NASA has started to piece together the Europa Clipper

Europa holds mysteries – and maybe even life

Most of Jupiter's 79 moons are small and not all that attractive to astronomers. But the four biggest — Io, Europa, Ganymede, and Callisto — are more like mini planets than desolate space rocks. In fact, they formed in parallel with Jupiter. All five objects emerged from the same cloud of gas and dust.

Europa stands out because it appears to have a crust of ice. Much like Earth, the rocky object has a metallic core at its center and "a skin of H₂O," according to Bob Pappalardo, NASA's project scientist for the mission.

He says that the "liquid water ocean" sits beneath roughly 13 miles of ice.

Researchers are so excited about the Europa Clipper mission that the moon's ocean could have astrobiological implications. If we find life in our Solar System — no matter what strange shape it takes — it might be on Europa.

The water is probably "in contact with the rock below, so nutrients can seep into that ocean and potentially serve as a fuel for life," Pappalardo says. Since no light can penetrate the thick later of ice, life on Europa won't rely on photosynthesis. Instead, researchers think life on the moon sustains itself on chemical reactions, as some forms of life on Earth do.

Teams on two continents are building the Europa Clipper 

The craft will carry nine scientific instruments, all designed and constructed at sites across the U.S. and Europe. The first one arrived at JPL last week when a team from the Southwest Research Institute in San Antonio, Texas, delivered an ultraviolet spectrograph. The instrument will provide data on the plumes of gas (and possibly water vapor) that the Galileo spacecraft has spied shooting up through cracks in the surface ice.

Europa Clipper's main module, which is the size of an SUV, is being finished by a team at Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. It's scheduled to arrive at JPL in the spring.

Europa Clipper was initially designed to travel aboard NASA's new Space Launch System rockets. Last summer, the agency announced it would instead pay Elon Musk's SpaceX $178 million to send the spacecraft on its journey.