“In space, no one can hear you scream” was the tagline of the 1979 box office film success Alien. And it’s true. Sound waves propagate mechanically as a vibration and therefore need a medium — solid, liquid or gas — to travel through. Although interplanetary (and interstellar) space is not completely empty, gas molecules and dust grains are so sparsely distributed that they do not form a continuous medium that would enable sound waves to be transmitted directly.
But there are many locations in the solar system where it might actually be quite noisy. Such places will have a medium through which sound waves can be transmitted — for example, an atmosphere or an ocean. And we have only started to explore what they sound like.
NASA announced that its next mission to Mars, the Mars 2020 lander, will carry a microphone so that the soundscape of the planet can be recorded. This is not the first time that a microphone has been sent to Mars — the US Planetary Society sponsored a microphone on the Mars Polar Lander mission in 1999. Unfortunately, the spacecraft crashed before any recordings could be transmitted. A microphone was also part of one of the instruments on the Phoenix Lander of 2008, but because of concerns about an interface problem with the landing system, the instrument was not switched on.
The point of an experiment like this is to use the sound to infer how the pressure of Titan’s atmosphere changes with depth. This can then be used to build a circulation model for Titan, similar to the ones we use on Earth to forecast the weather and understand changes in climate. Here's audio from Titan's haze:
And, at a time when ESA’s Rosetta mission is drawing to a close, we should remember that its target comet, 67P Churyumov-Gerasimenko, was singing out into the void as it approached the sun. We also heard the thud of the comet lander Philae’s arrival when it touched down on the comet in November 2014.
There are soundscapes of other solar system bodies including Jupiter and the rings of Saturn. But these are not direct audio recordings — they are a conversion of electromagnetic vibrations into audio signals. They sound pretty weird.
You only have to imagine being in a desert to realize the variety of sounds a microphone on the surface of Mars could record — and how they can be interpreted. First of all, the wind, whistling across the planetary landscape — how fast is it travelling? How often does it vary in speed or direction? What does a dust devil sound like? Or a dust storm? What about the crack of thunder associated with a lightning bolt? Or the variation in pressure during an electric storm? Once the wind drops, the gentle sounds that break the silence can be heard: the settling of dust grains disturbed by the wind.
There are several engineering advantages to having a microphone carried by a rover on Mars. As the vehicle trundles across the landscape, we might hear the noise of crashing gears, and realise that sand had clogged the wheels. This would allow engineers to diagnose problems more efficiently, and work out strategies to ameliorate or avoid them.
We have heard some sounds of a rover on Mars already: NASA released audio from the Opportunity Rover’s 11-year marathon. But like the sounds of Jupiter and Saturn’s rings, these sounds were not recorded directly — they are a conversion of the vibrations of the rover into audio as it travelled across the surface. The microphone on the Mars 2020 mission will be the first to pick up the sounds of Mars directly and transmit them to Earth.
What is interesting about the proposal for the microphone is the instrument into which it will be incorporated. It’s not an accelerometer, as on Titan and the previous Mars microphones, but on an instrument that is designed to measure the chemical composition of the rocks and soil by vapourising them: a Laser-Induced Breakdown Spectrometer. This works by firing a laser at a target, which “explodes” as a plasma and creates a very sharp pressure wave — the acoustic signal of which is proportional to the mass of sample being destroyed. Using the microphone to set up, calibrate and focus the laser will help improve the instrument. But at the same time, a whole raft of new sounds from the surface of the Red Planet will be picked up.
So where else might it be interesting to listen? I’d like to hear Europa or Enceladus, the respective moons of Jupiter and Saturn. They both have an ice-covered surface, below which is a deep ocean. Imagine what a microphone might pick up as a spacecraft penetrated the ice. The groaning of the icebergs as they moved against each other. The suck and pluck of more mushy ice as it percolated up through the cracks. The sudden whoosh of an ice geyser. And then into the ocean below. Waves slapping against the base of the icesheet. Water of different temperatures mixing — what does that sound like? Will there be bubbles? And perhaps as the penetrator settles onto the ocean floor, we might hear an unexpected crab scuttle past.