Some sounds picked up by the balloons have a clear source; a low murmur, similar to the sighing heard when a seashell is cradled to your ear, is the distant sound of ocean waves slamming into each other. But other intermittent crackles defy explanation.

“We spend an inordinate amount of time arguing about what things are,” said Siddharth Krishnamoorthy, a research technologist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

“I’ve been doing it for about 10 years now, and, you know, the fact that there’s mysterious sounds that I don’t understand is troubling, but it’s not like a revelation,” said Daniel Bowman, a principal scientist at Sandia National Laboratories who builds and launches the solar-powered balloons. Bowman is presenting his latest work this week at the 184th meeting of the Acoustical Society of America in Chicago.

“But it’s funny to speak with folks like you who are like, ‘Wait, what? You’re hearing stuff?’ And no, we don’t know what it is.”

Understanding the sounds is more than an intellectual pursuit. Scientists launching balloons on Earth want to use our planet as a test bed for interpreting balloon-based recordings so we can one day send similar balloons to study other worlds.

$50 of shipping tape, sheets of plastic and charcoal powder

Floating balloons into Earth’s stratosphere turns out to be a surprisingly low-tech enterprise.

Bowman constructs solar-powered hot-air balloons using ordinary materials found at a hardware store: sheets of painters’ plastic and shipping tape, with charcoal powder sprinkled inside. To avoid his creations’ being mistaken for spy balloons or interfering with aircraft, he gives the Federal Aviation Administration a heads-up about his experiments.

The sun heats the air inside the balloon, making it less dense than the outside air, and away it goes. The balloon reaches the stratosphere before the sun sets, when the change in temperature causes it to descend. The balloons go wherever the wind blows them — one traveled all the way from central New Mexico to the outskirts of Houston.

Bowman initially got interested in using balloons to measure volcanic eruptions in spots on Earth that don’t have great surveillance. A balloon floating above a volcanically active region could detect the sound waves generated by an eruption and warn airplanes to alter their flight paths.

How do Yellowstone’s seismic vibrations sound on the flute? Oddly beautiful.

But the balloons have more recently been used as sky-based earthquake monitors. The seismic waves generated by earthquakes move through the earth, but also create sound waves that move through the air. Scientists have recently proved that it’s possible to detect a range of quakes from near or far using this method.

A network of balloons was able to measure a large, 7.3-magnitude quake from nearly 2,000 miles away. From closer up, balloons have been able to detect much smaller quakes.

Proving that such measurements can work and are accurate on a planet that scientists mostly understand will provide ground truth for future extraterrestrial measurements.

The challenge is “to figure out how to do this on Earth, and then we’ll export it to Venus,” Bowman said.

Balloons above Venus

In the 1980s, Soviet scientists launched balloons above Venus as part of the Vega 1 and 2 missions. The balloons, aimed at making atmospheric measurements, collected and transmitted data for about 46 hours.

Now that the science of balloon-based acoustics has advanced, researchers want to go back. The question of whether Venus is volcanically active has become central in trying to understand why, despite being so similar to Earth, it turned out so uninhabitable. The activity inside planets’ interiors is closely linked to how conditions on the surface evolve and contributes to their atmospheres.

But on a planet where the surface is so hot, and the pressure so high, ground-based seismic instruments won’t last long enough to do the work. The atmosphere, on the other hand, is a bit gentler — more temperate, with more tolerable pressures at the heights where a Venus balloon powered by helium would float, somewhere around 30 miles up.

And the density of the atmosphere comes with an advantage for air-based seismic measurements: It will be easier to detect the pressure waves from seismic events.

Krishnamoorthy said that if a magnitude-5 temblor were to shake Venus, for example, the atmospheric signal picked up by a balloon would be about 60 times as strong as one generated by an equivalent quake on Earth.

That’s led Krishnamoorthy and Bowman to collaborate on a proposal for helium-powered balloons that could fly on a theoretical future Venus mission. For now, though, their focus is on understanding Earth, where multiple data streams can help them sleuth out what a signal means.

Some of the sounds are human-generated and can easily be ruled out, such as the distant rumble of a train or wind turbines. Other times, scientists send balloons over known noisy events to see what they sound like up in the stratosphere — trying to catch thunder if a storm is forecast, or launching a balloon near an area where there’s lots of fracking to see if they can measure an earthquake.

In one experiment, scientists tethered a balloon in place and then repeatedly dropped a seismic hammer, which weighed 13 metric tons, onto the ground to generate small earthquakes for instruments on the balloon to detect.

“On Venus, you won’t have this great infrastructure on the ground” to cross-check balloon readings, Krishnamoorthy said. “You’re flying a little bit blind.”