With a layered inner structure including a liquid iron core, a thin oxygen-rich atmosphere, an ice-shrouded global ocean with surface plumes that may spew signs of life, and an induced magnetic field—Jupiter’s fourth largest moon, Europa, has greater resemblance to a planet than a gas-giant’s moon.
Understanding where Europa’s 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 Gregor Steinbrügge, a researcher at Stanford’s School of Earth, Energy & Environmental Sciences about powerful eruptions on Jupiter’s icy ocean moon, Europa, that blast out as miles-high plumes that may contain signs of extraterrestrial life.
The Big Question
The key question astrobiologists are asking are where in Europa would they originate –from the depths of Europa’s 40-100 mile deep global ocean or from water pockets embedded in the icy shell itself? It’s the latter according to new evidence from researchers at Stanford University, the University of Arizona, the University of Texas and NASA’s Jet Propulsion Laboratory.
In earlier research at the California Institute of Technology, planetary scientist Mike Brown, conjectured that the rocky bottom of Europa’s vast ocean may be almost like an Earth-like miniature, with plate tectonics, continents, deep trenches, and active spreading centers.
The Ocean –An Earth-Like Miniature
“Think about mid-ocean ridges on Earth,’ Brown wrote on his blog, “with their black smokers belching scalding nutrient-rich waters into a sea floor teaming with life that is surviving on these chemicals. It doesn’t take much of an imagination to picture the same sort of rich chemical soup in Europa’s ocean leading to the evolution of some sort of life, living off of the internal energy generated inside of Europa’s core. If you’re looking for Europa’s whales – which many of my friends and I often joke that we are – this is the world you want to look for them on.”
Using images collected by the NASA spacecraft Galileo, researchers at Stanford University developed a model to explain how a combination of freezing and pressurization could lead to a cryovolcanic eruption, or a burst of water. The results, published Nov. 10 in Geophysical Research Letters, have implications for the habitability of Europa’s underlying ocean—and may explain eruptions on other icy bodies in the solar system.
The artist’s conception of Jupiter’s icy moon Europa above shows a hypothesized cryovolcanic eruption, in which briny water from within the icy shell blasts into space. A new model of this process on Europa may also explain plumes on other icy bodies. ( Justice Blaine Wainwright)
Europa Life –Detected by Spacecraft?
Scientists have speculated that the vast ocean hidden beneath Europa’s icy crust could contain elements necessary to support life. But short of sending a submersible to the moon to explore, it’s difficult to know for sure,. say the Stanford researchers. “That’s one reason Europa’s plumes have garnered so much interest, they say. “If the eruptions are coming from the subsurface ocean, the elements could be more easily detected by a spacecraft like the one planned for NASA’s upcoming Europa Clipper mission.”
Plumes from Within the Ocean’s Ice Shell –Diminished Odds of Habitability
But if the plumes originate in the moon’s 10 to 15 miles (15 to 25 kilometers) thick icy shell, they may be less hospitable to life, because it is more difficult to sustain the chemical energy to power life there. In this case, the chances of detecting habitability from space are diminished.
Manannán –18-mile-wide Crater –The Research Epicenter
The researchers focused their analyses on Manannán, an 18-mile-wide crater on Europa named after a warrior and king of the Otherworld in Irish mythology that was created by an impact with another celestial 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 Don Blankenship, senior research scientist at the University of Texas Institute for Geophysics (UTIG) and principal investigator of the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) instrument that will fly on Europa Clipper. “The polar and planetary sciences team at UTIG are all currently dedicated to evaluating the ability of this instrument to test those hypotheses.”
The 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,” Steinbrügge said. “It can move along thermal gradients, from cold to warm, and not only in the down direction as pulled by gravity.”
A Mile-High Plume Blast –Imaged by Galileo Spacecraft
The 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 a mile high. The eruption of this plume left a distinguishing mark: a spider-shaped feature on Europa’s surface that was observed by NASA’s space probe Galileo imaging and incorporated in the researchers’ model.
During a 2000 fly-by of Europa, Galileo may have witnessed a plume of water. A group of scientists including researchers from the Max Planck Institute for Solar System Research (MPS) in Germany have now found evidence of this event.
“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.
Larger Plumes Based on Hubble & Galileo Data
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, the researchers say. But the process modeled for the Manannán eruption could happen on other icy bodies—even without an impact event. In 2016, NASA’s Hubble Space Telescope took ultraviolet images of Europa transiting across the disk of Jupiter. Out of ten observations, Hubble saw what may be water vapor plumes on three of the images.
“Brine pocket migration is not uniquely applicable to Europan craters,” Voigt said. “Instead the mechanism might provide explanations on other icy bodies where thermal gradients exist.”
One-fifth as Salty as Earth’s Ocean
The 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.
The findings may be discouraging to astrobiologists hoping Europa’s erupting plumes might contain clues about the internal ocean’s capacity to support life, given the implication that plumes do not have to connect to Europa’s ocean. However, the new model offers insights toward untangling Europa’s complex surface features, which are subject to hydrological processes, the pull of Jupiter’s gravity and hidden tectonic forces 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, an assistant professor of geophysics at Stanford. “It opens up a whole new way of thinking about what’s happening with water near the surface.”
Source: Brine Migration and Impact‐Induced Cryovolcanism on Europa, Geophysical Research Letters (2020). DOI: 10.1029/2020GL090797
The Daily Galaxy, Max Goldberg, via Stanford University
image at top of page: NASA’s Goddard Space Flight Center/David Ladd