“It’s doubtful that it’s completely quiet on Europa,” said Steve Vance, a geophysicist at NASA’s Jet Propulsion Laboratory. Vance presented the results of how seismic investigations can shed light on the interior of icy worlds at the Astrobiology Science Conference in Mesa, Arizona, in April.
A global ocean is thought to lie beneath Europa’s icy crust. As Jupiter tugs on the moon with its enormous mass, it pulls and stretches the world enough to melt the ice that would otherwise remain solid. An ocean may also lie beneath two of the planet’s other large moons, Callisto and Ganymede, though their water layer is thought to be far shallower than Europa’s.
In the mid-2020s, NASA plans to launch Europa Clipper, an orbiter that will circle the moon and study its potential habitability. Currently scientists are pushing to follow the orbiter with a lander. Vance argued for the addition of an accelerometer to the lander to measure potential icequakes and study the depths of the ocean.
Britney Schmidt, a planetary scientist with a strong interest in Europa at the Georgia Institute of Technology who was not involved in Vance’s research, agreed. “Europa has plenty of tides,” she said. These tides pull and stretch the ice, causing it to shift over time. “There should be something going on.
On Earth, seismometers around the globe can monitor and track earthquakes. With multiple locations, the instruments can provide extremely precise information about the size and location of a shakeup. By calculating how long it takes a seismic wave to travel through the crust to two (or more) locations, geologists can determine the planet’s composition and spot any interior structures that lie along the path.
Things will be more challenging on icy moons. Because weight is limited and the competition for instruments is fierce, initial visitors to worlds like Europa will most likely contain only a single accelerometer. Accelerometers, which detect ground movement, are more general than earthquake-tracking seismometers. Though less sensitive than the alternative, accelerometers can study localized disturbances in greater depth.
An accelerometer placed on the surface of Europa — with, say, a lander — should be able to measure the thickness of the ice and the ocean beneath. It could even capture the sound of waves lapping against the ice. These sloshing ocean sounds could reveal how much turbulence occurs between the two, possibly driven by geologic activity further beneath the water.
But we may not have to probe all the way to the ocean to find liquid water. In 2011, Schimdt proposed that features on Europa’s surface could be related to subsurface lakes of liquid. By comparing the features to those found in Antarctica, her team suggested that as stress fractures in the ice fill with water, the weakened material collapses, creating the observed features. That water could come from shallow lakes — lakes that an accelerometer could spot.
An accelerometer could also reveal insights about the moon’s fracture-covered surface. As the Europa orbits Jupiter, the changing gravitational forces cause it to stretch and relax, cracking the crust. An accelerometer could identify these changes and use them to determine the characteristics of the icy outer layer.
At a recent meeting discussing the proposed Europa lander, Schmidt suggested that the spacecraft could use its arrival to measure the nature of the surface. While most instruments would likely be turned off during the landing, if an accelerometer was powered up, it’s possible that it could detect the blasted off skyhook proposed to lower the lander, much like the system used to lower Curiosity to Mars’ surface. After the lander touched down, the jettisoned skyhook would land on another part of the moon. If the accelerometer could record the crash, whose position and velocity could be calculated, it could provide even more insights into the nature of Europa.
With all of its water, Europa is one of the strongest contenders in the solar system as a habitat for life to evolve. If the deep ocean contains hydrothermal vents, they could be sources of the heat and energy that life needs to thrive. On Earth, underwater vents are one of the leading areas where life may have first evolved.
An accelerometer can help track just how welcoming the underground ocean might be.
“Using geophysics to measure habitability is a crucial part to getting context for any life we might find,” Vance said. By understanding how the water interacts with the crust, as well as whether or not the crust contains subsurface lakes, an accelerometer could help scientists better understand what’s happening beneath Europa without having to drill.
A similar instrument could be taken to Saturn’s moon Enceladus, where geysers revealed the presence of another global ocean. Other moons could have their own water layers, though whether they exist as single massive bodies of water or layers of water and rock remains open to debate
Using models, Vance and his colleagues determined that each moon should have its own voice, based on the arrangement of water and rock.
“Each ocean world has its own unique seismic signature,” he said.
Of course, to measure those signatures requires getting not only a lander, but an accelerometer to the surface as well. While Clipper has the go-ahead, a Europa lander is still in the proposal stages and far from certain. That means the future remains shrouded for the instrument that could probe its depths.
