A Family Portrait

The Voyagers took a part of the Solar System that had been studied briefly and in little detail, and fleshed it out into a family portrait of four giant planets, their ring systems and magnetic fields, plus forty-eight of their moons. The additions and revisions have been enough to cause textbooks on astronomy to be rewritten.13 Let’s see what was learned about major members of the family.

First up was Jupiter. This mighty gas giant is three times more massive than any other planet and 320 times more massive than the Earth (figure 4.2). The Voyagers reached Jupiter in 1979, with a separation of four months. Even after centuries of telescopic ob­servations, there were surprises. The Great Red Spot was revealed to be a huge anticyclone, large enough to swallow the Earth, with eddies and smaller storms around its periphery. It had changed color from orange to dark brown in the six years since the Pio­neer flybys. As they passed behind the planet the Voyagers saw lightning illuminating the darkness of the night-side atmosphere.14 Voyager 1 discovered a very faint ring system around Jupiter made of dust ejected from the inner moons after high-velocity impacts.15 The rings are far less dramatic than Saturn’s but are equally inter­esting scientifically. The main ring circles from 122,000 to 129,000 kilometers away from the center of the planet.

Voyager 1 discovered two substantial new moons of Jupiter: Thebe and Metis. Both are irregular in shape; Thebe is 70 miles in its largest dimension and Metis is only 35 miles long.16 Voyager 2 got into the act by discovering Adrastea, which is no bigger than a small town, 15 miles across. However, the real excitement came from Io, Jupiter’s closest moon and the fourth largest moon in the Solar System. This strange-looking rock, with its mottled yellow-

brown surface, looking like a moldy orange, is the most geolog­ically active world in the Solar System. The Voyagers saw nine erupting volcanoes between them, marking the first time active volcanism had been seen anywhere other than the Earth. Plumes shoot out of the volcanoes at up to 2,000 mph and rise 300 miles above Io’s surface.17 There are more than 400 volcanoes dotting the moon’s surface, not all of which are active.

Why is Io so lively? Normally, moons are geologically dead be­cause there’s not enough radioactive heating from their interior rocks to drive tectonic activity. But Io is in a gravitational “tug of war” with nearby Jupiter and the other three Galilean moons: Ganymede, Callisto, and Europa. This incessant pulling and push­ing heats up the interior of the moon, like a racquetball heats up when it’s flexed repeatedly. Io is distorted by the stretching force of Jupiter’s gravity, and this departure from a sphere is called a tidal bulge. It bulges by up to 300 feet, which is enormous compared to the roughly one-foot stretching of the Earth’s solid mass by the Moon. Sulfur and oxygen atoms ejected by the volcanoes create a torus of plasma around Jupiter, and these ionized atoms have been detected millions of miles away, right to the edge of Jupiter’s mag­netosphere. Lava flows episodically paint the surface red, orange, and yellow, and fizz off enough material to coat the entire surface with an inch of sulfur every year.

Galileo’s other “children of Jupiter” turned out to have distinct personalities as well (plate 5). Ganymede is the largest moon in the Solar System, even larger than Mercury and more than twice as big as Pluto, which has seemed poor and misbegotten since astrono­mers demoted it to the status of a dwarf planet in 2006. Gany­mede has terrain that’s partially cratered and partially grooved, which is thought to indicate tectonic processes. Rocks mixed with ice comprise the top layer. More recent observations point to a liquid ocean under the icy crust.18 The Voyagers saw huge impact craters on Callisto, with a couple indicating impacts almost large enough to blast the moon into fragments. However, the craters were strangely smooth, with almost no topographic relief, indicat­ing that water ice at one point had flowed over them and filled them in. Europa attracted keen attention from the mission sci­entists when the first images came back. Low-resolution pictures from Voyager 1 showed linear features crisscrossing the surface. Higher resolution pictures from Voyager 2 increased the puzzle be­cause the features were so flat they couldn’t have been created by the familiar terrestrial process of slabs of crust sliding and collid­ing with each other. The only plausible explanation was that they were ice floes. Other observations pointed to a liquid ocean tens of kilometers deep under the few kilometers of ice. Europa vaulted into its current position as one of the most compelling targets for a future lander.19

The Voyager flybys of Saturn took place in 1980 and 1981. They got four times closer to Saturn’s cloud-tops than they did to

Jupiter’s upper atmosphere.20 Saturn’s magnificent rings were the source of many puzzles and a few surprises. Viewed up close, the rings had incredibly detailed structure. Not only were there con­centric rings and gaps ranging from large and fuzzy to razor-sharp, but the cameras revealed radial spokes, kinks, and delicate braids. Time-lapse photography proved that some of these features came and went. The rings were made of mixed icy and rocky particles ranging from microscopic to house-sized, shepherded by Saturn’s many small and irregularly shaped moons.

The rings of Saturn and the other gas giants are the result of an amazingly subtle gravitational dance. With no choreographer other than Newton’s Law of Gravity, a disk of rocky and dusty material can naturally develop very complex structure. A reso­nance occurs for any pair of orbits where the periods are related by two small integers. In that case the two objects have a boosted interaction that can cause them to rearrange their position, cause one to be ejected from the system, or cause either one to clear out small particles in a ring at a particular radius. Some resonances are stable, such as the orbits of Jupiter’s moons Ganymede, Europa, and Io, which have orbital periods related by the ratio 1:2:4. Think of a child on a swing; you can sustain or increase their motion not only by pushing them once per cycle of their motion, but also every second cycle or every third cycle, and so on. Although the principles of orbital resonance are understood, not all the subtle features in Saturn’s rings have yet been explained. Voyager also earmarked Saturn’s large moon Titan as a place to return to. Titan has a thick atmosphere of nitrogen and was inferred to have bod­ies of liquid ethane and methane on its surface. As we’ll see in the next chapter, Cassini did give Titan the attention it deserved twenty years later.

Very little was known about dark and shadowy family members Uranus and Neptune before Voyager. The highlights of Voyager 2’s solo flybys of the two outermost planets included the discovery that the magnetic fields are tilted far from their rotation axes. The new data trebled the number of moons of Uranus from 5 to 15 (27 are now known, most named after characters in the plays of Wil­liam Shakespeare and a few named after characters in Alexander Pope’s poem “The Rape of the Lock”), and it doubled the num­ber of moons of Neptune from three to seven (thirteen are now known, named after Greek and Roman water gods). Miranda, the innermost of Uranus’s five large moons, is a bizarre object. Even though it’s only 300 miles in diameter, it has huge canyons and terraces ten miles high and mixed young and old surfaces. Voyager scientists thought it might have been the pieces of a smashed moon that came back together, but now it’s thought that Miranda’s to­pography arose from tidal heating at a time when it was in a much more eccentric orbit than it is now. Neptune rounds out the fam­ily portrait. This frigid and gloomy planet has howling winds of 1,200 mph and a Great Dark Spot similar in size to Jupiter’s Great Red Spot.21 Its large moon Triton might have been kept molten for a billion years after its capture by Neptune; geysers on its surface spew soot and nitrogen gas into its sparse atmosphere. Triton is the coldest place in the Solar System: -391 °F, a temperature at which the air we breathe would freeze solid.