Hubble's view of huge plumes of water vapor in 2013, data shown superimposed on a photo of Europa
Scientists are almost positive that below Europa's icy exterior, there's about 3 billion cubic kilometers of water sloshing around in a subsurface ocean. That's more water than we have here on Earth. And because life on Earth requires water, the Jovian moon is one of the top spots in our solar system where scientists would like to search for alien life.
The trouble is, Europa's ocean is thought to be buried under about 62 miles of solid ice. Which is why it was so exciting, in 2013, when Hubble spied water vapor above Europa. This water vapor may be erupting in plumes from Europa's surface, and if those plumes are shooting up from the inner ocean, a spacecraft could potentially sample the ocean simply by swooping through the plumes--no drilling rig required.