Jupiter’s Satellite System
In a sense, the Voyager Mission revealed a new planetary system. Astronomers had long been fascinated by the large Galilean satellites of Jupiter, but they had only looked from afar, watching the dancing points of light in their telescopes, and, occasionally, as the atmosphere steadied, seeing these points resolve themselves into tiny disks before dissolving again in the turbulence of the terrestrial atmosphere. Much had been learned from telescopic studies, but not until the Voyager flights had we truly seen the Galilean satellites. The historic hours as Voyager 1 cruised past each satellite on March 5 and 6, 1979, fundamentally altered our perspective. Four new worlds were revealed, as diverse and fascinating as the more familiar terrestrial planets. Although not yet household words, the names Io, Europa, Ganymede and Callisto have now been added to Mercury, Venus, Moon, and Mars in the lexicon of important “Earth-sized” bodies in the solar system.
Jupiter has fifteen known satellites, counting the two new satellites discovered by Voyager. These moons vary greatly in size, composition, and orbit. The four outermost satellites, Sinope, Pasiphae, Carme, and Ananke, circle the planet in retrograde orbits of high inclination; their distances from Jupiter vary between 20 and 24 million kilometers (290 RJ to 333 RJ). These small bodies, none more than 50 kilometers in diameter, require nearly two years for each orbit of Jupiter. It is possible that they are captured asteroids, but so little is known about them that astronomers cannot tell if their surface properties resemble those of asteroids, or if these four satellites are even similar to each other.
The next group of Jovian satellites consists of four small difficult-to-observe objects. These are Lysithea, Elara, Himalia, and Leda, the latter discovered by Charles Kowal of Hale Observatories in 1974. They have similar orbits, varying in distance from Jupiter between 11 and 12 million kilometers (about 160 RJ). Like the outer group, these satellites have orbits of high inclination; unlike the outer group, they move in the proper, prograde direction around Jupiter. The largest, Himalia (170 kilometers in diameter) and Elara (80 kilometers diameter), are known to be very dark, rocky objects, and it seems probable that the others are similar. It is unlikely that the census of the outer groups of irregular satellites is complete, and new satellites less than 10 kilometers in diameter will probably be discovered.
The Jovian system is dominated, of course, by the large Galilean satellites, which vary in size from just smaller than the Moon (Europa) to nearly as large as Mars (Ganymede). These satellites are in regular, nearly circular orbits in the same plane as the equator of Jupiter, and all four lie within the inner magnetosphere of Jupiter, where they interact strongly with energetic charged particles and plasma. Most of this chapter will be devoted to a discussion of these fascinating worlds.
We now know of three additional small satellites inside the orbit of Io, orbiting close to Jupiter. The first, Amalthea, was discovered in 1892; it orbits Jupiter in just twelve hours at a distance of 181 000 kilometers (2.55 RJ). A smaller object, Adrastea (officially 1979J1 for the first new satellite of Jupiter discovered in 1979), is much closer, at 134 000 kilometers (1.76 RJ). As described in Chapter 7, it skirts the outer edge of the ring, circling Jupiter in just over seven hours. The inner satellite moves faster than Jupiter’s rotation; seen from the planet, it would rise in the west and set in the east. Both Amalthea and Adrastea are buried deep within the inner magnetosphere where they are continually bombarded by energetic electrons, protons, and ions. Depletion of the Jovian radiation belt particles was observed at the orbits of both satellites by Pioneer 11, which went much closer to Jupiter than the Voyagers, testifying to the intensity of the interaction between these objects and their surroundings.
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 |