Callisto was revealed by the Voyager cameras to be a heavily cratered and hence geologically inactive world. This mosaic of Voyager 1 images, obtained on March 6 from a distance of about 400 000 kilometers, shows surface detail as small as 10 kilometers across. The prominent old impact feature Valhalla has a central bright spot about 600 kilometers across, probably representing the original impact basin. The concentric bright rings extend outward about 1500 kilometers from the impact center. [260-450]
| SATELLITES OF JUPITER | |||||
|---|---|---|---|---|---|
| Distance From Jupiter | |||||
| Name | 10³ kilometers | Jupiter Radii | Period (days) | Year of Discovery | |
| Adrastea | J14 | 134 | 1.76 | 0.30 | 1979 |
| Amalthea | J5 | 181 | 2.55 | 0.49 | 1892 |
| 1979J2 | J15 | 222 | 3.11 | 0.67 | 1980 |
| Io | J1 | 422 | 5.95 | 1.77 | 1610 |
| Europa | J2 | 671 | 9.47 | 3.55 | 1610 |
| Ganymede | J3 | 1070 | 15.10 | 7.15 | 1610 |
| Callisto | J4 | 1880 | 26.60 | 16.70 | 1610 |
| Leda | J13 | 11 110 | 156 | 240 | 1974 |
| Himalia | J6 | 11 470 | 161 | 251 | 1904 |
| Lysithea | J10 | 11 710 | 164 | 260 | 1938 |
| Elara | J7 | 11 740 | 165 | 260 | 1904 |
| Ananke | J12 | 20 700 | 291 | 617 | 1951 |
| Carme | J11 | 22 350 | 314 | 692 | 1938 |
| Pasiphae | J8 | 23 300 | 327 | 735 | 1908 |
| Sinope | J9 | 23 700 | 333 | 758 | 1914 |
The state of the interiors of the Galilean satellites can be judged from their sizes and densities. These cross-sectional views represent the best guess following the Voyager flybys as to the composition and structure of the objects. Io, with a density equal to that of the Moon and a long history of volcanic activity, is a dry, rocky object. Europa is less dense, and it probably has a global ocean of ice as much as 100 kilometers thick over a rocky interior. Ganymede and Callisto both have densities near 2 grams per cubic centimeter, suggesting a composition about half water and half rock. There is probably a rocky core surrounded by an icy mantle.
Io Active volcanoes Sulfur and frozen SO₂ Molten silicate interior Europa Global fracture patterns Ice crust Rocky interior Ganymede Fresh craters expose ice Young grooved terrain with intricate fracture Old, dark cratered areas Ice crust Water or ice mantle Callisto Large basins reduced by ice flow Fresh craters expose ice Ice/rock crust Water or ice mantle Moon Mercury
Long after the flybys of Jupiter, continued analysis of Voyager images revealed another new satellite, Jupiter’s fifteenth. Initially designated 1979J2, the unexpected new satellite orbits the planet at 3.17 RJ, between Io and Amalthea. Stephen Synnott of the JPL Optical Navigation Team discovered the satellite on pictures taken during the Voyager 1 events on March 5, 1979, while searching for additional images of satellite 1979J1. It is about 75 kilometers in diameter, but nothing else is known about its physical properties.
Together, the 15 satellites circling giant Jupiter form a mini-solar system. Perhaps the outer, irregular satellites were captured or resulted from the catastrophic collisions of one or more larger satellites with passing asteroids. The inner seven satellites constitute a coherent system, almost certainly formed together with Jupiter and sharing a common 4.5-billion-year history. They are fascinating as individual worlds, and also as brothers and sisters, and the study of their interrelationships undoubtedly will provide insights into the general problems of planetary formation and evolution.
| SIZES AND DENSITIES OF THE GALILEAN SATELLITES | ||
|---|---|---|
| Name | Diameter (kilometers) | Density (grams per cubic centimeter) |
| Io | 3640 | 3.5 |
| Europa | 3130 | 3.0 |
| Ganymede | 5270 | 1.9 |
| Callisto | 4840 | 1.8 |
Callisto
Callisto is the least active geologically of the Galilean satellites. Basically a dead world, it bears the scars of innumerable meteoric impacts, with virtually no sign of major internal activity. Callisto is a world of craters, and to understand it we must explore the role that cratering plays in molding planetary surfaces.