At first glance it is stratosphere From Earth it seems calm and still. But when the researchers started balloons Powered by solar energy, they discovered a hidden acoustic world with mysterious sounds of unknown origin at an altitude of 20 kilometers.
The sounds are “infrasound”, inaudible to the human ear just as light in the infrared spectrum is invisible to the human eye. Recorded with special instruments and accelerated thousands of times, they sound like muffled whispers.
Some sounds picked up by balloons have a definite source: a low murmur, like the sigh you hear when you put a seashell to your ear, is the distant sound of crashing sea waves. But other intermittent crackling noises cannot be explained.
Scientists launching balloons to Earth want to use our planet as a testing ground for interpreting records so we can one day send similar balloons to explore other worlds.
“We spend a lot of time arguing about what things are,” said Siddharth Krishnamoorthy, a research technologist at the Jet Propulsion Laboratory NASA in Pasadena, California.
“I’ve been doing this for about 10 years, and the fact that there are mysterious sounds that I don’t understand is disturbing, but not really a revelation,” said Daniel Bowman, a scientist at the Sandia Laboratory, which builds and launches the solarpowered balloons. Bowman is presenting his latest work at the 184th Acoustical Society of America meeting in Chicago this week.
“But it’s fun to talk to people like you who say, ‘Wait a minute, what’s that like?’ Do you hear things?’ Yes we are. But no, we don’t know what it is.
Understanding sounds is more than an intellectual pursuit. Scientists who launch balloons to Earth want to use our planet as a testing ground to interpret recordings so we can one day send similar balloons to explore other worlds.
$50 worth of duct tape, plastic sheeting, and charcoal dust
The venture of levitating balloons in the Earth’s stratosphere is surprisingly lowtech.
Bowman builds solarpowered hotair balloons from common materials found at hardware stores: painter’s plastic, tape, charcoal dust. To prevent his creations from being mistaken for spy balloons or interfering with airplanes, he informs the Federal Aviation Administration of his experiments.
The sun heats the air inside the balloon, making it less dense than the air outside, and then it escapes. The balloon reaches the stratosphere before sunset and then descends due to the change in temperature. Balloons fly where the wind blows them one traveled from central New Mexico to the outskirts of Houston.
Bowman was originally interested in using balloons to measure volcanic eruptions in places on Earth that are poorly monitored. A balloon hovering over a volcanically active region could detect the sound waves generated by an eruption and warn planes to change their flight path.
More recently, balloons have been used as earthquake monitors. The seismic waves generated by earthquakes travel through the earth, but they also create sound waves that travel through the air. Scientists have proved that using this method it is possible to detect a number of earthquakes from close and from a distance.
A balloon network managed to measure a large 7.3 magnitude earthquake almost 3,200 kilometers away. Up close, the balloons registered much smaller earthquakes.
Evidence that such measurements work and are accurate will provide background information for future extraterrestrial measurements.
The challenge is “figuring out how to do that on Earth and then exporting it to Venus,” Bowman said.
In the 1980s, Soviet scientists released balloons over the sea Venus as part of the Vega 1 and 2 missions. Designed for atmospheric measurements, the balloons collected and transmitted data for about 46 hours.
Now that the science of balloon acoustics has advanced, researchers want to take a step back. The question of whether Venus is volcanically active is central to trying to understand why, despite being so similar to Earth, it has become an uninhabitable planet. The activity inside planets is closely related to how surface conditions evolve and affect their atmosphere.
But on a planet where the surface is very hot and the pressure is very high, groundbased seismic instruments don’t last long enough to do the job. The atmosphere, on the other hand, is a bit milder more temperate, with more tolerable pressures at altitudes where a heliumpowered balloon of Venus would float, say about 30 miles or so.
And the density of the atmosphere brings an advantage to airborne seismic measurements: pressure waves from seismic events are easier to detect.
Krishnamoorthy said that if a magnitude 5 quake were to shake Venus, for example, the atmospheric signal picked up by a balloon would be about 60 times stronger than that produced by a corresponding earthquake on Earth.
This prompted Krishnamoorthy and Bowman to work together on a proposal for heliumpowered balloons that could fly to Venus on a hypothetical future mission. For now, though, their focus is on understanding the Earth, where different streams of data can help them figure out what a given signal means.
Some of the sounds are humanmade and can be easily tuned out, such as the distant rumble of a train or wind turbines. In other cases, scientists send balloons over known, noisy events to see what they sound like in the stratosphere—trying to catch thunder when a thunderstorm is forecast, or launching a balloon near an area where it is hydraulic fracking operations (fracking) often occur.
In one experiment, scientists attached a balloon and then repeatedly released a seismic hammer weighing 13 tons, creating small earthquakes that could be detected by instruments on the balloon.
“On Venus, we won’t have this large infrastructure on the ground” to verify the balloon readings, Krishnamoorthy said. “Let’s fly a little blind.” / TRANSLATION BY RENATO PRELORENTZOU