Previous estimates had suggested the moon's crust might be tens if not hundreds of kilometers thick-too thick, that is, to allow direct exploration of its potentially life-friendly ocean anytime soon.
In 1997, when the Galileo spacecraft was flying some 124 miles above the surface of Europa, the team in charge did not suspect that the satellite had gone through a plume of water vapor from the icy moon.
Europa, a moon of Jupiter thought to harbor a warm, saltwater ocean sloshing beneath a thick, icy crust, has always been considered one of the best spots in the Solar System to look for alien beings. So the team reanalysed Galileo's magnetic data with modern computers and techniques, including a simulation by Zianzhe Jia, a space scientist at the University of MI, of what a plume would do to Galileo's instruments. What they found is that magnetometer readings and radio signals also showed anomalies when the craft flew over the area of the purported plumes determined by Hubble.
"During Galileo, we'd always known there was something weird during this flyby", Cynthia Phillips, a scientist at NASA's Jet Propulsion Laboratory in Pasadena, California, was quoted as saying. But that still means an orbiting spacecraft, like the Europa Clipper mission that's tentatively scheduled to launch in the early 2020s, could sample a plume and get a glimpse of what lies beneath the moon's ruddy, crisscrossed rind. No other flybys picked up evidence of these eruptions, though this particular one was the closest that the spacecraft came to Europa's surface.
"When we first saw those images, I think a lot of us in the community were very excited", says planetary scientists Xianzhe Jia from the University of MI. In the sequence of numbers produced by those two instruments, they immediately spotted something unusual: Anomalous blips, lasting about three minutes, centred around Galileo's closest approach to the moon. Until now, scientists thought a probe would need to land on the icy moon to investigate, but the latest research suggests the plumes could be sampled by an orbiter. Other researchers revisited the data as they planned for future NASA missions to revisit the planet, creating a computer simulation of the conditions when Europa might eject water into its atmosphere. This plume of water points toward an environment on the moon which would be habitable by human beings. Finally, those charged particles would interact with the surrounding magnetic field, generating fluctuations detectable by Galileo.
Jia and his colleagues are now working on the magnetic field and plasma instruments for two future missions aimed at studying Jupiter and its moons.
"We know that Europa has a lot of the ingredients necessary for life as we know it". Such a sea could provide shelter to bacterial life akin to that found in the depths of Earth's oceans. Because geologic processes move material from underground to the surface, the reverse may also be happening, transporting surface materials highly oxidized by Jupiter's harsh radiation down through the ice.
Hunt for clues in the universe to answer one of humanity's biggest questions: Are we alone? "You have to grab the ice grains that are there and see what's inside Europa", Cable says.
"If we can take samples directly from the interior of Europa, we can see more clearly if it has the ingredients for life", said Robert Pappalardo, a scientist at NASA's Europa Clipper mission, a mission that could be launched in June 2022.