Scientists baffled as balloons in stratosphere record mysterious sounds of ‘completely unknown’ origin

Hot air balloons launched by scientists have recorded mysterious sounds of “completely unknown” origin, high in the Earth’s atmosphere.

Large 6-7-metre-long balloons were sent to the stratosphere – the relatively calm layer of Earth’s atmosphere which is rarely disturbed by planes or turbulence – by researchers, including Daniel Bowman of Sandia National Laboratories in the US.

In this layer of the Earth’s outer atmosphere, scientific instruments on balloons can pick up a range of sounds that are unheard elsewhere, including the natural sounds of colliding ocean waves and thunder, as well as human-made ones like wind turbines or explosions.

While the balloons could detect such human and environmental sounds, researchers reported in a presentation at the 184th Meeting of the Acoustical Society of America, that they also managed to record some strange sounds that could not be identified.

“[In the stratosphere] there are mysterious infrasound signals that occur a few times per hour on some flights, but the source of these is completely unknown,” Dr Bowman said in a statement.

The balloons, originally designed to monitor volcanoes on Earth, can collect data and detect low-frequency, infrasound using micro barometers.

Researchers tracked the routes of these balloons using GPS as they can sometimes sail for hundreds of miles and land in hard-to-reach places.

“Our balloons are basically giant plastic bags with some charcoal dust on the inside to make them dark. We build them using painter’s plastic from the hardware store, shipping tape, and charcoal powder from pyrotechnic supply stores,” Dr Bowman said.

“When the sun shines on the dark balloons, the air inside heats up and becomes buoyant. This passive solar power is enough to bring the balloons from the surface to over 20km (66,000 ft) in the sky,” he explained.

Scientists said their balloons can also help explore other planets.

They theorised the use of these balloons to observe Venus’s seismic and volcanic activity through its thick atmosphere.

“A new generation of Venus balloons is now being designed that can last over 100 days and can change their altitude to navigate different layers of Venus’s atmosphere,” the scientists wrote in the study.

As part of the next phase of experiments, they hope to catalog signals that are analogous to those on Venus, and develop tools that can automatically identify signals of interest.

“Conducting this experiment from a balloon floating at an altitude of 50-60km above the surface of Venus provides a significantly extended observation period, surpassing the lifespan of any spacecraft landed on the surface with current technology,” they said.