August 10, 2022: If you want to detect an earthquake on Venus–good luck. The planet’s surface is hot enough to melt lead, and the atmospheric pressure is crushing. No ground-based seismometer could possibly survive.

What’s an extraterrestrial seismologist to do? Launch a balloon.

A new paper just published in the Geophysical Research Letters reports the detection of a magnitude 7.3 earthquake by a fleet of balloons floating through the stratosphere above Indonesia’s Flores Sea. Onboard infrasound sensors registered acoustic waves rippling upward from the sea surface below, proving that, here on Earth, balloons can be used as seismometers.

“The same technique should work in the atmosphere of Venus,” says Raphael Garcia, the study’s lead author and a planetary scientist at the Institut Supérieur de l’Aéronatique et de l’Espace of the University of Toulouse. “Balloon-based sensors could float high above Venus’s deadly surface, collecting data at a safe distance.”

In the fall of 2021, the Centre National d’Etudes Spatiales (CNES) launched a fleet of 16 balloons from Mahé Island in the Seychelles archipelago. Unlike ordinary weather balloons, which explode in a matter of hours, these were “superpressure balloons,” which can remain aloft for months. Stratospheric winds carried them over the Flores Sea just in time for the temblor.

Four balloons picked up the undersea quake on Dec. 14, 2021. Combining their signals, researchers pinpointed the epicenter within 300 km, the magnitude of the quake within 0.8 units, and its onset within 50 seconds. Furthermore, waveforms recorded by the infrasound sensors were detailed enough to sense structures in the Earth 100 km deep.

Garcia would like to do the same thing on Venus. “We know nothing of its interior,” he says. “We don’t know how it’s made inside, and seismology is one of the best tools to figure that out.”

Seismic balloons could come in handy on our own planet, too. “Balloons could be used to cover ocean regions where conventional seismometers are not yet deployed,” notes Garcia.  “Another advantage: Balloons may be rapidly deployed just after a big quake for monitoring aftershocks.”

The test flights have already unearthed a curiosity in South America. On Nov. 28, 2021, just one of the balloons detected a magnitude 7.5 earthquake in northern Peru. The infrasound frequency, 0.23 Hz, was higher than expected; for comparison, the Flores Sea quake registered a more typical 0.085–0.125 Hz. Garcia’s team believes the high pitch may have been caused by a “ringing” of sediments in the Amazonian basin.

Sensing earthquakes from the stratosphere is relatively new. Researchers at Caltech and the Jet Propulsion Laboratory did it for the first time in July 2019. Garcia’s study marks the first time an earthquake was detected by more than one balloon. It won’t be the last.

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