Solar-powered balloons launched into Earth’s stratosphere have recorded a series of mysterious noises, and scientists cannot pinpoint their origins.
The noises, detected by specialized instruments 70,000 feet above the Earth’s surface, are known as infrasound because they are so low that they are inaudible to human ears. Selected from a wave of hidden low-frequency sounds, including thunder, ocean waves, rocket launches, cities, wind turbines, and even planes, trains, and cars, the strange infrasounds have so far defied explanation.
“[In the stratosphere,] there are mysterious infrasound signals that occur several times an hour on some flights, but the source of these is completely unknown,” said the lead researcher. daniel archer (opens in a new tab)senior scientist at Sandia National Laboratories in New Mexico, said in a statement (opens in a new tab).
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Starting about 9 miles (14.5 km) above Earth’s surface and extending upward to a height of about 31 miles (50 km), the stratosphere is the layer of the atmosphere above our own. Full of ultraviolet blocking ozone, the stratosphere is a calm place, with little turbulence. Most sounds at this altitude originate from ultra-low frequency reverberations from the Earth’s surface.
Amateur scientists and researchers have been sending balloons into the stratosphere since the 1890s. One of the first experiments with microphone balloons, the top-secret Project Mogul military experiment designed to detect sounds from Soviet atomic bomb tests in the late 1940s, crashed in Roswell, New Mexico in 1947, leading to a UFO-inspired cover-up Conspiracy theories for this day.
To test the soundscape of the stratosphere, Bowman and his colleagues built a series of 23-foot-wide (7-meter) plastic balloons, affixing them with infrasound sensors called microbarometers and adding coal dust. The darkening property of charcoal allows sunlight to heat the air inside the balloon, causing the balloons to float.
“Our balloons are basically giant plastic bags with a little bit of charcoal dust inside to darken them. We build them out of painter’s plastic from the hardware store, duct tape, and charcoal dust from pyrotechnic supply stores,” Bowman said. “When the sun shines on the dark balloons, the air inside heats up and becomes buoyant. This passive solar energy is enough to take the balloons from the surface more than 20 km (66,000 feet) into the sky.”
Starting with their first balloon launch in 2016, the researchers sent 50 balloons into the sky to sample the rumbles and low rumbles of the stratosphere. The researchers initially set out to record the sounds of volcanic eruptions, but they also studied the other sounds they picked up, tracking their balloons across hundreds-of-mile flight paths using GPS.
It was during these flights that the researchers picked up the sounds: low, recurring rumbles whose signals could not be traced. Scientists have a few ideas about what these mysterious noises might be, and they range from a previously undetected form of atmospheric turbulence to echoes from below that have been distorted beyond recognition.
The researchers say they will continue to investigate sounds in the stratosphere, tracing more sounds to their points of origin and studying their variability across seasons and in different regions of the world.
The researchers presented their findings May 11 at the 184th Meeting of the Acoustical Society of America (opens in a new tab) In Chicago.