Voyage to Jupiter

by David Morrison; Jane Samz

Ganymede (longitude 320°).

CHAPTER 9
FOUR NEW WORLDS

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 R_J to 333 R_J). 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 R_J). 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 R_J). 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 R_J). 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.