that these echoes bounce around at partly reflects the length of the speaker’s windpipe. Our brains use this cue to help discern the size of people we hear speaking or singing (the assumption being that bigger humans have bigger windpipes).
On Mars the carbon dioxide-rich atmosphere makes sound travel slower; consequently, these echoes would bounce around more slowly and sound like they came from a much bigger windpipe. “According to our ears,” Leighton said, “will.i.am would sound about 1.4 times taller,” or slightly more than eight feet tall. The mechanism behind the phenomenon is similar to what happens when people breathe in helium and speak, but in this case the effect is the opposite.
Leighton knows all this because he recently developed software to alter sounds so they seem like they’re being produced in the atmosphere of an alien planet. As Public Radio International reported in 2008, Leighton and his colleagues morphed the sound of an organ playing Bach to make it seem like it was being played on Mars, Venus, and Titan (a moon that orbits Saturn). This year, they used their software to emulate speech on these worlds for, among other reasons, a show at the Astrium Planetarium near Winchester, U.K. Being able to simulate what things sound like on other planets, Leighton said, could help future space explorers detect the acoustical signals of dangerous environmental events, like a rock slide on Mars.