Early History of the Satellites

The inner eight moons of Jupiter are commonly thought to have originated in much the same way as the planet itself. Just as the primordial solar nebula is believed to have broken up into accreting planetesimals, which became the planets, and a central condensation, which became the Sun, the accumulation of material into a protoplanetary cloud at Jupiter's orbit ultimately led to the formation of the planet and its inner moons. The analogy goes further. The high temperature of the forming Jupiter apparently prevented volatile substances from condensing at the distances of the innermost moons. Hence, Ganymede and Callisto, the most distant of the inner eight moons, represent the volatile-rich outer bodies in this system.

The origin of the numerous small outer moons, with their orbits of high eccentricities and inclinations, is thought to be quite different. They are members of the population of irregular satellites in the solar system and are most likely captured objects. Their ultimate origin, however, remains unclear. The parent bodies of these moons may have first formed in the outer nebula of Jupiter, strayed away, and then been recaptured; alternately, they may have formed independently in the solar nebula itself and then been captured. In either case, the capture process apparently caused the parent bodies to break up, forming the debris observed today as the outer irregular moons. Ongoing studies of these objects and their possible relatives among the Trojan asteroids may provide the answer to their origin.

Saturn

In the night sky Saturn is easily visible to the unaided eye as a nontwinkling point of light, and, when viewed in even a small telescope, the planet girded with its magnificent rings is arguably the most sublime object in the solar system. Saturn is designated by the symbol

Saturn's name comes from the Roman god of agriculture, who is equated with the Greek deity Cronus, one of the Titans and the father of Zeus (the Roman god Jupiter). As the farthest of the planets known to ancient observers, Saturn also was noted to be the slowest-moving. At a distance from the Sun that is 9.5 times as far as Earth's, Saturn takes nearly 30 Earth years to make one solar revolution. The Italian astronomer Galileo in 1610 was the first to observe Saturn with a telescope. Although he saw a strangeness in Saturn's appearance, the low resolution of his instrument did not allow him to discern the true nature of the planet's rings.

Saturn is the second largest planet of the solar system in mass and size and the sixth in distance from the Sun. It occupies almost 60 percent of Jupiter's volume but has only about one-third of its mass and the lowest mean density—about 70 percent that of water—of any known object in the solar system. Hypothetically, Saturn would float in an ocean large enough to hold it. Both Saturn and Jupiter resemble stars in that their bulk chemical composition is dominated by hydrogen. Also, as is the case for Jupiter, the tremendous pressure in Saturn's deep interior maintains the hydrogen there in a fluid metallic state.

Saturn's structure and evolutionary history, however, differ significantly from those of its larger counterpart. Like the other giant, or Jovian, planets—Jupiter, Uranus, and Neptune—Saturn has extensive systems of moons (natural satellites) and rings, which may provide clues to its origin and evolution as well as to those of the solar system. Saturn's moon Titan is distinguished from all other moons in the solar system by the presence of a significant atmosphere, one that is denser than that of any of the terrestrial planets except Venus.

The greatest advances in knowledge of Saturn, as well as of most of the other planets, have come from deep-space probes. Four spacecraft have visited the Saturnian system—Pioneer 11 in 1979, Voyagers 1 and 2 in the two years following, and, after an almost quarter-century hiatus, Cassini-Huygens beginning in 2004. The first three missions were short-term flybys, but Cassini went into orbit around Saturn for several years of investigations, while its Huygens probe parachuted through the atmosphere of Titan and reached its surface, becoming the first spacecraft to land on a moon other than Earth's.

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