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Europa, the sixth of Jupiter’s moons and the fourth largest, has an underground ocean covered in an icy shell. Despite evidence of plumes on the frozen moon, no surface features have been definitively identified as a source to date. It is also unknown whether the activity originates from near-surface water reservoirs within Europa’s ice shell or whether it originates from the global ocean below. In a new study, a team of US planetary researchers examined an impact crater called Manannán and found that the fracture system located at its center is consistent with the formation of a brine tank near the surface; when the last pocket of water in the center of the crater started to freeze, the overpressurization resulted in a cryo-volcanic eruption that placed brine on the surface.
This artist’s conception of Europe shows a hypothetical cryo-volcanic eruption, in which brackish water from inside the frozen shell explodes into space. Image credit: Justice Blaine Wainwright.
“Understanding where these water plumes come from is very important to know if future explorers of Europa could have the ability to actually detect life from space without probing the ocean of Europa,” said lead co-author, the Dr. Gregor Steinbrügge, a postdoctoral researcher in the Department of Geophysics at Stanford University.
Dr Steinbrügge and colleagues focused their analyzes on Manannán, a 29 km (18 mi) wide crater on Europa that was created by impact with another object some tens of millions of years ago.
Reasoning that such a collision would generate an enormous amount of heat, they modeled how the melting and subsequent freezing of a pocket of water inside the frozen shell could cause the water to erupt.
“The comet or asteroid that hit the ice shell was basically a great experiment we’re using to build hypotheses to test,” said co-author Dr. Don Blankenship, a scientist at the University’s Institute of Geophysics. 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 turns to ice during the later stages of the impact, pockets of water with higher salinity could be created on the lunar surface.
Additionally, these pockets of salt water can migrate laterally through Europa’s ice shell melting adjacent regions of less brackish ice and consequently become even saltier in the process.
“We have developed a way that a water bladder can move sideways – and this is very important. It can move along thermal gradients, from cold to hot, and not just in the downward direction as pushed by gravity, “said Dr. Steinbrügge.
The surface of Europa looms in this newly reworked color view; the image scale is 1.6 km per pixel; north on Europa is on the right. Image credit: NASA / JPL-Caltech / SETI Institute.
The new model predicts that when a migrating brine pocket reached the center of the Manannán crater, it froze and began to freeze, generating pressure that ultimately resulted in a plume, estimated to be over 1.5 km high. (1 miles).
The eruption of this plume left a distinctive mark: a spider-like feature on Europa’s surface that was observed by NASA’s Galileo spacecraft and incorporated into the model.
“Although the plumes generated by the migration of the brine pockets would not provide a direct view of the ocean of Europa, our results suggest that the Europa ice shell itself is very dynamic,” said co-lead author Joana Voigt. , graduate research assistant at the University of Arizona, Tucson.
The relatively small size of the plume that would form at Manannán indicates that the impact craters probably cannot explain the source of other larger plumes on Europa that have been hypothesized based on data from Hubble and Galileo. But the process modeled for the Manannán eruption could take place on other icy bodies, even without an impact event.
This study also provides estimates of how salty Europa’s ice surface and ocean may be, which in turn could affect the transparency of its ice shell to radar waves.
Calculations, based on Galileo imaging from 1995 to 1997, show that Europa’s ocean may be about a fifth saltier than Earth’s ocean, a factor that will improve the Europa Clipper mission’s sonar’s ability to collect data from the inside.
“This makes Europe’s shallow subsurface a much more exciting place to think about,” said co-author Dr. Dustin Schroeder, a research fellow 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 going on with the water near the surface.”
The team document was published in the journal Geophysical Research Letters.
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G. Steinbrugge et al. Brine migration and impact-induced cryvolcanism on Europa. Geophysical Research Letters, published online on November 5, 2020; doi: 10.1029 / 2020GL090797
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