Cutaway views of the possible internal structures of the Galilean satellites. Ganymede is at the lower left, Callisto at the lower right, Io on the upper left, and Europa on the upper right. The surfaces of the satellites are mosaics of images obtained in 1979 by NASA's Voyager spacecraft, and the interior characteristics are inferred from gravity field and magnetic field measurements by NASA's Galileo spacecraft. The satellites are shown according to their actual relative sizes. Ganymede's radius is 2634 kilometers (km); Callisto's is slightly smaller at 2403 km; Io's radius is 1821 km, similar to the 1738 km radius of our Moon; Europa's radius is 1565 km, not too much smaller than our Moon's radius. With the exception of Callisto, all the satellites have metallic (iron, nickel) cores (shown in gray) drawn to the correct relative size. Again, with the exception of Callisto, all the cores are surrounded by rock (shown in brown) shells. Io's rock or silicate shell extends to the surface, while the rock layers of Ganymede and Europa (drawn to correct relative scale) are in turn surrounded by shells of water in ice or liquid form (shown in blue and white and drawn to the correct relative scale). Callisto is shown as a relatively uniform mixture of comparable amounts of ice and rock. The surface layers of Ganymede and Callisto are shown as white to indicate that they may differ from the underlying ice/rock layers in a variety of ways including, for example, the percentage of rock they contain. The white surface layer on Europa could have similar significance, although it could also suggest an ice layer overlying a liquid water ocean. Galileo images of Europa suggest that a liquid water ocean might now underlie a surface ice layer several to ten kilometers thick; however, this evidence is also consistent with the existence of a liquid water ocean in the past. It is not certain if there is a liquid water ocean on Europa at present. For more info go to: http://galileo.jpl.nasa.gov/moons/.
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.
The interior of Io is heated by the constant squeezing and distortion of Io in Jupiter's powerful gravitational grip. Io is the most geologically active body in the Solar System, and though it is less than a third of Earth's size, it generates twice as much heat as the Earth. "Jupiter's massive gravity field distorts the shape of Io in the same way that tides are raised in Earth's oceans by the gravitational tugs of the Sun and Moon." As Io orbits Jupiter, these so-called "body tides" rise and fall due to subtle changes in Io's orbit which in turn are caused by the gravitational nudges from Europa and Ganymede, other moons of Jupiter.
As a result, Io is squeezed like a rubber ball. Friction created by this action heats and melts rock within Io to produce the volcanoes and lava flows seen all over its surface, and huge geysers that spew sulfur dioxide onto Io's landscape.
The Galileo spacecraft discovered a large, dense core within Io. During the spacecraft's flyby within 559 miles of the moon on Dec. 7, 1995, as Galileo passed by the moon on its way to enter orbit around Jupiter, precise measurements of the spacecraft's radio signal revealed small deviations in Galileo's trajectory caused by the effects of Io's own gravity field. From these data, Galileo scientists have determined that Io has a two-layer structure. At the center is a metallic core, probably made of iron and iron sulfide, about 560 miles in radius, which is overlain by a mantle of partially molten rock and crust. The core was probably formed from heating in the interior of the moon, either when it originally formed or as a result of the perpetual tidal heating driving its volcanoes.
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.
Geologic Action on Europa
Looking at the pictures from Galileo, we see evidence of geologic action on Europa. Small blocks of crust float like icebergs over an invisible sea. Some blocks are tilted, others rotated out of place. Dark bands of ice and rock spread outward from a central ridge. What is the cause for this activity? In a gravitational tug of war of incredible dimensions, Europa is pulled in different directions by Jupiter and by the planet's other moons in a process called tidal flexing. Over one Europan day, it stretches and compresses up to several tens of meters. The outer surface of Europa is a rigid sphere. Imagine Earth covered by a blanket of ice that traps the oceans below. In the course of a day, these oceans rise and fall. This is what happens to Europa. The flexing of Europa's surface continues until the brittle crust cracks. We don't know what happens when the crust fractures. The process may be slow and steady, advancing only centimeters at a time - or, it may cause ice volcanoes or geysers to erupt violently, showering the surface with material from below.
Is There Life in the Oceans of Europa?
Another interesting possibility arises from this tidal flexing of Europa. Heat generated by the expansion and contraction may be enough to melt part of the crust underneath the surface, creating lakes or oceans below. The possibility of liquid water just below Europa's surface naturally leads to the question of whether life could have evolved there. Scientists have discovered marine life on Earth that thrives in the deep ocean near hydrothermal vents. This discovery provides us with a model for how similar organisms might survive on Europa. However, liquid water is just one of life's key ingredients. Many other factors, including organic material and a continuous energy source, must be present. Even if there is no ocean currently on Europa, one may have existed in the past, perhaps leaving fossilized remains to be found by a future mission.
GANYMEDE:
Ganymede is the largest satellite in the solar system with a diameter of 5,268 km (3270 miles). It is larger than Mercury and Pluto, and three-quarters the size of Mars.
Composition
Ganymede is most likely composed of a rocky core with a water/ice mantle and a crust of rock and ice. Its low density of 1.94 gm/cm3,indicates that the core takes up about 50% of the satellite's diameter. Ganymede's mantle is most likely composed of ice and silicates, and its crust is probably a thick layer of water ice.
CALLISTO:
Callisto is almost the size of Mercury. It is the outermost of the Galilean satellites. It has the lowest density of the Galilean satellites (1.86 grams/cubic centimeter). Its interior is probably similar to Ganymede except the inner rocky core is smaller, and this core is surrounded by a large icy mantle. Callisto's surface is the darkest of the Galileans, but it is twice as bright as our own Moon.
Landscape
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. 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).