This week, in the still, silent desert near Pahrump, Nevada, researchers set off an artificial earthquake. It shook the ground and, less obviously, the air, allowing NASA scientists to listen for the vibrations with balloons floating overhead. If the technology can be shifted to Venus, it could be the first to detect earthquakes there, which could provide important clues about the interior of our sister planet and why it evolved so differently from our own.
“We’ve never made a direct seismic measurement on Venus,” says Siddharth Krishnamoorthy, an experiment team member at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California. “There is a lot balloons can offer in terms of unlocking some major questions about the planet.”
For the 19 December test, U.S. Department of Energy researchers set off a 50-ton chemical explosion roughly 300 meters underground to generate a magnitude-3 or -4 tremor—partly to verify the agency’s ability to detect underground nuclear explosions. But researchers also lofted two helium-filled balloons over the site, one tethered and another free floating, each a few hundred meters above the ground. The balloons carried barometers to measure changes in atmospheric pressure and detect the earthquake’s infrasound waves, low-frequency acoustic vibrations below the threshold of human hearing.
A similar setup could one day float high in the atmosphere of Venus. At the planet’s surface, conditions are infernal: Temperatures are high enough to melt lead, and pressures are so overwhelming that they would crush a submarine. It would be hard for any lander to survive long enough to detect a tremor. But 50 kilometers above the surface, temperatures and pressures are remarkably clement, perfect for a long-lived balloon (aside from a touch of sulfuric acid in the greenhouse atmosphere, which is 96% carbon dioxide). In 1985, the Soviet Union showed it could be done, flying two balloons for 2.5 days in this layer. They only stopped recording data when their batteries ran out.
Balloons could detect tremors from such as high perch because Venus’s atmosphere is so much thicker than Earth’s: Waves would transfer better from the ground into the air and travel more readily. Based on preliminary calculations, the team believes it could detect venusian quakes as small as magnitude 2 from that height. That goal was advanced by an initial desert test last year—dropping 13-ton weights onto the desert floor from a height of 1.5 meters—that proved instruments could pick up infrasound waves from the shaking and infer the direction of the quake.
Krishnamoorthy says he and colleagues are now trying to detect stronger seismic sources at larger distances, as they did this week, to better tease out the quake’s signature from the environmental noise. The team next plans to loft balloons over Oklahoma, where thousands of earthquakes have occurred in recent years, triggered by oil and gas activity. That could allow the group to detect shaking coming from much deeper underground.
Translating the tests to Venus could be somewhat tricky, however, says planetary scientist Ralph Lorenz of the University of Arizona in Tucson. The timing and character of the test quakes were known to the experimenters, and it might be a challenge to separate a signal from the background clatter on blustery Venus, where winds are supersonic.
It could be that Venus is seismically quiet, an important negative result that would force researchers to reevaluate their models of the planet’s interior. But many scientists think heat is still trying to escape the planet, potentially in ways that shake the surface. Scuff marks across its surface today point to stretching and strain that could be causing tremors, although many scientists think the planet has not had plate tectonics for a long time, if ever—one reason why the planet has ended up with a runaway greenhouse effect. On Earth, tectonic plate motion is responsible for most earthquakes and it also helps bury carbon deep within the mantle, buffering the planet from global warming.
This makes Venus our sister, but not our twin, says geologist Paul Byrne of North Carolina State University in Raleigh. He says a gauge of tectonic activity would provide clues to Venus’s interior structure and past history, perhaps explaining why it lacks a magnetic field like Earth’s or at what point its water disappeared. Knowing why Venus went down such a different path than our own could also help in understanding the glut of rocky exoplanets now being found around other stars. Byrne points out that alien astronomers peering at our solar system from far away would be hard pressed to say whether Earth or Venus hold life.