IO:
There are no impact craters on Io. The surface of Io must be younger than a millions years old, and is continually being resurfaced by volcanic activity. Also, the surface is very colorful, mottled with red, yellow, white and orange black markings. The surface composition on Io consists largely of sulfur with deposits of frozen sulfur dioxide. The surface on Io is mostly flat plains rising no more than 1 km. Moutain ranges up to 9 km high have also been observed.
EUROPA:
Voyager pictures show pale-yellow icy plains with red and brown mottled regions. Long cracks run for thousands of kilometers over the surface. On Earth, these cracks would indicate such features as tall mountains and deep canyons. But none of these features are higher than a few kilometers on Europa, making it one of the smoothest objects in our Solar System.
If we look at the surface more closely, as we have with the instruments on the Galileo spacecraft, we see some fascinating features. Europa looks like broken glass that is repaired by an icy glue oozing up from below. Low ridges, straight and curved, crisscross the surface. Flows and fractures, pits and frozen "puddles" - all hint at a unique geologic history. Large circular features could be the sites of impacts or the result of upwelling of material from beneath the surface. Making sense of this chaotic landscape is a challenge to planetary scientists. Some of our questions are: "How old is the surface? How were the cracks and other features made? What is under the ice?" To answer them, we collect data and make careful obervations, applying what we know about geology, physics, and chemistry. Geologists figure out the age of a surface by counting the impact craters formed where comets, meteorites, and other debris hit the surface.
Comparison to Our Moon
Earth's Moon has young and old craters literally everywhere, which tells us that it has been geologically inactive for more than a billion years. Earth has been impacted at least as many times as the Moon, but Earth's surface has been smoothed by active geological processes such as plate tectonics and volcanic flows, and by constant weathering. Like our 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, which is unlikely, relatively recent events must have smoothed over the craters.
GANYMEDE:
Ganymede has had a complex geological histroy. It has mountains, valleys, craters and lava flows. Ganymede is mottled by both light and dark regions. It is heavily cratered, especially in the dark regions, implying ancient origin. The bright regions show a different kind of terrain - one which is grooved with ridges and troughs. These features form complex patterns, have a vertical relief of a few hundred meters, and run for thousands of kilometers. The grooved features were apparently formed more recently than the dark cratered area, perhaps by tension from global tectonic processes. The real reason is unknown; however, local crust spreading does appear to have taken place, causing the crust to shear and separate.
CALLISTO:
Callisto is the most heavily cratered object in the solar system. It is thought to be a long dead world, with a nearly complete absence of any geologic activity on its surface. In fact, Callisto is the only body greater than 1000 km in diameter in the solar system that has shown no signs of undergoing any extensive resurfacing since impacts have molded its surface. Meteorites have punctured holes in the crust, causing water to spread over the surface and forming bright rays and rings around the crater. With a surface age of about 4 billion years, Callisto has the oldest landscape in the solar system.
Courtesy: The Jet Propulsion Laboratory, Pasadena, CA which manages the Galileo mission for NASA's Office of Space Science, Washington, DC. JPL is an operating division of California Institute of Technology (Caltech).