Seismic Missions Could Reveal the Solar System’s Underworlds
Besides their successful landing on the moon, the astronauts of Apollo 11 made another historic “first” in July 1969 when Buzz Aldrin radioed a message back to Earth: “Houston, the passive seismometer has been deployed manually.” That seismic experiment was the first ever set on the lunar surface. Several more would be placed during later Apollo missions, and collectively, they gave what remains the best-yet view of our sister satellite’s underworld. Yet despite this initial success and a few subsequent ill-fated attempts by the U.S. and the Soviet Union, interplanetary seismology remained on the sidelines of space exploration for the rest of the 20th century. Now, however, it’s getting a makeover for the new millennium. In 2018 NASA’s InSight mission carried a seismometer to Mars. Its fresh data has transformed this research area from a fringe pursuit to a vibrant, established subfield of planetary science. New seismometers are currently being developed for deployment across the solar system, from our moon to the far-flung icy satellites of Jupiter and Saturn.
“This has the potential to be the beginning of a new golden age” in which scientists will peel back the layers on moons and planets alike to glimpse their hidden innards, says Mark Panning, a planetary seismologist at NASA’s Jet Propulsion Laboratory (JPL).
Earth is, relatively speaking, a very active planet of shifting, sliding tectonic plates, erupting volcanoes and crust-shattering quakes. These powerful events produce seismic waves, which reverberate through our planet’s interior. Seismometers can track these seismic waves to reveal their propagation, intensity and sources. These instruments routinely record seismic waves traversing Earth’s crust and mantle and even bouncing off our planet’s core, yielding otherwise-unobtainable information about the subsurface.
These same sorts of observations can be used to look inside other worlds and see how their geological guts compare to our own. The Apollo missions did as much for the moon, discovering it was, like Earth, separated into layers, with a core, mantle and crust. “This showed that the moon is differentiated,” says Angela Marusiak, a geophysicist at the U.S. Geological Survey. “There is a core deep down, so we know the moon did, at one point, have magnetic protection.” As an added benefit, some of those map-making seismic waves came from meteorites striking the lunar surface, allowing scientists to learn something about essentially every world orbiting the sun. “The meteorite impacts were really important because it tells us the cratering rate on the moon,” Marusiak says. “We can use crater counting to age different things, not just on the moon but other bodies in the solar system.”
A Seismic Shift
After Apollo, the hoped-for next giant leap in interplanetary seismology simply fizzled out. NASA’s Viking 1 and 2 landers both carried seismometers when they touched down on Mars in 1976. Sadly, neither lander’s kit delivered solid results: Viking 1’s failed entirely, and the results from Viking 2 were inconclusive. “The seismometer was on top of the lander, and it wasn’t protected from the wind,” Marusiak says. Later, in 1982, seismometers on the Soviet Union’s Venera 13 and 14 landers detected hints of volcanic tremors on Venus. But those landers were very short-lived, each barely surviving for about two hours and one hour, respectively, before succumbing to the planet’s harsh surface conditions.
Seismology fell off the radar in the following decades despite numerous attempts by scientists to include seismometers on various planetary missions. “There’s been a lot of bad luck and changes of plans,” Panning says. The first signs of rejuvenation came in 2014, when Europe’s Philae lander bounced down onto Comet 67P/Churyumov-Gerasimenko. The small lander used three accelerometers to track the waves produced by a thermal probe that was hammered into the surface, discovering that about 20 centimeters beneath the comet’s crusty exterior was soft, fluffy material “like fresh snow,” says Martin Knapmeyer of the German Aerospace Center, who led the experiment. This was the first unequivocal extraterrestrial seismic reading since 1972’s Apollo 17 mission.
But in 2018, when NASA’s InSight lander launched to Mars, everything changed. Following its landing later that year, a robotic arm deployed an extremely sensitive seismometer, which, in a nod to the failures of Viking 1 and 2, included a shield to protect against the Martian wind. The experiment was a runaway success. To date, InSight has detected more than 1,300 marsquakes, including a monster magnitude 5.0 quake earlier this year that mission scientists are still poring over. And while the lander is now running out of power as its dust-covered solar panels struggle to gather enough sunlight, its legacy is secured. “Seismologists have had a hard time getting their science sold,” says Thomas Zurbuchen, associate administrator for the Science Mission Directorate at NASA Headquarters in Washington, D.C. “The success of InSight has really changed that.”
Already work is afoot on the next seismometer that will be sent to space. It is an instrument led by Panning called the Farside Seismic Suite (FSS) that will ride on a NASA-contracted commercial lander to the far side of the moon in 2025, NASA revealed last week. “We will get seismic data from the far side of the moon for the first time ever,” Panning says. This distant half of the moon is relatively unblemished, compared with the near side. For reasons unknown, it bears far fewer dark spots painted by ancient outpourings of lava. The answer may lie under the lunar surface. “All of the Apollo landings, and therefore all of the quakes detected, were on the near side of the moon,” Panning says. “It’s reasonable to question whether [seismic activity of] the far side will look the same as the near side. There could be differences that we don’t know about.”
Next on the list is another moon seismometer—but not one for our own natural satellite. Launching in 2027, NASA’s hotly anticipated Dragonfly mission will travel to Saturn’s moon Titan, the only other known body in the solar system with lakes and seas on its surface (although they contain cryogenically cold petroleumlike goop rather than liquid water). Dragonfly is a nuclear-powered rotorcraft that will fly through the skies of Titan and land in multiple locations after its arrival in 2034, studying the composition of the surface, taking images and looking for possible signs of life all the while. But it will also include “geophones” on its landing rails that can detect seismic waves, as well as a more sensitive dedicated seismometer built by Japan’s space agency that can be lowered to the surface on a winch.
Titan is thought to have layers of ice below its surface, as well as a global liquid-water ocean. If this buried ocean is in direct contact with underlying layers of silicate rock—something only a seismometer can readily reveal—it could have been fed nutrients that may have allowed life to arise. If Titan’s interior has a different arrangement, such as another layer of ice underneath the ocean, the prospects for life could dim. “If there’s ice underneath the ocean, there’s going to be a barrier between the water and the rock,” says Andrea Bryant of the University of Chicago, who recently presented an analysis of possible seismicity that Dragonfly may detect. “That would mean there’s not going to be this exchange of minerals.” It’s not clear how successful attempts at seismology on Titan will be, given uncertainties about the dynamics of its interior. We know this moon is pushed and pulled by Saturn in its orbit, but whether this will produce detectable seismic waves that can map its inner workings is anybody’s guess. “This is one of the leading challenges of putting the mission together,” says Ralph Lorenz, Dragonfly’s chief architect at the Johns Hopkins University Applied Physics Laboratory (APL). “It would be great to say it’s guaranteed, but it will depend on the presence of seismic sources and the background noise.”
A seismometer has been earmarked for Enceladus, too, another of Saturn’s icy moons that is also a promising, potentially habitable locale. In April the latest Planetary Science Decadal Survey directed NASA to develop an orbiter and lander to visit this world later this century. Such an “Orbilander” mission could include a seismometer to look beneath the moon’s icy shell at the suspected ocean that may harbor life. “We know that material is coming up from the subsurface ocean and being ejected into space,” says Shannon MacKenzie of APL, who led the Orbilander proposal. “What’s the driving mechanism? What’s the plumbing structure of Enceladus’s plumes, and how are they sustained? Those are questions that seismology can shed some light on.” Such investigations are applicable to Jupiter’s ocean-bearing icy moon Europa, too, another location scientists hope to one day investigate with a seismometer-bearing lander.
Surveying the Solar System
Closer to home, Venus remains another compelling target. A seismometer sent there would not only reveal the internal structure of a third planet after Earth and Mars but also elucidate whether Venus remains volcanically active. This is a key question determining much of the planet’s past and future evolution—the absence of current volcanism, for instance, could point to a strange geologic cycle of violence unfolding across much of Venus’s history. “People have proposed that episodically, every few hundred million years, you might have a massive resurfacing [event] where a lot of magma comes out,” says Colin Wilson, a planetary scientist at the University of Oxford. “There’s a huge range of possibilities for the way the crust of the planet may be moving and changing. Seismicity would be a way of telling [more about] that.”
Upcoming orbiters from the U.S. and Europe are set to reinvigorate the study of Venus, and many planetary scientists consider a future lander there inevitable. With temperatures of hundreds of degrees Celsius and crushing atmospheric pressures at the planet’s unforgiving surface, however, there are significant challenges to designing a machine that could survive long enough to conduct meaningful seismology. Another possibility might be to study venusquakes from the skies. In experiments conducted in 2019, Siddharth Krishnamoorthy of JPL and his colleagues demonstrated balloon-borne seismology on Earth, lofting barometers on four high-altitude balloons over the open expanse of eastern California. Incredibly, one of the balloons picked up a 4.2 magnitude quake by detecting the resulting pressure waves as they travelled through the atmosphere. This was the first-ever detection of an earthquake by a balloon-borne instrument and proof that the technique could be used in the clouds of Venus, which are a heavenlike oasis compared to the planet’s hellish surface. “The main advantage is: we can do this now,” Krishnamoorthy says. “We don’t need high-temperature electronics that are perhaps decades away.”
Other locations are alluring, too. Some scientists dream of seismic probes for Mercury, which appears to have an oversized core, possibly produced by one or more giant impacts in the early solar system. The dwarf planet Ceres in the asteroid belt is of interest as well, in part because of hints that it, too, harbors a liquid ocean beneath its surface. Researchers are currently investigating the possibility of mounting a sample-return mission to the dwarf planet, based on a recommendation from the Planetary Science Decadal Survey released in April. “It would be crucial to do a survey of the tectonic activity of the region where you are taking the sample from,” says Simon Stähler, a seismologist at the Swiss Federal Institute of Technology in Zurich, who co-authored a recent preprint paper reviewing seismology in the solar system. “You want to know whether that region is geologically dead and the samples are billions of years old or whether you have cryovolcanism that makes the samples very young. I hope at least a simple seismometer could be on this mission.”
Thanks to the success of InSight, seismology has found itself propelled back into the limelight. Its unparalleled ability to unlock the secrets hiding within alien worlds, even playing a surprising role in the search for life, might make it a near-mandatory companion for any future lander mission. “The same set of rules seem to be governing seismology across Earth, the moon and Mars at least,” Krishnamoorthy says. “That itself is saying something pretty profound.” Oft-forgotten, seismology’s time in the sun has arrived. What will we discover lurking in the deep? “There is quite some momentum here,” Stähler says. “This is happening. And that is awesome.”