Saturn, Uranus and Neptune

Voyagers 1 and 2 reached Saturn in 1980 and 1981, respectively. The second-largest planet in the solar system, Saturn has only a third of the mass of Jupiter, though it is still more than 90 times as massive as the Earth. Saturn sports a ring system that has been visible to telescopes on Earth for centuries. Voyager 1's close-up images revealed, however, that the planet's seven major rings are divided into hundreds of thousands of subrings, each divided by a narrow space, like the grooves on a phonograph record. The rocky and icy particles making up the rings are organized in this way by the effects of Saturn's gravity.

Voyager data on Saturn's atmosphere revealed little helium in comparison with Jupiter's atmosphere. Saturn has a colder interior than Jupiter—cold enough so that helium in the atmosphere liquefies. Since helium is denser than hydrogen, it falls through the atmosphere in the form of little droplets until it reaches—and becomes part of—the planet's dense core. The result is that there is now little helium remaining in the higher layers of the atmosphere.

Perhaps the most extraordinary part of the Voyager mission was Voyager 1's exploration of Titan, the largest of Saturn's 18 known moons. Scientists knew that Titan had an atmosphere, but they had not been able to determine its extent or density. Voyager 1's cameras showed that Titan's surface is completely hidden by a thick, orange haze suspended in the atmosphere. The probe revealed that nitrogen is the most abundant gas in the atmosphere. The temperature of Titan is so low—a frigid 178 °C (289 °F) at its surface—that water is completely frozen out of the atmosphere. At that temperature, however, methane—also plentiful around Titan—can exist as either a liquid or a gas, and it may form clouds in the atmosphere, much as water does on Earth.

The Voyager probes also detected many organic molecules in Titan's atmosphere. These probably form when ultraviolet light from the sun breaks apart methane and nitrogen molecules, some of which then recombine to form more complex substances, including hydrocarbons (molecular chains of carbon and hydrogen) and nitriles (molecular chains of carbon, hydrogen, and nitrogen). This photochemical process creates a “smog” that condenses and settles onto the moon's surface. Scientists speculate that pools of liquid organic molecules, or clusters of solid organic molecules, may exist on Titan's surface. Life on Earth may have arisen from reactions in similar collections of organic molecules, and some of the same reactions could be occurring on Titan.

After leaving Saturn, Voyager 1's course took it out of the solar system and into deep space. Voyager 2 continued on to Uranus and Neptune, reaching those planets in 1986 and 1989. Both of these large, gaseous planets are less than a fourth the mass of Saturn, but they still contain thick atmospheres of hydrogen and helium. They are so far from the sun—19 and 30 times the Earth's distance, respectively—that they receive little heat from solar radiation. At Uranus, Voyager 2 discovered a hazy, sluggish atmosphere. At Neptune, it found an atmosphere containing clouds and storms whose patterns change continuously. They concluded that Neptune's atmosphere must be powered by a much stronger source of internal heat, a result confirmed by Voyager 2 measurements.

Voyager 2 also found the magnetic fields of both Uranus and Neptune to be unusual. Unlike the magnetic fields of Earth, Saturn, and Jupiter, which are roughly aligned with the planets' axis of rotation, the fields of Uranus and Neptune are tilted. In Neptune's case, the field is shifted well away from the axis of the planet. The origin of these odd shifts remains poorly understood.

Voyager 2 discovered that Neptune has at least eight moons; only two had been known of previously. Neptune's largest moon, Triton, contained its own surprises. Voyager found that a bright cap of nitrogen and methane ice at the moon's south pole prevents sunlight from heating the surface, keeping temperatures below 233 °C (388 °F), the coldest surface yet measured by a probe. But despite such low temperatures, there is activity on Triton. Voyager spotted two geysers that were shooting plumes of dark dust to an altitude of 8 kilometers (5 miles). Scientists speculate that the eruptions are powered by underground pockets of nitrogen that are warmed just enough by sunlight to expand and burst through the surface. The plumes were an unexpected final discovery before Voyager 2 followed its sister ship on a perpetual journey beyond the solar system.