It turns out that you can detect an earthquake from a balloon

On July 4, 2019, as America celebrated its independence with explosions in the sky, the land around Ridgecrest, California decided to join the party: a 6.4-magnitude earthquake shook the region. Two days later, it was hit by a 7.1-magnitude earthquake, and tens of thousands of aftershocks followed in the weeks that followed. California, which is no stranger to vibrations, has a dense network of ground seismometers – and they were busy that summer.

At the same time, a small group of scientists, also in the Golden State, were trying to eavesdrop on these grumbles in a somewhat unorthodox and counterintuitive way: balloons high above their heads.

Seismographs are usually stuck to solid ground for a reason. The seismic waves they measure come from earthquakes, and travel through the Earth, so it makes sense that Earth-connected instruments are well placed to detect these vibrations. Why make the task more difficult by cutting off contact with the ground to listen for seismic activity from above?

Well, it just so happens that there is a place that is completely inappropriate for traditional seismology. There, the Earth ripens at 900 degrees Fahrenheit, and the atmospheric pressure equals one mile under water. Any seismometer placed there, if it survived the crash with pressure, would quickly heat up and melt. Welcome to Venus – a world that is currently adept at quickly dumping any robotic envoy we dare to set on its volcanic wasteland. The Soviet Union’s lander Venera 13, which visited Venus in 1982, is the current record holder, having lasted only 127 minutes before dying.

The flower certainly does not look friendly. NASA/JPL

However, there are parts of Venus’s turbulent, acid skies that are not relentlessly stressed while also being more forgiving. This could make these spots a relatively welcome place to send out a balloon-height seismic spy device, which might be able to detect earthquakes below. If so, scientists can document the geological activity of our neighbors and compare it to ours, in an effort to better understand the behavior of both.

But to know that such a scheme will work on Venus, it needs to work on Earth first. That’s why, in the summer of 2019, scientists were looking at clouds over California, while those abundant earthquakes stirred the ground beneath their feet. They have now reported their findings in the journal Geophysical Research Letters. Spoiler alert: Next stop, Venus.

Sending balloons to Venus is not a new idea. It’s been done before: the Soviet Union dropped two of them into the atmosphere in 1985, equipped with sensors to track their movements while studying the weather. They drifted through the night in 150-mph winds before their batteries ran out, 46 hours after their deployment, just as the sun was beginning to set. Then it diminished out of thin air, and would never be seen again.

The balloons are darkened slightly to warm the air inside, and raise them up until dusk, when they have cooled and descended. NASA/JPL-Caltech

As fascinating as these missions were, none of their scientific instruments were designed to detect seismic activity. Although the thick atmosphere of Venus prevents us from seeing it, there is a strong chance that we will discover it.

It is almost certain that many of its volcanoes are alive and kicked. An atmospheric seismometer suspended, say, 35 miles above the surface of an eagle, says Jennifer Jackson, a geologist at Caltech and a co-author of the study, can hear all kinds of volcanic eruptions, from mountains of active volcanic rock to structural collapse cauldrons. magma; Recent work suggests that country-sized patches of Venus’ crust vibrate like ice sheets, creating lots of earthquakes. Meteors could also be heard exploding in the sky.

“You can listen to the heartbeat of a planet,” says Paul Byrne, a planetary scientist at North Carolina State University, who was not involved in the work. And since different journeys of seismic waves tell scientists the properties of the rocks they pass through, an atmospheric seismometer can be used to create a picture of Venus’s secret, shimmering lower surface—a place we know almost nothing about.

But you can’t just attach a regular seismometer to a balloon to capture all of these vibrations. Remember, they were made to be in contact with the earth. However, earthquakes produce identical sound waves that travel through the air. They tend to be ultrasonic – that is, below the range of human hearing. But these sound waves change the pressure of the air they’re moving through, so you can use barometers, or pressure sensors, to detect these infrasounds from a distance.

Pressure sensors that scientists used to detect earthquakes from above the sky. Courtesy Siddharth Krishnamurthy

Scientists already use ultrasound on Earth to hear exploding space rocks, distant thunderstorms, magma gurgling inside volcanoes, and tectonic shivering. So why not do the same on Venus? So this small but dedicated team of international researchers is designing their very own earthquake trapping balloons.

The problem is that natural earthquakes are unpredictable, which makes it difficult to test the effectiveness of the plan. So the researchers decided to make their own. Sometimes they used a seismic hammer – a giant piston-like device that hits the ground. On other occasions, they buried and detonated explosives. Infrasound detectors connected to tethered balloons detected both types of artificial earthquakes. But can they work for the real thing?

While considering their options, including a trip to Oklahoma — where man-made earthquakes from the oil and natural gas industry are common — the Ridgecrest earthquake sequence began. They moved to seize the opportunity.

The balloon designs were led by study co-author Daniel Bowman, a geophysicist at Sandia National Laboratories in Albuquerque, New Mexico. Their versatility lies in their striking simplicity: they’re made of tape and plastic caps, and they cost no more than $30 apiece.

The balloons used in the experiment are lightweight and inexpensive. Courtesy Siddharth Krishnamurthy

Charcoal powder is used to darken the balloon material so that it absorbs sunlight and heats the air inside and causes it to rise. When night falls, the air cools and thickens, and the balloons return to the ground. All you have to do is stick some tools on it and leave it on, says Quentin Brisseau, a seismologist with the Norwegian Seismic Matrix and lead author of the study.

The team packed up and left two balloons for an infrasound investigation in California on July 22, 2019, and two more on August 9, as they ascended between 11 and 15 miles above the Earth’s surface. What they were hoping for was a powerful aftershock. Most of these tremors are imperceptible to people. Only a strong vibration would create an ultrasonic signal large enough to penetrate other noises emitted by the environment around them.

On July 22, nature threw them a bone. One of the balloons was hit by a subsonic explosion from below. California seismometers indicate that a 4.2-magnitude earthquake struck 50 miles away. Careful processing of the data confirmed that infrasound was indeed tectonic in origin. The simulations showed that the infrasound blast reached the balloon at exactly the right time, based on the distance to the earthquake source.

Balloons fall to the ground at dusk, as the air inside them cools. Scientists believe that a copy of Venus can be kept aloft for 60 days. Courtesy Siddharth Krishnamurthy

They did: the first time an earthquake was detected from a rising balloon. It might just be a single measurement, but the team found they could use it to start putting together a picture of that earthquake’s location underground.

This success story inadvertently reversed an old anecdote. The balloons sent on July 22 were “named for a hare and a tortoise,” says Siddharth Krishnamurthy, a research technical expert at NASA’s Jet Propulsion Laboratory and co-author of the study. The turtle, which had little to no charcoal pigmentation, took its time to rise into the air. For 15 minutes, he sat 100 feet off the ground, then slowly rose as his temperature gradually rose. Hare, in part because of his great pigmentation, shot as if there was no tomorrow.

But Hare had a problem: its rapid ascent dislodged too much air and made too much noise, perhaps too much to allow him to hear any earthquakes. The tortoise, which was flying much more gently, was in a perfect listening position for the vocal yawn. Slow and steady won the race.

There is still a lot to sort out before trying this on Venus horizons. This field test proved that earthquakes can be detected from a height. But using balloons to determine the epicenter presents another challenge entirely. After all, Venus wouldn’t have the Earth’s seismometers to help.

During future experiments, including those in Oklahoma, the team will associate multiple gauges with each balloon. Sound waves from earthquakes will hit each of these barometers at slightly different times, which can be used to approximate the direction the waves came from. It is an airborne earthquake detection in stereo.

The Mojave Desert, inhospitable as it is, doesn’t hold a candle to what machines might encounter on Venus. But experience has shown that seismic activity can be detected from the balloon. NASA/JPL-Caltech

However, it is already clear that balloons over Venus are “a great idea,” Byrne says. “It is elegant. They are particularly attractive to Venus.” This is because the planet’s atmosphere is decidedly dense, which means that sound waves from earthquakes will travel more efficiently through the sky, allowing them to reach those virtual balloons easily and at a detectable level.

Add a few tools to it, and you’ve got a floating science lab. Even with the technology available today, says Jackson, they believe one of these stations could stay there for at least 60 days.

But will this ever happen? Until this month, it wasn’t particularly bearable. But now that we know that NASA is sending two spacecraft to Venus, and the European Space Agency is sending a third, so the odds have increased somewhat. Two of these missions will orbit the planet, while another will plunge into the atmosphere, taking readings until you perish on the surface. They’ll make game-changing discoveries, sure, but not the kind of measurements a long-range balloon flight through the atmosphere can make.

“I very much hope there will be Venus balloons in the future,” says Krishnamurthy. “But nobody guesses whether it will happen or not.”

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