Researchers recently tested whether a balloon-borne sensor could listen for venusquakes to learn about the planet's makeup.

On Earth, a network of seismology ground stations identify the epicenter and strength of quakes. The new study demonstrates how a balloon equipped with sensors can do the same from the air. Photograph: NASA

One sent by the Russian Vega mission in the 1980s survived only 56 minutes on the planet’s surface. (Its predecessors, the Soviet Venera series of landers, also died abruptly.) This all might seem to make exploring Venus a dead end. Perhaps because of its lack of hospitality, the planet has been significantly less studied than Mars. For a sister planet, we simply don’t know much about the place.

But before future Venusian explorers lose hope, they’d do well to look up. Another part of the Vega mission is considered one of the great successes of space exploration: its balloons, which floated through the Venusian middle atmosphere 54 kilometers above the planet’s volcanic plains. Up there, it was practically balmy. Temperatures were around 80 degrees Fahrenheit, with pressure similar to Earth’s surface. The balloons voyaged almost 7,000 miles around the planet and survived more than two Earth days before their batteries died. Their limited sensors provided direct and indirect data on temperature, pressure, wind, light level, cloud particles, and altitude. (A modern analysis of the mission revealed that they might have floated through a drizzle of sulfuric acid rain.)

So while NASA has flirted with building a steampunk mechanical rover that might withstand the surface heat and pressure, they’re also thinking about exploring by air. What might they discover up in the planet’s sky? “It is by no means difficult to imagine an indigenous biology in the clouds of Venus,” Carl Sagan once wrote. (Perhaps, he said, one with a float-bladder sac and a taste for water and minerals.) Last year, scientists detected the signature of what may be phosphine, a potential sign of anaerobic life―or maybe just volcanoes―in the atmosphere, sparking a debate about alien life nearly as heated as the planet’s surface.

“Flying an aerial platform at Venus would be hugely advantageous,” Paul Byrne, a planetary geologist at North Carolina State University, told WIRED by email. Though not associated with the NASA team’s balloon research, Byrne feels the idea could deliver a host of new data to researchers, from chemical measurements of the planet’s atmosphere to evidence of a weak modern-day magnetic field to infrared images of the surface. “A Venus balloon? Oh yes, please.”

Which helps explain why, in 2019, a team of NASA JPL, Caltech, and international scientists found themselves scrambling to build a balloon that could accomplish one of the big objectives of a future Venusian airborne mission―eavesdropping on the planet’s seismology. Their findings, published in May in Geophysical Research Letters, show how capturing low-frequency sound waves in the atmosphere caused by earthquakes on our own planet is great practice for listening to venusquakes.

After the 2019 Ridgecrest quake, scientists rushed to build ultra-lightweight heliotrope balloons that could carry seismic sensing equipment aloft. Photograph: Gerry Walsh/NASA

Seismology is about waves. The epicenter of an earthquake is like a stone dropped into a pond. The disturbance ripples outward along Earth’s crust. That movement translates into a pressure change in the air just above the ground. This produces infrasound waves (long, slow sound waves so low that humans can’t hear them) that travel through the atmosphere both straight up from the epicenter (epicentral waves) and above seismic waves as they travel along the earth (surface waves).

On Earth, a network of seismology ground stations uses sensors to detect these waves, and to identify the epicenter and strength of quakes. The new study demonstrates how a balloon equipped with sensors can do the same from the air. A balloon-borne barometer that captures only epicentral or ground infrasound waves can lend some insight into a quake’s location and strength. One that captures both might tell what the crust of a planet looks like. That could prove useful in scoping out the surface of a planet we can’t actually see.

(Seismological data also works for ones we can see. Marsquake readings from the InSight lander have been invaluable in mapping the Martian crust.)

To prove that studying Venus’ seismology from the air was possible, the team planned a flight campaign in Oklahoma—where earthquakes are frequent, probably due to fracking—to test out whether they could hear the infrasound of Earth’s rumblings from high up in the atmosphere. But when the Ridgecrest series of earthquakes struck near JPL’s Los Angeles home base, in 2019, triggering thousands of small aftershocks, senior program manager James Cutts, research technologist Siddharth Krishnamoorthy, and others on the team sensed an opportunity. “This had to be done quickly, since the later it got, the weaker and less numerous the aftershocks were,” Krishnamoorthy says.

Problem: They didn’t have balloons yet. Over a frantic 16 days, they scrambled to build four ultralight “heliotropes,” simple balloons about 20 feet in diameter and 12 feet tall, made using plastic sheeting and tape. The heliotropes—named Tortoise, Hare, Hare 2, and CrazyCat—rose into the stratosphere as the sun heated the air inside their charcoal-covered plastic balloon “envelopes.” They floated freely with the breeze, each with a barometric sensor package hanging from a tether below, listening for the very faint sounds of an aftershock.

On July 22, 2019, the ground shook with that aftershock. As it passed below the balloons, it produced surface infrasound wave disturbances that traveled upward 4.8 kilometers and hit Tortoise’s barometer, registering as a series of tiny pressure changes. These changes were so small that it took Krishnamoorthy months of data analysis after the flight to see them. But there they were: Tiny wave profiles neatly matching quake readings from four ground-based seismometer stations in the area near the balloons. They matched computer models of infrasound propagation from the aftershock, too. Tortoise had heard the quake.

But could a balloon capture seismic infrasound while floating in the atmosphere of Venus? There, the balloon would be flying much, much higher—about 50 kilometers rather than 5. At that altitude, Venus’ acid clouds might attenuate the infrasound waves, making them slightly harder to detect, says Andi Petculescu, a theoretical acoustics physicist at the University of Louisiana Lafayette. (What does Venus sound like? Here’s his study on what Bach might sound like on Earth, Titan, Venus, and Mars, due to different sound wave attenuation factors.)

Yet other factors would work in the balloon’s favor. Though Venusian winds blow steadily at more than 200 miles per hour, a balloon at stable altitude should remain relatively “quiet” as it breezes along. (Imagine the calm of being on a hot air balloon, which is traveling at the same speed as the wind.) Because of the super-thick Venus atmosphere, Byrne writes, Venus’ surface is coupled to that atmosphere some 60 times more effectively than Earth’s is—which means that the energy from a quake will be much more readily transmitted into the atmosphere on Venus, making it a prime locale for floating a seismometer.

A future Venus balloon would also be a lot more complex than the homemade heliotrope. Imagine instead something like the tennis-court-sized, self-navigating balloons designed by Google’s recently shuttered “Loon” program, which aimed to use high-altitude balloons to beam internet worldwide. (Though the program was shut down, the balloons worked well, successfully providing internet connectivity to Puerto Rico in 2017 after Hurricane Maria damaged telecommunications infrastructure, and restoring emergency phone service after devastating flooding in Peru.)

A concept image for a balloon that could carry seismic infrasound sensing equipment over Venus. Illustration: Tibor Balint

A Venus mission would most likely include a variable-altitude “aerobot” in combination with a planetary orbiter for transmission relay to Earth, says James Cutts, a planetary scientist at JPL. Unlike the Vega balloons, which stayed at one altitude, a Venus aerobot would consist of two balloon “envelopes” that could exchange helium gas to increase or decrease buoyancy to rise or fall by roughly 10 kilometers. (This would also allow them to fly at night, unlike the heliotropes used in the California study.)

The aerobot would be powered by a solar array and rechargeable battery and could carry a payload of between 100 and 200 kilograms in a gondola hanging below—a major upgrade from the Russians’ 8-kilogram balloon. Cutts imagines a mission length of 100 days, during which the balloon would ride Venusian winds to circumnavigate the planet some 20 times or more.

A 2020 NASA flagship mission concept study report included an aerobot as part of a larger potential package including a lander, orbiter, and small two satellites that would be delivered to Venus by a Falcon 9 Heavy Expendable rocket. The aerobot would separate from the orbiter and drop into the Venusian atmosphere, protected from friction and heat by aeroshells, along with parachutes to slow down the craft’s initial descent and inflate its balloons. “Venus’ atmosphere is more compact than Earth’s,” Cutts says. “Some people have compared [the craft’s entry] to dropping a vehicle off a six-story building into a bowl of cement.” This deceleration might produce 50 g’s of force—a rough landing, but not the roughest NASA has tried.

Once deployed and inflated, the aerobot could listen for seismic activity using barometers similar to the ones heliotropes use to listen on Earth, beaming a treasure trove of data back home. “We know virtually nothing about venusquakes,” says Byrne. “In fact, we don't even know for sure that they happen—although I think everyone presumes that they must. Detecting one would give us firm evidence that Venus is geologically active. And then, depending on the type of quake, and its location and magnitude, and how those characteristics compare with the structures we can see with existing radar image data, we would quickly learn a huge amount about the nature and behavior of tectonic processes on Venus.”

An onboard aerosol mass spectrometer could also study the planet’s gas clouds, looking for traces of phosphine and other chemicals of interest. A digital holographic microscope could study individual particles of interest. (Liquid droplet, ice crystal, or something else?) The aerobot might tow a second probe that could measure data at lower, more dangerous altitudes. Perhaps the aerobot would trigger its two-balloon system to occasionally dip lower in the atmosphere and take measurements before rising again to the relative safety of the acidic sky.

In the next two years, Cutts and his team plan to build subscale versions of aerobots, testing them on Earth during deployment and inflation. They hope to conduct a long-duration flight test of a prototype, too. Sensitive components of the craft’s arrays could be tested in simulations of Venus’ acidic clouds. According to NASA’s concept mission study, a best-case scenario could see a launch date set for 2031 and an aerobot entering Venus’ atmosphere by 2034.

Just a couple of weeks ago, the team traveled to Oklahoma to attempt more seismic measurements from their heliotropes. A simplified balloon cruising above the prairie, listening for frack-caused quakes, might seem a far cry from an aerial robot grabbing data in another planet’s atmosphere. But we already put a seismometer on Mars. Maybe floating one through the Venusian sky isn’t as far away as we think.