Venus is a notoriously hellish world, and that makes it challenging to explore. The surface pressure is crippling and the average temperature is 864 degrees Fahrenheit. The ground spits up lava and the skies rain acid. As a result, no landers have ever lasted longer than a few hours on the surface, with Venera 13 holding the record at 127 minutes.
But there may be another way to explore Venus—with balloons. The Venusian atmosphere is actually a lot more welcoming than its surface, and more importantly, its high atmospheric density can transmit valuable information about seismic activity deep within the planet too.
This mission concept was developed by a team of multidisciplinary specialists, in a workshop sponsored by the Keck Institute for Space Studies. Their detailed report will be presented today at the 23rd meeting of the Seismological Society of America (SSA) by team leads Stephen Arrowsmith of the Los Alamos National Laboratory, Philippe Lognonné of the University of Paris, and Jim Cutts of NASA's Jet Propulsion Laboratory.
Venus's intense atmospheric density has presented obstacles for past voyages to the planet, but Arrowsmith, Lognonné, and Cutts propose in their report that it would be a valuable asset for a balloon mission. "The most important property of the Venus atmosphere is its density," Cutts told me over email.
"This means that a much larger fraction of the seismic energy passes from the solid planet into the atmosphere than would be case on Earth, for example," he continued. In other words, balloons deployed into Venus's atmosphere would be able to detect the planet's seismic contortions in unprecedented detail.
"Since the atmosphere of Venus at the surface is about 60 times as dense as that of the Earth the coupling is 60 times better," Cutts wrote—and, not only does the dense cloud cover on Venus conduct the fallout of these surface disruptions, it actually intensifies them too.
"Once the waves enter the atmosphere as infrasonic waves, they experience an amplification as they rise," he said. "By the time they get to 50 [kilometers in] altitude where the pressure is 100 times less than an the surface, the pressure wave has increased by a factor of 10."
Clearly, balloons soaring high in the Venusian skies would have no lack of data to work with in reconstructing the dynamics of the geological events below. In fact, this amplification effect is expected to deliver a surplus of data from the complex activity on the surface. "One challenge in fact will be discriminating between the different kinds of events," Cutts told me.
"Volcanic eruptions are expected to have a unique acoustic signature," he said. "Storms will be distinguished because they tend to propagate horizontally, whereas the seismic event will arrive vertically from below."
The information gathered could have a wide range of implications; at the moment we know next to nothing about Venus's internal mechanics. Learning how the planet ticks is a crucial step not just for planetary science, but for the ongoing quest to move our species off-Earth—perhaps, even, to the Venusian clouds.
Along those lines, the balloon mission concept could also be used to explore other atmospheric worlds in our solar system, like Saturn's moon Titan.
"Titan is a prime target for balloon exploration," Cutts told me, citing a recent NASA/ESA joint concept called the Titan Saturn System Mission (TSSM). "Recent technical developments should enable light gas (helium or hydrogen) balloons with very long lifetime and capable of making repeated visits to the surface."
Balloons were the vehicles that first opened the skies to humans on Earth, kickstarting the many off-Earth adventures humankind has accomplished since. Now, it seems that they may also play an important role in expanding our knowledge of our neighboring worlds, starting with the closest and most stubbornly evasive: Venus.