Evidence Mounts for Ocean on Jupiter's Moon Europa

Hubble has been searching for further evidence of water plumes shooting off the moon's surface.

Caption: Artist's concept of a plume of water vapor blasting off the icy surface of the Jovian moon Europa, located about 500 million miles from the sun. Credits: NASA/ESA/SWRI

NASA on Monday will release additional evidence of an underground ocean on Jupiter's moon Europa, a prime candidate in the search for life beyond Earth.

Scientists' first clue that Europa has an ocean came from NASA's twin Voyager probes, which flew by Jupiter in 1979.

Pictures taken by the probes showed that Europa's surface was relatively smooth, brighter than Earth's moon and crisscrossed with bands and ridges.

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"Some of the dark bands had opposite sides that matched each other extremely well, like pieces of a jigsaw puzzle. These cracks had separated, and dark, icy material appeared to have flowed into the opened gaps, suggesting that the surface had been active at some time in the past," NASA said in 2014.

Some of the longest linear features did not fit predicted patterns of fractures that should be created by tides as Europa orbits Jupiter. Instead, the patterns matched what would happen if Europa's surface could move independently, without being locked to the interior, such as what would happen if a layer of liquid or slightly warmer ice existed between the crust and the deep interior, NASA said.

Related studies showed that Europa may have had a warm interior at some point in the past, and could still retain heat today, a finding that raised the prospect of a global subsurface ocean.

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The next mission to Jupiter was NASA's Galileo spacecraft, which toured the giant planet and its entourage of moons from 1995 to 2003. As part of its extensive study, Galileo made 12 close flybys of Europa, the second closest large moon to Jupiter after Io.

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"One of Galileo's most important measurements showed how Jupiter's magnetic field was disrupted in the space around Europa. This measurement strongly implied that a special type of magnetic field is being created (or induced) within Europa by a deep layer of some electrically conductive fluid beneath the surface.

"Based on Europa's icy composition, scientists think the most likely material to create this magnetic signature is a global ocean of salty water," NASA said.

Then in 2012, images from the Hubble Space Telescope showed a cloud of water vapor towering over Europa's south pole, providing the first strong evidence of water plumes erupting off the moon's surface.

"If those plumes are connected with the subsurface water ocean we are confident exists under Europa's crust, then this means that future investigations can directly investigate the chemical makeup of Europa's potentially habitable environment without drilling through layers of ice. And that is tremendously exciting," lead scientist Lorenz Roth, who was with the Southwest Research Institute in San Antonio at the time, said in a statement.

The plumes were seen just once, in December 2012. Scientists won extra time on Hubble to keep searching.

Astronomers also have re-examined archived Galileo images and discovered that Europa's surface has clay-like minerals that may contain organics. Other analysis shows that Europa has active plate tectonics, a process that could seed the water with nutrients.

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Another study of archived Galileo images determined that lumpy patches of ice on Europa are likely caused by huge lakes of water trapped relatively close to the surface. The researchers theorized that the lakes formed from surface ice interacting with a deep, briny warm ocean that sits on top of a rocky mantle.

The key piece of evidence was a 62-mile wide circular region called Thera Macula, which is about the size of Lake Ontario.

"The surface has dropped down by about 400- to 600 meters in this whole area. The big icebergs have kind of cracked up, and they're floating along higher than that lowest point ... That tells us the material below this feature on the surface is still liquid. So there's a giant lake," lead scientist Britney Schmidt said in a March 2012 EarthSky podcast.

She estimated the lake to be 2- to 2.5 miles deep, about the depth of Earth's oceans.

Schmidt, who is now an assistant professor at Georgia Institute of Technology in Atlanta, along with William Sparks, who heads the team using Hubble to search for Europa plumes, are among the scientists scheduled to discuss the new findings at a NASA press conference on Monday.