YOU are entering an alien soundscape. The first thing you notice is a chorus of curious pings – or perhaps you’d call them chirps. Underneath their bright staccato is an almost ominous roar. And faintly, in the distance, could that be the whistle of a railway train?
In a few years, sounds like these might be proclaiming good news for life on Europa, the pale moon of Jupiter that may be one of the most hospitable spots in the solar system. Although its surface is an airless landscape of cracked ice, all the evidence says that beneath that bleak shell is a liquid water ocean stretching hundreds of kilometres down to the rocky mantle below. If life can thrive on Earth’s ocean floors, feeding on the chemicals that gush from the rocks, why not on Europa too?
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NASA is already putting together a mission to this intriguing world. Europa Clipper should launch in the early 2020s, and when it arrives in orbit around Jupiter it will repeatedly swoop over the moon, picking up valuable magnetic and gravitational information about its structure. And there are tentative plans for a second mission that would land on Europa’s surface. According to a report published in February, the lander’s panoply of instruments should include a small seismometer – a simple device that would give scientists an ear on Europa’s inner workings.
This could enable them to learn more about its ice crust, work out the chemistry of the ocean and the rocks beneath, and perhaps pick up the music of active geysers on the surface and volcanoes erupting on the sea floor. Put it all together, and we could get a much better idea of whether Europa is a healthy spot for life.
“So now we need to know what we’ll be listening for,” says Mark Panning, a member of the mission team based at the University of Florida in Gainesville. To help them make the most of any data sent back, Panning and his colleagues have simulated the vibrations a seismometer might pick up on Europa, and turned the result into sound files.
“Previous studies tried to do detailed modelling,” says Panning. “A lot of it was very dependent on assumptions about how the ice behaves – a lot of unknowns thrown together.” His team chose instead to focus on the big picture, by working out how much energy was likely to go into the seismic processes that shake Europa’s crust.
It all starts with the tide. Just as our moon exerts a pull on Earth, Jupiter’s gravity stretches out Europa. Because the moon’s orbit is slightly elongated, and it wobbles from side to side as it travels round Jupiter, this tidal distortion is constantly shifting, repeatedly stretching and squeezing Europa in different directions. These contortions heat up the moon enough to keep its inner ocean from freezing solid. They should also create cracks in the ice shell, or icequakes, a process whose total energy can be worked out without too many assumptions.
Panning and his collaborators assumed that Europa’s icequakes follow a similar pattern to tremors on our planet and moon. As seismometers on Earth and those left behind by the Apollo missions have shown, quakes on both bodies have the same probability distribution, with those one notch higher on the magnitude scale happening one-tenth as often. This relationship meant the team could divide the total tidal energy felt by Europa into a plausible set of icequakes.
Team member Simon Stähler at the Ludwig Maximilian University of Munich in Germany then modelled how vibrations from each quake travel through the ice crust and ocean before being picked up by a seismometer. Finally, he speeded up the recording 500 times, to shift low, slow quake vibrations up into the audible range. This last step wasn’t strictly necessary for science purposes, says Panning. “Making it audible was just fun.”
The resulting audio file consists of a series of pings, like the sound of pebbles being thrown onto a frozen lake. This is no coincidence. Europa’s ice crust is kilometres thick, but it vibrates in roughly the same way as ice on a lake, by bending. Bending waves of this type move faster when they have higher frequency, so the high-pitched sounds reach the seismometer before the lower notes – resulting in that characteristic ping.
Listen to the sound of our own planet and you’d hear something quite different. Earth’s rocky crust doesn’t sit on a liquid layer, but instead on a thick, rocky mantle. This solid sandwich won’t bend, so the energy released by earthquakes takes the form of waves that either travel deep through the planet or cling close to the surface. The upshot is that when the seismic waves from a cluster of earthquakes are sped up, you hear a series of percussive cracks from fast-travelling deeper body waves, each followed by the boom of slower surface waves, which then rise in pitch.
Because we don’t know exactly how thick Europa’s ice crust is, the team ran simulations for icequakes travelling through layers either 5 or 20 kilometres deep. “The pings in the thinner ice shell are more dispersed,” says Panning, with each one lasting longer.
And while the ice cracks, Europa’s dark ocean roars. Simulations show that its currents are probably turbulent, with eddies moving at up to 2 metres per second. These would push up against the crust, sending out seismic waves of their own. The team has now added this effect, resulting in a distant, deep rushing sound.
“Cracks in Europa’s icy crust would sound like pebbles thrown on a frozen lake”
Hidden under these sounds will be subtler signals that can be used to work out Europa’s structure. As well as the pings produced by the bending ice sheets, each icequake sends out two other types of wave. There are compression waves, like sound in air, and shear waves – a side-to-side shaking. These are much lower in amplitude than the bending waves, making them inaudible on the sound files, but their reflection at boundaries within the moon could give measures of crust thickness and ocean depth. Oil companies use similar principles – though relying on explosives rather than cracking ice – to look for promising spots to drill, says Panning. Without a seismometer in place at present, we can only get a hazy view inside Europa and other moons, by measuring their magnetic and gravitational fields from a distance. Unlike seismology, these methods can’t clearly identify sharp boundaries within a body.
Seismic data could also reveal whether Europa’s seabed is smooth or rough and rugged. And combined with gravity and magnetic measurements, they could give us a taste of how salty the ocean is, because sound travels faster in saltier water.
Audio guide
Planting a seismometer on Europa’s surface may even make it possible to look deeper. Some of the sound of an icequake will penetrate the mantle, reach the metal core that may exist within and bounce back. If the mission is lucky, there might even be a big quake way down in the mantle, which would set the whole moon ringing and reveal its deep structure more clearly.
In the absence of that, or an unlikely large cometary impact, icequakes will probably be the loudest thing the proposed lander will hear during the few weeks of its battery-powered life. But some fainter, more intriguing voices might also make themselves heard. The Hubble Space Telescope has spotted plumes of water vapour rising from Europa, similar to those puffed out by geysers on Saturn’s moon Enceladus. Geysers on Earth have a characteristic sound, with a base note overlaid by a series of harmonics. “When you speed it up, it sounds like a train whistle,” says Panning. His colleague Steven Vance, at the Jet Propulsion Laboratory in Pasadena, California, says that capturing this sound with a seismometer should allow us to work out the size of the water chambers that feed these plumes, and gain some insight into how the geysers work.
And if all the other noise doesn’t drown it out, any volcano on the sea floor could add its own whistling to the choir, suggesting a steady flow of hydrogen and methane into the ocean. That would be encouraging news for Europa’s habitability, because bacteria on Earth eat these substances. Hydrogen and methane might be created when hot fluids chemically alter rocks in the mantle, leaving behind less dense minerals with a distinct seismological signature.
Although these chemical morsels could power life as we know it, they won’t be enough on their own. Hydrogen and methane can only be digested if there is also a supply of oxygen, which is scarce on most worlds. Oxygen is abundant on Earth only because of our long history of photosynthesis. While plant life on Europa is a slim possibility, this moon has another source of the element: charged particles from Jupiter’s radiation belts generate free oxygen by breaking up water molecules on Europa’s icy surface.
A big question is how much of this life-giving gas would find its way to the ocean below. Again, a seismometer could help to give us an answer. For a start, the thickness of the icy crust matters, because a thinner layer of ice should be relatively permeable. Seismology should also reveal whether water is seeping through the ice and whether the bottom bit of the crust consists of convecting slush. Both these things would affect how rapidly the crust’s structure changes, and how long it takes oxygen formed at the surface to reach the sea. “There might also be plate-tectonic-like behaviour on Europa,” says Vance. That would be a direct route for oxygen to ride down into the ocean on subducting plates of ice.
With all the deep knowledge on offer from such a small device, future missions to other outer moons might want to include a seismometer too. Enceladus would be a good target, says Vance. “We know it’s seismically active and we know where to land.”
He is also interested in the biggest ice moons, including Jupiter’s Ganymede and Callisto. His calculations suggest these objects could resemble multi-decker sandwiches, with oceans stacked beneath oceans, separated by exotic forms of ice that only exist at high pressure. “You could only find these with seismology,” says Vance.
Saturn’s biggest moon may be especially appealing from a sonic point of view. “We know Titan should be seismically active, from the methane atmosphere,” says Vance. “We could also hear water sloshing around the lakes.”
The Europa lander has yet to be approved, and missions to other icy moons are even more distant prospects, but a seismometer will soon be en route to a slightly closer target. An instrument called SEIS is due to launch on the InSight mission to Mars next year, aiming to show us the deep structure of the Red Planet. Astronomers are all ears.
This article appeared in print under the headline “Send an ear”
