Europa is covered with a shell of ice. It has an internal source of heat from the tidal forces exerted by Jupiter. Europa races around Jupiter in an elliptical, 3.5-day orbit. When the moon is farthest from Jupiter and the gravitational squeeze of the giant planet eases up, the icy shell decompresses just enough to let subsurface water find a path to the surface and vent into space, scientists believe. They say the subsurface ocean could be 50 miles deep at least, containing more water than in all the oceans on Earth.
The geysers are squirting not only water into space, but also whatever else is in that subsurface ocean. Landing on Europa is too expensive, as is sending a probe down through the ice into the water below, but a spacecraft can learn much about the planet through high-resolution stereo imagery, ice-penetrating radar and other instruments. It also could fly through the plumes and, with an instrument called a mass spectrometer, analyze the ejected material.
“This is huge,” said JPL’s Robert Pappalardo, who has worked on plans for the Europa Clipper. “It now means that we could effectively sample Europa’s interior from flybys.”
Rather than orbiting Europa, the spacecraft would go into an orbit around Jupiter, spending most of its time outside the planet’s radiation field, and then swoop in repeatedly, with 34 flybys of Europa and nine of the moon Ganymede.I think this should be our top priority in space exploration, far above the dismal International Space Station or the fantasy of a manned mission to Mars. But right now the "budget environment" is even more challenging than the environs of Jupiter, and the Europa Clipper may not fly for decades.