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On Jupiter’s icy moon Europa, powerful eruptions could eject into space, raising questions among hopeful astrobiologists on Earth: What would have exploded from mile-high plumes?
Could they contain signs of extraterrestrial life? And where did they originate in Europe? A new explanation now points to a source closer to the icy surface than might be expected.
Rather than originating from the depths of Europe’s oceans, some eruptions could originate from pockets of water embedded in the frozen shell itself, according to new evidence from researchers at Stanford University, the University of Arizona, the University of Texas and the Jet NASA’s Propulsion Laboratory.
Using images collected by NASA’s Galileo spacecraft, the researchers developed a model to explain how a combination of freezing and pressurization could lead to a cryo-volcanic eruption or a water explosion. The findings, published Nov.10 in Geophysical Research Letters, have implications for the habitability of Europa’s underlying ocean and could explain eruptions on other icy bodies in the solar system.
Herald of life?
Scientists have speculated that the vast ocean hidden beneath Europa’s icy crust may contain elements necessary to sustain life. But unless you send a submarine to the moon to explore, it’s hard to know for sure. This is one of the reasons Europa’s plumes have garnered so much interest: if the eruptions come from the underground ocean, the elements could be more easily detected by a spacecraft like the one planned for NASA’s upcoming Europa Clipper mission.
But if the plumes originate in the icy shell of the moon, they may be less hospitable to life, because it is more difficult to sustain the chemical energy to fuel life there. In this case, the chances of detecting habitability from space are reduced.
“Understanding where these water plumes come from is very important to know if future explorers of Europe could have the ability to actually detect life from space without probing Europe’s ocean,” said lead author Gregor Steinbrügge. , postdoctoral fellow at Stanford’s School of Earth, Energy and Environmental Sciences (Stanford Earth).
The researchers focused their analyzes on Manannán, an 18-mile crater on Europa that was created by an impact with another celestial 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 big experiment that we’re using to build hypotheses to test,” said co-author Don Blankenship, senior researcher at the University of Texas Institute for Geophysics (UTIG ) and principal researcher of the instrument Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) which will fly to Europa Clipper. “The UTIG polar and planetary science team is currently working on evaluating the ability of this tool to test these hypotheses.”
The 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 in which a water pocket can move sideways – and this is very important,” said Steinbrügge. “It can move along thermal gradients, from cold to hot, and not just downward as attracted by gravity.”
A salty driver
The model predicts that when a migrating brine pocket reached the center of the Manannán crater, it froze and began to freeze, generating pressure that eventually produced a plume, estimated to be over a mile high. The eruption of this plume left a distinctive mark: a spider-like feature on the surface of Europa that was observed by Galileo’s images and incorporated into the researchers’ 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 likely cannot explain the source of other larger plumes on Europa that have been hypothesized based on data from Hubble and Galileo, the researchers say. But the process modeled for the Manannán eruption could take place on other icy bodies, even without an impact event.
“Brine pocket migration is not only applicable to European craters,” Voigt said. “Instead the mechanism could provide explanations for other frozen bodies where thermal gradients exist.”
The 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’s images from 1995 to 1997, show that Europa’s ocean may be about one-fifth saltier than Earth’s ocean, a factor that will improve the Europa Clipper mission’s sonar’s ability to collect data from its internal.
The results may be daunting to astrobiologists who hope that Europa’s erupting plumes may hold clues to the inner ocean’s ability to sustain life, given the implication that the plumes do not have to connect to Europa’s ocean. However, the new model offers insights into unraveling the complex surface features of Europa, which are subject to hydrological processes, the pull of Jupiter’s gravity, and tectonic forces hidden within the icy moon.
“This makes the shallow subsurface – the ice shell itself – a much more exciting place to think about,” said co-author Dustin Schroeder, assistant professor of geophysics at Stanford. “It opens up a whole new way of thinking about what’s happening with the water near the surface.”
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Schroeder is also an Assistant Professor, Courtesy, of Electrical Engineering and a Center Fellow, Courtesy, at the Stanford Woods Institute for the Environment. Co-authors include Krista Soderlund, Natalie Wolfenbarger, and Duncan Young of the University of Texas at Austin; Christopher Hamilton of the University of Arizona, Tucson; and Steven Vance of NASA’s Jet Propulsion Laboratory.
The research was supported by the G. Unger Vetlesen Foundation. Part of the work was performed by the Jet Propulsion Laboratory, Caltech, under a contract with NASA.
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