In the search for life beyond Earth, subsurface bodies of water in the outer Solar System are some of the most important targets. That’s why NASA and ESA are sending Europa Clipper and JUICE missions to Jupiter’s icy moon Europa: there is strong evidence that under a thick crust of ice, the moon harbors a global ocean that could potentially be habitable. But planetary scientists believe the ocean isn’t the only water on Europa. Based on observations from NASA’s Galileo orbiter, they believe salty liquid reservoirs may reside inside the moon’s shell.
Review of the mechanisms proposed in the literature to explain the emplacement of liquid reservoirs invoked to correspond to a variety of geological features at Europa’s surface. Image credit: Lesage et al., doi: 10.3847/PSJ/ac75bf.
Jupiter’s icy moon Europa possesses one of the youngest surfaces of the Solar System, with an average surface age estimated around 40 to 90 million years.
The great variety of geological features observed on Europa’s surface demonstrates widespread ongoing or geologically recent activity.
Additionally, it is very likely that Europa hosts a global liquid water ocean that is believed to be in contact with its rocky mantle and situated beneath an 20-50 km thick icy crust.
Based on surface observations and subsurface modeling, several studies have also hypothesized the potential presence of liquid reservoirs in the ice crust.
These reservoirs, if they exist, could represent the most accessible liquid water bodies in the outer Solar System and are key for the exploration of ocean worlds and the search for life beyond Earth.
Europa is the target of NASA’s upcoming Europa Clipper mission and one target of ESA’s upcoming JUICE mission.
In anticipation of the arrival of these spacecraft, it is important to address the question of whether or not liquid water could reach Europa’s surface and to constrain the sources of any potential observed flows.
“We demonstrated that plumes or cryolava flows could mean there are shallow liquid reservoirs below, which Europa Clipper would be able to detect,” said Dr. Elodie Lesage, Europa scientist at NASA’s Jet Propulsion Laboratory.
“Our results give new insights into how deep the water might be that’s driving surface activity, including plumes.”
“And the water should be shallow enough that it can be detected by multiple Europa Clipper instruments.”
The researchers’ computer modeling lays out a blueprint for what scientists might find inside the ice if they were to observe eruptions at the surface.
According to their models, they likely would detect reservoirs relatively close to the surface, in the upper 4-8 km (2.5-5 miles) of the crust, where the ice is coldest and most brittle.
That’s because the subsurface ice there doesn’t allow for expansion: as the pockets of water freeze and expand, they could break the surrounding ice and trigger eruptions, much like a can of soda in a freezer explodes. And pockets of water that do burst through would likely be wide and flat like pancakes.
Reservoirs deeper in the ice layer — with floors more than 8 km (5 miles) below the crust — would push against warmer ice surrounding them as they expand. That ice is soft enough to act as a cushion, absorbing the pressure rather than bursting.
Rather than acting like a can of soda, these pockets of water would behave more like a liquid-filled balloon, where the balloon simply stretches as the liquid within it freezes and expands.
“The new work shows that water bodies in the shallow subsurface could be unstable if stresses exceed the strength of the ice and could be associated with plumes rising above the surface,” said Dr. Don Blankenship, a researcher with the Institute for Geophysics at the University of Texas.
“That means Europa Clipper’s Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) instrument could be able to see water bodies in the same places that you see plumes.”
The team’s paper was published in the Planetary Science Journal.
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Elodie Lesage et al. 2022. Simulation of Freezing Cryomagma Reservoirs in Viscoelastic Ice Shells. Planet. Sci. J 3, 170; doi: 10.3847/PSJ/ac75bf
