Europa is the sixth-largest moon in the Solar System and Jupiter’s fourth-largest satellite. Despite its cracked and discolored appearance, it is the smoothest solid object in the Solar System; its highest peaks, of which there are few, only reach a few hundred meters tall, and large craters are rare. The crisscrossed streaks and splotches that run across the terrain are probably mostly due to relatively shallow fractures and different compositions of the ground, not the enormous canyons present on some other worlds. This smoothness means that the surface of the moon is fairly young, probably tens of millions of years old rather than billions, like some other objects. Somehow, the ice is being resurfaced and smoothed out faster than on many other worlds. The question of why that happens has not been definitively answered yet, but it probably has something to do with what’s below Europa’s surface. This frigid moon of Jupiter hides a tantalizing secret: a probable sea containing twice as much water as all of Earth’s oceans combined. While the icy shell is hard as a rock, the interior is warmer, heated by the flexing of its iron core and stone mantle. The side of Europa that’s closest to Jupiter experiences a stronger pull than the farther side, stretching the entire moon and probably causing the long cracks that run across its surface as well as heating the interior. The evidence that there is some sort of water or slush beneath Europa’s surface is fairly strong, based on models of how a buried ocean would affect the ice above it. Measurements of the moon’s magnetic field also hinted that there is some sort of electrically conductive fluid — such as salty water —flowing under the surface. There are even images from the Hubble Space Telescope that appear to show huge plumes of water vapor erupting from Europa’s south pole, although they are not high-resolution enough to be definitive. If the liquid ocean really is there, it could make Europa an especially promising place to look for life, because liquid water is crucial for life as we know it. But many mysteries remain about this huge moon, from the thickness of the ice shell to the details of what happens beneath it. Source - https://www.planetary.org/worlds/europa