The balloon was floating over the Pacific Ocean when the first sound waves hit. For 11 seconds, a small device hanging below the large transparent globe recorded sudden, jerky fluctuations in air pressure – echoes of an earthquake more than 1,700 miles away.
That scientific instrument was one of four floating over the Malay Archipelago on December 14, 2021. On that day, the quartet became the first network of devices to monitor an earthquake from the airresearchers report in the Aug. 16 issue Geophysical Investigation Letters.
The finding could help scientists track earthquakes in remote areas of Earth and also opens the door for specially equipped balloons to one day be sent to study the geology of other worlds, including our closest planetary neighbor.
“Venus is Earth’s sister planet, but it’s the evil twin,” says David Mimoun, a planetary scientist at the University of Toulouse in France. “We don’t know why the two planets are so different. That is why we need measures.
The idea of using balloons to study distant noises on Earth has its roots in the Cold War. In the 1940s, the US military launched a top-secret project to spy on Soviet nuclear weapons tests using microphones attached to balloons floating in the atmosphere. When the ground shakes, it releases low-frequency sound waves that can travel long distances in the atmosphere. The military planned to use the microphones to pick up the sound of the ground shaking from a nuclear explosion. But the project was ultimately deemed too costly and was abandoned, though not before one of the balloons crashed in New Mexico, giving rise to the Roswell conspiracy.
For decades, balloon science remained primarily in the realm of meteorology. Then, in the early 2000s, Mimoun and his colleagues began experimenting with using balloons for space exploration, specifically to study extraterrestrial earthquakes.
Analyzing tremors is one of the main ways scientists can learn about the interior of a planet. On worlds with thin atmospheres, like Mars or Earth’s moon, this usually means sending a lander to the surface and measure earthquakes directly on the ground (Serial number: 05/13/22).
But doing that on Venus isn’t really an option. The dense atmosphere means the planet’s surface has about the same pressure as Earth’s deep ocean, with temperatures averaging around 450° Celsius, hot enough to melt lead. “Basically, it’s hell,” says Mimoun.
The Landers have reached the surface of venus prior to (Serial Number: 6/19/76). But these probes lasted only a few hours before succumbing to extreme heat and pressure. The chances of measuring an earthquake in that short period of time are slim, says Siddharth Krishnamoorthy, a research technologist at NASA’s Jet Propulsion Laboratory in Pasadena, California, who was not involved in the study. So while radar images of Venus have revealed a world dotted with volcanoes, scientists still don’t know for sure whether Venus is geologically active, he says.
Scientists have previously experimented with the idea of detecting earthquakes on Venus using orbiters (Serial number: 9/02/05). But earthquake-detecting balloons have better resolution, Mimoun says, meaning they could provide the key to revealing life inside the planet. But first, Mimoun and his colleagues had to show they could design devices small enough to be carried by balloons but sensitive enough to detect earthquakes far below.
In 2021, the team attached microbarometers to 16 balloons launched from the Seychelles Islands, off the coast of East Africa. In December, four balloons, separated by thousands of kilometers, recorded similar low-frequency sound waves. These changes in air pressure resembled ground readings from a magnitude 7.3 earthquake near the Indonesian island of Flores, indicating that the sound waves were produced by the earthquake. The researchers were able to use changes in air pressure to pinpoint the earthquake’s epicenter and calculate its magnitude.
“This is a big step forward in demonstrating the usefulness of this technology,” says Paul Byrne, a planetary scientist at Washington University in St. Louis, who was not involved in the study.
Even without being able to detect earthquakes, the balloons, if designed to survive in Venus’s atmosphere, could detect changes in air pressure that reveal clues about volcanic eruptions and the planet’s mysterious highlands, says Byrne.
Venus is entering a renaissance of interest from space agencies. At least two NASA missions are planned to visit the planet by the end of this decade (Serial number: 2/6/21). Mimoun hopes earthquake-detecting balloons will be included in the next big mission, stressing that their data could help researchers understand why Earth and Venus, similar in size and distance from the sun, relative to the other planets, They have followed such different paths. .
“We have no idea,” says Mimoun. “So we have to go back.”