Jakarta –
A solar-powered balloon launched into Earth’s stratosphere recently has recorded a series of mysterious rumbles, and scientists are having a hard time pinpointing their origin.
Noise detected by special instruments above the Earth’s surface is known as infrasound because it is pitched so low that it is inaudible to the human ear.
This mysterious sound has been sorted out from among the hidden low-frequency sound waves, including thunder, ocean waves, rocket launches, wind turbines, even planes, trains, and cars. But the strange infrasound has so far not been explained.
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“In the stratosphere there are mysterious infrasound signals that occur several times per hour on some flights, but the source is completely unknown,” said lead researcher Daniel Bowman, senior scientist at Sandia National Laboratories in New Mexico, quoted from Live Science.
Starting about 14.5 km above the Earth’s surface and extending to an altitude of approximately 50 km, the stratosphere is the layer of the atmosphere above us.
This layer is filled with the ultraviolet barrier ozone. The stratosphere is a calm place, with little turbulence. The majority of sound at this altitude comes from very low-frequency echoes from the Earth’s surface.
Scientists and amateur investigators have been sending balloons into the stratosphere since the 1890s. One of the first microphoneic balloon experiments, Project Mogul’s top-secret military experiment designed to detect sound from Soviet atomic bomb tests in the late 1940s, crash-landed at Roswell, New Mexico in 1947, leading to the cover-up that inspired the UFO conspiracy theory to date.
To get a feel for the stratospheric soundscape, Bowman and his colleagues made a series of 7-meter-wide plastic balloons, strapped them to an infrasound sensor called a microbarometer and added charcoal powder. The dark nature of charcoal allows sunlight to heat the air inside the balloon, causing it to float.
“Our balloon is basically a giant plastic bag with charcoal dust in it to darken it. We made it using painter’s plastic from a hardware store, shipping tape, and charcoal powder from a pyrotechnics supply store,” says Bowman.
“When the Sun shines on a dark balloon, the air inside heats up and becomes light. This passive solar power is enough to carry the balloon from the surface to an altitude of more than 20 km in the sky,” he said.
Starting with their first balloon release in 2016, the researchers sent 50 balloons into the sky to sample the low thumps and rumble of the stratosphere. The researchers initially started recording sounds from volcanic eruptions, but studied other sounds they picked up as well, tracking their balloons across hundreds of kilometers of flight paths using GPS.
It was during this flight that the researchers caught sounds in the form of a low, repetitive rumble whose signal could not be traced. Scientists have had several guesses about these mysterious noises, ranging from previously undetected forms of atmospheric turbulence to echoes from below that have become unrecognizable.
Researchers say they will continue to investigate sounds in the stratosphere, tracking more sounds to their point of origin and studying their variability across seasons and different regions of the world.
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(rns/rns)
2023-08-18 09:30:58
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