Large circular features could be the sites of impacts, or result from the upwelling of material from below. Making sense of this chaotic landscape is a challenge to planetary scientists. Although the surface of Europa is a jumble of curious structures, it is the kind of puzzle that science can probably solve. To answer them we collect data and make careful observations, applying what we know about geology, physics, and chemistry. In a way, the instruments on board Galileo become an extension of our senses.
Geologists determine the relative age of a surface by counting the number of impact craters.
The Earth's moon is riddled with craters young and old, testimony to the fact that it is has been geologically inactive for a billion years or more. Earth itself has been impacted at least as many times as its moon, but Earth's surface has been altered by active geological processes such as plate tectonics and volcanism, and by constant weathering. Like our own moon, Jupiter's satellites Ganymede and Callisto are heavily cratered, evidence of very old and inactive surfaces. On Europa, however, only a few large craters have been identified. Unless Europa has somehow avoided these impacts, relatively recent events must have erased their signatures.