Europa, the sixth of Jupiter’s moons and the fourth largest, has a subsurface ocean covered by an icy shell. Despite evidence for plumes on the icy moon, no surface features have been definitively identified as their source to date. Furthermore, it remains unknown whether the activity originates from near-surface water reservoirs within Europa’s ice shell or if it is sourced from the underlying global ocean. In a new study, a team of U.S. planetary researchers looked at an impact crater called Manannán and found that the fracture system located in its center is consistent with the formation of a near-surface brine reservoir; as the final water pocket at the crater’s center started to freeze, overpressurization resulted in a cryovolcanic eruption that emplaced brine onto the surface.
This artist’s conception of Europa shows a hypothesized cryovolcanic eruption, in which briny water from within the icy shell blasts into space. Image credit: Justice Blaine Wainwright.
“Understanding where these water plumes are coming from is very important for knowing whether future Europa explorers could have a chance to actually detect life from space without probing Europa’s ocean,” said co-lead author Dr. Gregor Steinbrügge, a postdoctoral researcher in the Department of Geophysics at Stanford University.
Dr. Steinbrügge and colleagues focused their analyses on Manannán, a 29-km- (18-mile) wide crater on Europa that was created by an impact with another object some tens of millions of years ago.
Reasoning that such a collision would have generated a tremendous amount of heat, they modeled how melting and subsequent freezing of a water pocket within the icy shell could have caused the water to erupt.
“The comet or asteroid hitting the ice shell was basically a big experiment which we’re using to construct hypotheses to test,” said co-author Dr. Don Blankenship, a scientist in the Institute for Geophysics at the University of Texas and principal investigator of the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) instrument that will fly on NASA’s future Europa Clipper spacecraft.
The team’s model indicates that as Europa’s water transformed into ice during the later stages of the impact, pockets of water with increased salinity could be created in the moon’s surface.
Furthermore, these salty water pockets can migrate sideways through Europa’s ice shell by melting adjacent regions of less brackish ice, and consequently become even saltier in the process.
“We developed a way that a water pocket can move laterally — and that’s very important. It can move along thermal gradients, from cold to warm, and not only in the down direction as pulled by gravity,” Dr. Steinbrügge said.
The surface of Europa looms large in this newly-reprocessed color view; image scale is 1.6 km per pixel; north on Europa is at right. Image credit: NASA / JPL-Caltech / SETI Institute.
The new model predicts that when a migrating brine pocket reached the center of Manannán crater, it became stuck and began freezing, generating pressure that eventually resulted in a plume, estimated to have been over 1.5 km (1 miles) high.
The eruption of this plume left a distinguishing mark: a spider-shaped feature on Europa’s surface that was observed by NASA’s Galileo spacecraft and incorporated in the model.
“Even though plumes generated by brine pocket migration would not provide direct insight into Europa’s ocean, our findings suggest that Europa’s ice shell itself is very dynamic,” said co-lead author Joana Voigt, a graduate research assistant at the University of Arizona, Tucson.
The relatively small size of the plume that would form at Manannán indicates that impact craters probably can’t explain the source of other, larger plumes on Europa that have been hypothesized based on Hubble and Galileo data. But the process modeled for the Manannán eruption could happen on other icy bodies — even without an impact event.
This study also provides estimates of how salty Europa’s frozen surface and ocean may be, which in turn could affect the transparency of its ice shell to radar waves.
The calculations, based on imaging from Galileo from 1995 to 1997, show Europa’s ocean may be about one-fifth as salty as Earth’s ocean — a factor that will improve the capacity for the Europa Clipper mission’s radar sounder to collect data from its interior.
“This makes the shallow subsurface of Europa a much more exciting place to think about,” said co-author Dr. Dustin Schroeder, a researcher in the Department of Geophysics and the Department of Electrical Engineering at Stanford University.
“It opens up a whole new way of thinking about what’s happening with water near the surface.”
The team’s paper was published in the journal Geophysical Research Letters.
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G. Steinbrügge et al. Brine Migration and Impact-Induced Cryovolcanism on Europa. Geophysical Research Letters, published online November 5, 2020; doi: 10.1029/2020GL090797
