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Europa, the sixth of Jupiter’s moons and the fourth largest, has an underground ocean covered with an icy shell. Despite evidence of plumes on the frozen moon, no surface features have been definitively identified as their source to date. Additionally, it is still unclear whether the activity originates from near-surface water reservoirs in Europe’s ice shell or whether it originates from the underlying global ocean. In a new study, a team of American planetary researchers examined an impact crater called Manannán and found that the fracture system at its center is consistent with the formation of a brine reservoir near the surface; As the last pocket of water in the center of the crater began to freeze, the overpressure resulted in a cryovolcanic eruption that placed brine on the surface.
This artist’s Europa design shows a hypothetical cryovolcanic eruption, in which brackish water from inside the icy shell blows into space. Image credit: Judge Blaine Wainwright.
“Understanding where these plumes of water come from is very important to whether future explorers of Europe might have a chance to actually detect life from space without probing the ocean of Europe,” said the co – principal author, Dr Gregor Steinbrügge, postdoctoral researcher in the Department. of geophysics at Stanford University.
Dr Steinbrügge and his colleagues focused their analyzes on Manannán, a 29 km (18 miles) wide crater in Europe that was created by an impact with another object tens of millions of years ago.
Believing that such a collision would have generated an enormous amount of heat, they modeled how the melting and subsequent freezing of a pocket of water inside the icy shell could have caused the water to burst.
“The comet or asteroid hitting the ice shell was basically a great experiment that we use to build hypotheses to test,” said co-author Dr. Don Blankenship, a scientist at the Institute of Geophysics at the University of Texas and Principal Investigator. of the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) instrument which will fly on NASA’s future Europa Clipper spacecraft.
The team’s model indicates that as Europe’s water turned to ice during the later stages of impact, pockets of water with increased salinity could form on the surface. from the moon.
In addition, these pockets of salt water can migrate laterally through the ice shell of Europe, melting adjacent regions of less brackish ice, and therefore become even saltier in the process.
“We have developed a way in which a pocket of water can move sideways – and this is very important. It can move along thermal gradients, from cold to hot, and not just downward under gravity, ”said Dr Steinbrügge.
Europa’s surface occupies an important place in this recently restated 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 pocket of migrating brine reached the center of Manannán crater, it got stuck and started to freeze, generating pressure that eventually resulted in a plume, estimated to be over 1.5 km (1 mile) high.
The eruption of this plume left a distinctive mark: a spider-like feature on the surface of Europa that was observed by NASA’s Galileo spacecraft and incorporated into the model.
“Even though the plumes generated by the migration of brine pockets would not provide a direct glimpse of the European ocean, our results suggest that the Europa ice shell itself is very dynamic,” said Joana Voigt , co-lead author, 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 over Europe that have been assumed based on data from Hubble and Galileo. But the process modeled for the Manannán eruption could occur on other frozen bodies – even without an impact event.
This study also provides estimates of the salinity of Europe’s frozen surface and ocean, which in turn could affect the transparency of its ice shell to radar waves.
Calculations, based on Galileo imagery from 1995 to 1997, show that the European ocean could be about a fifth as salty as the Earth’s ocean – a factor that will improve the capability of the Europa Clipper mission’s radar sounder. to collect data from its interior.
“It makes Europa’s shallow subsurface 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 happens with water near the surface.”
The team’s article was published in the journal Geophysical research letters.
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G. Steinbrugge et al. Migration of brine and cryovolcanism induced by impact on Europe. Geophysical research letters, published online November 5, 2020; doi: 10.1029 / 2020GL090797
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