Saturn is the most distant planet we can see without a telescope. To observers on Earth, it seems to hover serenely, a jewel in the night sky some 1.4 billion kilometers (890 million miles) away. Up close, however, the view is anything but tranquil. Since 2004 the spacecraft Cassini has orbited Saturn, revealing a dynamic world of wind and lightning, rippling rings and a menagerie of moons.
The moon Titan, Saturn’s largest, holds surprises of its own. Cassini released the Huygens probe, which touched down gently on Titan’s surface in 2005—the first spacecraft ever to land on a moon other than our own. Unique among moons in our solar system, Titan has a dense atmosphere, weather systems, lakes of ethane and methane, and a landscape eerily like Earth’s. But Titan’s surface is frigid: -180˚ C (-290˚ F).
Cassini continues to record new vistas from the Saturn system. Thanks to the work of thousands of people at NASA, the European Space Agency, and the Italian Space Agency, we, too, have the chance to see this extraordinary planet, its rings and its moons at close range.
Saturn is a gas giant planet: it consists mainly of a thick atmosphere of hydrogen, helium and other gases. No spacecraft could land there. A space probe descending through the clouds would be crushed by intense pressure before it reached layers of liquid hydrogen and helium, then metallic hydrogen and, finally, Saturn’s dense core of ice and rock. In our solar system, Saturn is the second largest planet after Jupiter, another gas giant.
Observations of Saturn’s stormy lower atmosphere rank among Cassini’s most important accomplishments. But, in the deep freeze of the outer solar system, where does Saturn get the heat to drive this weather? Since the time of its formation, the planet has been slowly collapsing from its own gravity, a process that generates as much heat as Saturn receives from the distant Sun. The result: bizarre cyclones, high-speed winds and bolts of lightning.
The Cassini Spacecraft
Cassini was launched from Cape Canaveral in Florida on October 15, 1997. Rather than flying directly to Saturn, it first looped twice around the Sun, taking advantage of two gravity-assists from Venus and one from Earth to catapult it to the outer solar system, where it got another assist from Jupiter. After a nearly seven-year trip, Cassini reached Saturn in 2004.
This scale model is one-tenth the size of the actual spacecraft, which is 6.7 meters (22 feet) high and 4 meters (13 feet) wide—about the size of a truck. You don’t see solar panels on Cassini because at Saturn the Sun is 100 times fainter than on Earth—too weak to provide energy. Instead, the spacecraft’s instruments are powered by a radioisotope thermal generator.
Saturn’s dazzling rings are made mainly of ice, in chunks that range in size from marbles to school buses. Small amounts of rock and other material mixed in give them the pastel hues photographed by Cassini. This blend of particles orbits Saturn in a disk that is as spectacularly thin as it is wide. Edge-to-edge, Saturn’s rings stretch some 270,000 kilometers (165,000 miles)—two-thirds the distance from Earth to the moon. Yet in most places they are only about ten meters (about 30 feet) thick.
As Saturn’s rings whirl in formation around the planet, the particles gently jostle and bump each other, forming clumps in some places and thinning out in others. Moons drive ripples and waves across the rings with their gravity. Ever changing, the dynamics of this disk provide scientists with a model for understanding other spinning systems, like galaxies, the disks of material falling into black holes and quasars and even our early solar system.
Moons of Ice
Some 4.5 billion years ago, the planet Saturn formed in a disk of gas and dust orbiting the young Sun. At the same time, Saturn’s major moons took shape in a similar, smaller disk around the planet. These moons are all made primarily of the same material—water in the form of solid ice, mixed with some rock. Most also have circular orbits in Saturn’s equatorial plane and in the same direction as Saturn rotates. Together Saturn’s moons resemble a miniature planetary system, and scientists study them to learn how such systems form.
Images from Cassini reveal that—despite their similarities—Saturn’s moons are very different from one another. Cliffs, craters, mountains, valleys and rare patches of dark debris mark their icy faces. Of the five moons featured here, one is unlike the others. Phoebe, the most far-flung moon, orbits in the opposite direction. Scientists think Phoebe is a remnant of planet formation in the early solar system, snared in the outer reaches of Saturn’s gravity.
Titan and Enceladus
Most of Saturn’s moons are unimaginably cold and, over their billions of years in orbit, impacts have left them covered with craters. Balmy and crater-free by comparison, the two active moons Titan and Enceladus have surfaces shaped by internal heat. Titan also has weather systems that rain liquid methane onto the moon and erode its surface. On Enceladus, spectacular geysers of water vapor shoot from vents at its south pole.
Even more startling, both of these moons have the ingredients for life: a brew of organic molecules, water and sources of energy. In fact, scientists think that conditions on Titan, with its dense, nitrogen-rich atmosphere, resemble the environment of primordial Earth, although Titan is much, much colder. Titan’s surface looks eerily Earth-like as well—photographs taken as the Huygens probe parachuted to the moon’s surface revealed a landscape of rivers and lakes.
Photography in Space
Humans perceive visible light as the spectrum of colors in a rainbow. Light is electromagnetic radiation, or waves, and there are many wavelengths that humans cannot see—radio, infrared, ultraviolet and x-rays, for example. How different surfaces reflect different wavelengths of light tells scientists about the texture and composition of those surfaces—including the landscapes of planets and moons.
Cassini’s cameras often capture many images of the same subject, each in a different wavelength. In fact, one instrument—the visible and infrared mapping spectrometer, or VIMS—can record data simultaneously at 352 wavelengths. In order to see and interpret the data as images, scientists often assign every wavelength a “false” color. These colors might be shades of gray, or bright reds, greens and blues. Scientists also use high-contrast colors and image sharpening to highlight fine details.
The exhibition Spectacular Saturn was produced with assistance from NASA/JPL, SSI, Cornell University, the American Museum of Natural History, and the Eastman Kodak Company.