1. The Jovian Moons
Jupiter has 16 moons. By far the largest and best known are the 4 Galilean Moons, so named because they were discovered by Galileo

2. IO
After Io's romance with Jupiter, she was turned into a heifer, pursued by Juno's gadfly

3. There are no impact craters on Io
There are no impact craters on Io. The surface of Io must be younger than a Million years old, and is continually being resurfaced by volcanic activity.
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.

4. Io is the most active object geologically in the Solar System
It is covered with volcanic activity that spews various sulfur compounds onto the surface and accounts for the color and pocked nature of the surface
This volcanic activity does not originate in plate tectonic activity the way that much volcanic activity originates on the Earth. Its cause are the tidal forces associated with the 500 pound gorilla next door (Jupiter)

5. In 1979 an engineer responsible for navigation of the Voyager I spacecraft noticed a strange mushroom-like object on the limb of Io
It was realized that the camera had captured a volcano erupting on the surface of Io, the first live volcano found in the Solar System beyond the Earth

6. The Io Plasma Torus A torus of sodium gas along with sulfur ions is spread out over Io's orbit. This torus is so large that it has been observed from Earth. As Jupiter rotates, it takes its magnetic field around with it, sweeping past Io and stripping off about 1,000 kilograms (1 ton) of Io's material every second! This material becomes ionized in the magnetic field and forms a doughnut-shaped cloud of intense radiation referred to as a plasma torus. Some of the ions are pulled into Jupiter's atmosphere along the magnetic lines of force and create auroras in the planet's upper atmosphere. It is the ions escaping from this torus that inflate Jupiter's magnetosphere to over twice the size we would expect.

8. Discovery: Jan 7, 1610 by Galileo Galilei
Surface Composition: Sulphur
Diameter (km): 3,630
Mass (kg): 8.94x1022
Orbital period (days):
Rotational period (days):
Density (gm/cm3) 3.57
Orbit Speed (km/sec): 17.34

9. EUROPA
To elude the ever-watchful Juno, Jupiter approached Europa as a bull; she climbed upon his back, and the two swam off to Crete, where Europa became an object of worship

10. Discovery: Jan 7, 1610 by Galileo Galilei
Diameter (km): 3,138
Mass (kg): 4.8e22
Orbital period (days):
Rotational period (days):
Density (gm/cm3) 3.01
Surface Composition: Water Ice
Orbit Speed (km/sec): 13.74

11. The United States sent two spacecraft, Pioneer 10 and 11, to Jupiter in the early 1970's.
The twin Voyager spacecraft flew by Jupiter and its moons in 1979, giving us our first close-up view of Europa.

12. Galileo images hint at the possibility of liquid water beneath the icy crust of this moon. The bright white and bluish parts of Europa's surface are composed almost completely of water ice. In contrast, the brownish mottled regions on the right side of the image may be covered by salts (such as hydrated magnesium-sulfate) and an unknown red component. The yellowish mottled terrain on the left side of the image is caused by some other, unknown contaminant.

13. Long cracks run for thousands of kilometers over the surface
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
Europa is heated by tidal forces from Jupiter. The surface bends and flexes and water gushes to the surface and freezes

14. Europa is further from Jupiter than Io, so the tidal effect is less dramatic. In the case of Europa, the effect of the tides induced by Jupiter is to heat the interior of Europa sufficiently to keep the surface soft. Thus, no vertical relief features can survive for long on Europa's surface, explaining its smooth appearance.

15. WHAT DO YOU THINK?

16. GANYMEDE
Ganymede, a "handsome youth" attracted Jupiter's attention too, who whisked the boy off to become a cupbearer to the gods
Zeus, made a place for him among the stars as Aquarius – the Water Bearer. There he still stands, smiling, pouring nectar and shielded to this day by the wing of the Eagle constellation.

17. Ganymede is larger than the planet Mercury but only half its mass
Ganymede is larger than the planet Mercury but only half its mass. It is the largest Jovian satellite, and the largest moon in the solar system
Its diameter is 5270 km, much larger than our own Moon (3476 kilometers).
Ganymede is the third of the Galilean moons

18. Bright frost is visible at the north and south poles
Bright frost is visible at the north and south poles. The very bright icy impact crater, Tros, is near the center of the image in a region known as Phrygia Sulcus. The dark area to the northwest of Tros is Perrine Regio; the dark terrain to the south and southeast is Nicholson Regio.
Ganymede's surface is characterized by a high degree of crustal deformation

19. Ganymede's surface is a roughly equal mix of two types of terrain: very old, highly cratered dark regions (left), and somewhat younger (but still ancient) lighter regions marked with an extensive array of grooves and ridges (right). Their origin is clearly of a tectonic nature, but the details are unknown. In this respect, Ganymede may be more similar to the Earth than either Venus or Mars (though there is no evidence of recent tectonic activity).
The brownish-gray color is believed due to mixtures of rocky materials and ice. Geologically recent impact craters and their ejecta are indicated by bright regions

20. New and old terrain
The structure of Ganymede

21. Ganymede is composed of silicate rock and water ice, with an ice crust floating over a slushy mantle that may contain a layer of liquid water
Preliminary indications from the Galileo probe data suggest that Ganymede is differentiated into a three layer structure: a small molten iron or iron/sulfur core surrounded by a rocky silicate mantle with a icy shell on top.

22. CALLISTO
The Galilean satellites are all named after objects of mythological Jupiter's wide-ranging fancies. Callisto was a beautiful maiden who enticed Jupiter, thereby invoking the wrath of Juno, Jupiter's wife. She turned Callisto into a bear
Callisto and her son Arcas transformed as the Great Bear and the Bear Warden. (c) 1995 Visula Language. All rights reserved.

23. With a diameter of over 4,800 km (2,985 miles), Callisto is the third largest satellite in the solar system and is almost the size of Mercury
Discovery: Jan 7, 1610 by Galileo Galilei
Orbit Speed (km/sec): 8.21
Mass (kg): 1.077e23
Rotational period (days):
Orbital period (days):

24. Callisto is the outermost of the Galilean satellites, and orbits beyonds Jupiter's main radiation belts. It has the lowest density of the Galilean satellites (1.86 grams/cubic centimeter).
Callisto is also made of ice and is heavily cratered. It is the least reflective of the four, but brighter than our moon. It is hard to see from Earth. The surface temperature on Callisto varies from a high of -118° F to a low of -193°.

25. Callisto is the most heavily cratered object in the solar system
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.
The large Valhalla multi-ring structure (visible near the center of the image) has a diameter of about 4,000 kilometers, making it one of the largest impact features in the Solar System

26. It is only weakly influenced by Jupiter-induced tides
It is only weakly influenced by Jupiter-induced tides. The evidence suggests that the crust of Callisto probably froze as it was formed, and essentially nothing has happened geologically on Callisto for 4.5 billion years except for the influence of large and small meteor impacts.

27. The Other Moons of Jupiter
Jupiter has a dozen moons other than the large Galilean satellites. These are much smaller than the Galilean moons

28. University of Hawaii astronomers announce the discovery of 11 new satellites of Jupiter. These new satellites, when added to the eleven discovered the previous year by the Hawaii team, bring the total of known Jupiter satellites to 39. Jupiter now has more known satellites than any other planet.
The new satellites were discovered during mid-December of 2001