Europa water currents. Computer artwork showing the surface structure of Europa, one of Jupiter's moons. Europa has a small metal core (not seen) surrounded by a large layer of rock (brown, bottom). The surface is thought to consist of an ocean of liquid water (blue) covered by a thick layer of ice (top). It is thought the water beneath the surface ice is kept liquid through volcanic activity and heat from tidal effects induced in Europa's interior by Jupiter's gravitational field. This sets up convection currents (arrows) that keep the water circulating, preventing it from freezing at all but the uppermost layers.