| Jupiter has 16 moons. By far the largest and best known are the 4 Galilean Moons, so named because they were discovered by Galileo. Indeed, the 4 Galilean moons are not difficult to see from Earth with even small telescopes. The adjacent image shows a Voyager montage of the Galilean moons. |
| | The Other Moons of Jupiter |
| | Jupiter has a dozen moons other than the large Galilean satellites. These are much smaller than the Galilean moons. An example is Amalthea, shown in the image to the left, which orbits inside the orbit of Io (its color may be due to particles from the volcanoes on Io). |
| | Many of these moons may be asteroids captured in the immense gravitational field of Jupiter. Images of these satellites may be found at the Planetary Picture List. |
| | The Moon Europa: Cue Ball of the Satellites |
| | Europa, like Io, is heated internally by the tidal tug-of-war with Jupiter, but because Europa is further from Jupiter than Io 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. |
| | Europa, pictured in the image to the right, may be the smoothest solid body in the Solar System. It appears to be covered with a frozen water ocean and has very few craters. As the adjacent image indicates, it is covered with cracks, but these seem to have little vertical relief (Ref). Details of the crazed cracks criss-crossing Europa's frozen surface are apparent in this mosaic of the Galileo spacecraft's latest images of Jupiter's ice-covered moon shown to the right. |
| | Here is a further discussion of these Cracked Ice Plains of Europa. |
| | The Moon Ganymede: Laboratory for Tectonic Motion |
| | he figure shown to the right is a Galileo Spacecraft image of the third Galilean moon, Ganymede, which has the distinction of being the largest moon in the Solar System. Its diameter is 5270 km, much larger than our own Moon (3476 kilometers). This moon is also largely covered by a frozen water ocean. Portions of its surface look young geologically and portions look old. It has large grooves in places, the crust is broken into fragments, and there is evidence for past plate tectonic activity. This represents the first conclusive evidence of plate tectonics in the Solar System beyond the Earth. |
| | Detailed Surface Features |
| | he following two images show more detailed features on Ganymede. |
| | Tectonic features on the surface of Ganymede |
| | The Galileo Regio region on Ganymede. |
| | This figure is a mosaic of four Galileo images of the Galileo Regio region on Ganymede overlayed on the data obtained by the Voyager 2 spacecraft. These Galileo images show fine details of the dark terrain that makes up about half of the surface. Ancient impact craters indicate the great age of the terrain, dating back several billion years (more info). |
| | Young Surface and Old Surface |
| | In the adjacent image the dark areas are older, more heavily cratered regions and the light areas are younger, tectonically deformed regions. 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 (more info). |
| | History and Structure of Ganymede |
| | The figure adjacent left shows a model of Ganymede. We believe that Ganymede, further from Jupiter and less heated by tidal effects, is frozen more solid than Europa, but its surface and subsurface is still somewhat plastic because of tidal heating. The past plate tectonic activity that we see evidence for may have resulted from the heat generated by earlier tidal coupling to Jupiter. |
| | The Moon Io: A Seething Interior and Active Surface |
| | The adjacent figure (Ref) shows the best Galileo Orbiter image yet of Io's full disk. How did this innermost of the Galilean satellites come to look like a pepperoni pizza? The answer is that 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. |
| | As we shall see, this volcanic activity does not originate in plate tectonic activity the way that much volcanic activity originates on the Earth. For Io, we shall see that instead it is tidal forces associated with the 500 pound gorilla next door (Jupiter) that are the culprit. |
| | The Io Torus and the Io Flux Tube |
| | As we have already seen Io influences and is strongly influenced by Jupiter's magnetic field. The moon orbits within the field, and material ejected from its surface by volcanoes (and by the charged particle bombardment from the material already trapped in Jupiter's magnetic field) is a primary source of the charged particles in the Jovian magnetic field. These particles (primarily sulfur and oxygen) have a high concentration in a doughnut shaped region surrounding Io's orbit called the Io Torus. Here is a movie simulating the Io Plasma Torus. |
| | As Io moves around its orbit in the strong magnetic field of Jupiter and through this plasma torus, a huge electrical current is set up between Io and Jupiter in a cylinder of highly concentrated magnetic flux called the Io Flux Tube. The Flux Tube has a power output of about 2 trillion watts, comparable to the amount of all manmade power produced on Earth. It is responsible for bursts of radio frequency radiation long detected on Earth. |
| | In 1979 an engineer responsible for navigation of the Voyager I spacecraft noticed a strange mushroom-like object on the limb of Io, in an image of stars used to check the positioning of the spacecraft. After ruling out trivial explanations, 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. Subsequent spacecraft (Voyager II and Galileo) have documented extensive vulcanism on the surface of Io, as illustrated in the following three images. |
| | The rightmost image above is of the large shield volcano Ra Patera, as recently photographed by the Galileo spacecraft. Some volcanoes seen by Voyager are no longer erupting, but there are new volcanoes that have begun erupting in the 17 years between Voyager and Galileo. |
| | The image adjacent left shows the region near the volcano Pele, as photographed by Galileo in 1996. The large red ring around Pele is thought to indicate recent volcanic activity. The red color indicates the presence of Sulfur, although how the Sulfur was produced is not precisely known. |
| | Volcano on Io's limb discovered by Voyager |
| | Volcano erupting on Io as observed by Voyager |
| | Galileo image of volcano erupting on Io |
| | The Surface and Interior of Io |
| | Indeed, we now know that Io has the youngest surface in the Solar System. Present estimates are that it is so active geologically that its surface is completely redone on a timescale of about a million years---a twinkling of the eye on geological timescales. This makes Io the most active object geologically in the Solar System. In addition to the intense volcanic activity, Io's surface is being continuously eroded by a bombardment of charged particles trapped in Jupiter's magnetic field. |
| | The source of this geological activity is the tidal effect of Jupiter on Io as it orbits the giant planet. These tidal forces are so large that Io's surface is pulled upwards and downwards by hundreds of meters in each rotation. Just like bending a paper clip heats it, these tidal distortions keep much of the interior of Io in a molten state. Thus, Io is a thin crust (primarily of sulfur) sitting on a molten interior. It is this molten interior breaking through to the surface that produces the volcanoes of Io, and the spectacular surface appearance as captured in the adjacent picture of a sequence of Hubble Space Telescope images. |
| | 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. After Io's romance with Jupiter, she was turned into a heifer, pursued by Juno's gadfly (I'm not making this up!). To elude the ever-watchful Juno, Jupiter approached Europa as a bull; she climbed upon his back, and the two flew off to Crete, where Europa became an object of worship. Ganymede, a "handsome youth" attracted Jupiter's attention too, who whisked the boy off to become a cupbearer to the gods. The stuff of soap opera, right here in your astronomy class! |
| | Proximity to Jupiter is Destiny |
| | The Galilean moons are all between 3000 and 5000 km in diameter, and they differ markedly in structure. For most objects in the Solar System we believe that their mass is the single most important factor governing their structure. For the Galilean Moons, we will find that the nearness to Jupiter is the primary determining factor in their structure. Thus, for most planets and moons in the Solar System mass is destiny, but for the Galilean satellites proximity to Jupiter is destiny and mass is secondary. |
| | The Moon Callisto: Geologically Dead |
| | The image on the right shows the moon Callisto, the fourth Galilean satellite. Callisto, like Ganymede and Europa, is covered by a frozen water ocean, but unlike the other Galilean satellites, Callisto is saturated with craters, suggesting very old terrain. There is no evidence for plate tectonics. |
| | The Valhalla Impact Crater |
| | The image on the left (Ref) shows an enormous impact feature on Callisto called Valhalla. This gigantic meteor impact basin appears to have concentric waves around it with a diameter of about 3000 km. It is likely that the waves were formed when the surface ice was melted by a gigantic meteor impact and then quickly refroze, capturing the waves in a refrozen ice surface. |
| | Maximal Cratering Density |
| | The image left shows a closeup with more detail of the heavy cratering. The cratering density on Callisto is essentially maximal, meaning that no higher density of craters could be formed because the formation of a new crater would on the average destroy one old crater. |
| | Callisto, the most distant Galilean satellite, 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. |
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