“Trying to detect naturally occurring earthquakes from balloons is a challenge, and when you first look at the data, you can feel disappointed, as most low-magnitude quakes don’t produce strong sound waves in the atmosphere,” said Quentin Brissaud, a seismologist at Caltech’s Seismological Laboratory and the Norwegian Seismic Array (NORSAR) in Oslo, Norway. “All kinds of environmental noise is detected; even the balloons themselves generate noise.”
During previous tests, the researchers detected the acoustic signals from seismic waves generated by a seismic hammer (a heavy mass that is dropped to the ground), as well as explosives detonated on the ground below tethered balloons. But could the researchers do the same with free-floating balloons above a natural earthquake? The main challenge among others: There was no guarantee an earthquake would even happen while the balloons were aloft.
On July 22, they had a lucky break: Ground-based seismometers registered a magnitude 4.2 aftershock nearly 50 miles (80 kilometers) away. About 32 seconds later, one balloon detected a low-frequency acoustic vibration – a type of sound wave below the threshold of human hearing called infrasound – wash over it as it was ascending to an altitude of nearly 3 miles (4.8 kilometers). Through analysis and comparisons with computer models and simulations, the researchers confirmed that they had, for the first time, detected a naturally occurring earthquake from a balloon-borne instrument.
“Because there is such a dense network of seismometer ground stations in Southern California, we were able to get the ‘ground truth’ as to timing of the quake and its location,” said Brissaud, the study’s lead author. “The wave we detected was strongly correlated with nearby ground stations, and when compared to modeled data, that convinced us – we had heard an earthquake.”
The researchers will continue flying the balloons over seismically active regions to become more familiar with the infrasound signatures associated with these events. By adding several barometers to the same balloon and flying multiple balloons at once, they hope to pinpoint where a quake occurs without needing confirmation from ground stations.
From California to Venus
Sending balloons to Venus has already been proven feasible. The two Vega mission balloons deployed there in 1985 by a Soviet-led cooperative transmitted data for over 46 hours. Neither carried instruments to detect seismic activity. Now this study demonstrates that the technique for detecting infrasound at Venus may be possible as well. In fact, because Venus’ atmosphere is much denser than Earth’s, sound waves travel far more efficiently.
“The acoustic coupling of quakes into the atmosphere is calculated to be 60 times stronger on Venus than on Earth, meaning it should be easier to detect venusquakes from the cool layers of Venus’ atmosphere between 50 to 60 kilometers [about 31 to 37 miles] in altitude,” said JPL technologist Siddharth Krishnamoorthy, principal investigator of the analysis effort. “We should be able to detect venusquakes, volcanic processes, and outgassing events while characterizing the levels of activity.”
What interests Krishnamoorthy the most about flying balloons on Venus is that scientists could use them to drift over regions that look like they should be seismically active based on satellite observations and find out whether they really are. “If we drift over a hotspot, or what looks like a volcano from orbit, the balloon would be able to listen for acoustic clues to work out if it’s indeed acting like a terrestrial volcano,” said Krishnamoorthy, who was also technical lead for the Ridgecrest balloon campaign. “In this way, balloons could provide the ground truth for satellite measurements.”
While the Venus balloon team continues to explore those possibilities, colleagues at NASA will be moving ahead with two missions the agency recently selected to go to Venus between 2028 and 2030: VERITAS will study the planet’s surface and interior, and DAVINCI+ will study its atmosphere. ESA (European Space Agency) has also announced its own mission to Venus, EnVision. These missions will offer new clues as to why the once-Earth-like planet became so inhospitable.