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.
RELATED: Hubble Discovers Water Plumes Over Europa
"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.
RELATED: Europa's Epsom Salt May Indicate Ocean Life
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.