Solar-powered balloons floating in the stratosphere have recorded low-frequency sounds of mysterious origin.
“When we started flying balloons years ago, we didn’t really know what we would hear,” he says. daniel archer at Sandia National Laboratories in New Mexico. “We learned to identify the sounds of explosions, meteorite crashes, airplanes, electrical storms, and cities. But pretty much every time we send out balloons, we find sounds we can’t identify.”
Bowman and his colleagues measured infrasound signals (sounds with a frequency so low that they are inaudible to human ears) using solar-powered balloons floating 20 kilometers high.
The researchers built balloons about 7 meters wide and made of thin plastic. They filled the balloons with coal dust, which is heated by sunlight and makes the balloon float. Unlike weather balloons, which soar until they explode, these solar-powered balloons hovered in the stratosphere for hours, carrying infrasound sensors for hundreds of miles. The researchers deployed more than 50 balloons over the course of seven years starting in 2016.
The data they collected showed that the stratosphere sounds quite different from Earth’s surface. On the ground, infrasound sensors pick up signals that have been deflected by descending winds, but the balloons floated above those winds, recording signs of turbulence elsewhere in the atmosphere and infrasonic sounds from sea storms. However, Bowman says that many infrasound signals from the stratosphere did not have an obvious origin. He presented the work at a meeting of the Acoustical Society of America in Chicago, Illinois, on May 11.
These mysterious signals could be related to types of atmospheric turbulence that have never been recorded before, but infrasound in the stratosphere has rarely been explored before, so it’s hard to guess, Bowman says.
He says one of the first such balloon studies was a US Army Air Forces experiment codenamed Project Mogul, which sought to detect infrasound signals from nuclear weapons tests in the Union. Soviet Union in the 1940s. One of the Project Mogul balloons crashed in Roswell, New Mexico, bringing the top-secret program into public view. The cover-up to hide the balloon’s purpose led to UFO conspiracies, and most data from subsequent balloon flights, ending in the 1960s, remained classified, Bowman says.
Roger Waxler at the University of Mississippi says he is not surprised by the enigmatic infrasound signals that appear in recordings from the stratosphere. “In the field, he can place sensors in arrays and know exactly where they are relative to each other, which helps calculate where infrasound is coming from. With balloons, they just go where they go,” he says.
Bowman is collaborating with NASA to develop similar balloon technology for an even less explored place: the clouds of Venus. He and his colleagues want to adapt their solar balloons to record infrasound on the surface of Venus, which could help record the planet’s seismic activity.