Artist's concept of the proposed Jupiter Icy Moons Orbiter, an ambitious mission which would orbit three planet-sized moons of Jupiter -- Callisto, Ganymede and Europa -- which may harbor vast oceans beneath their icy surfaces. NASA's Galileo spacecraft found evidence that Jupiter's large icy moons appear to have three ingredients considered essential for life: water, energy and the necessary chemical contents. The evidence from Galileo suggests melted water on Europa has been in contact with the surface in geologically recent times and may still lie relatively close to the surface.