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Backlit view of the Saturn system

On 19 July 2013, NASA's Cassini spacecraft slipped into Saturn's shadow and turned to image the planet, seven of its moons, its inner rings—and, in the background, our home planet, Earth. This rare backlit view of the Saturn system allows scientists to see details in the rings and Saturn’s atmosphere.

NASA

Remembering a ringed world

Before coming to Science I was an editor at National Geographic, where I wrote a story on the first phase of Cassini’s historic encounter with Saturn. That journey ended last September, when the two-story-tall interplanetary probe dove into the yellowish haze of the giant planet’s upper atmosphere. Cassini was perhaps the most successful planetary mission ever conducted. Exploring Saturn, its gaudy rings and retinue of over 60 moons for 13 years, Cassini was like a voyage back in time, offering humanity clues to how the solar system took shape and gave rise to life. On the anniversary of Cassini’s finish, as Science publishes papers from the final phase of the mission, here are a few recollections of this remarkable exploration.

This computer image shows Cassini-Huygens during the Saturn Orbit Insertion maneuver, just after the main engine has begun firingDavid Seal NASA/JPL/Caltech

This computer image shows Cassini-Huygens during the Saturn Orbit Insertion maneuver, just after the main engine has begun firing. After 90 minutes, Cassini is slowed enough to be captured by Saturn’s gravity.

On 1 July 2004, I was at Cassini Mission Control at the Jet Propulsion Laboratory in Pasadena, California, when Cassini arrived at Saturn, ending a 7-year, 2.2-billion-mile journey. Traveling at 68,000 miles an hour, the spacecraft had to shed speed so that Saturn’s gravity could capture it. Firing engines, it dropped to within 13,000 miles of the planet’s butterscotch clouds after a daring passage through a wide gap in the outer rings. At that speed, hitting a single pebble could have ended the $3.4 billion mission.

Titan and Titan's atmosphereNASA

Larger than Mercury, smaller than Mars, Titan is the only moon with a thick atmosphere, a hazy methane veil above a world that resembles a distant cousin of Earth.

The most intriguing of Saturn’s moons is the largest: Titan. In December 2004, 6 months after arriving at Saturn, Cassini launched the crown jewel of the mission, a saucer-shaped probe named Huygens that had ridden piggyback from Earth. Three weeks later, Huygens plunged into Titan’s smoggy atmosphere. Pummeled by air friction, the probe’s heat shield reached a temperature of thousands of degrees. Gradually, Huygens slowed and cooled. Parachutes opened, the heat shield dropped away, and Huygens drifted like a leaf on the winds of Titan, cameras and microphones recording the weather on this distant world. The first picture from the ground appeared—a garish orange landscape, strewn with rocks.

A surprising landscape of liquid-eroded hillsNASA

Pictures made during Huygens’ descent revealed a surprising landscape of liquid-eroded hills, suggesting weather systems like those of Earth’s, with chilly methane replacing water.

Other Huygens pictures from the descent showed hills carved by methane rain. Later observations from Cassini revealed seas and lakes of the chilly compound. A stunning picture emerged of a complex world, the only one in the solar system other than our own with stable liquid on the surface. Titan turned out to be a kind of frozen time capsule, with some conditions like those found on the very early Earth.

Saturn’s gossamer rings divide into hundreds of delicate bands, with gaps separating the major rings.NASA

Saturn’s gossamer rings divide into hundreds of delicate bands, with gaps separating the major rings.

Saturn’s most prominent feature is the dazzling ring system. From edge to edge, the main rings span some 200,000 miles, nearly the distance from Earth to the moon. Yet the thickness of these bands of icy rubble averages only 150 feet. How the rings formed remains a mystery, although some scientists speculate that Saturn’s gravity tore apart an icy moon or a comet, strewing debris that provided the raw material. In Saturn’s rings today, tiny moons play the role of the planets. Each moon’s gravitational tug is minute. Yet the moons’ gravity helps maintain the rings by keeping the particles from straying from their orbits. A moon can also carve a gap between rings, and its gravity can send waves of density rippling through a nearby ring, like traffic speeding and slowing on a crowded freeway.

EnceladusNASA

The greatest surprise came from one of the smallest moons. Icy Enceladus is just 300 miles across. The southern hemisphere is curiously smooth, suggesting some activity is paving over impact craters like those in the north.

Bright as a beacon, ice-covered Enceladus reflects more light than any other body in the solar system. In 1980, Voyager images showed only a few large craters marring the moon’s surface, leading scientists to suspect that geologic processes were somehow erasing the scars. As the distant sun silhouetted Enceladus, Cassini made images of the tiny moon that showed geyser-like eruptions of water vapor and ice particles shooting far into space. Could this modest moon harbor life? Life as we know it requires liquid water, energy, and organic molecules. Life might be hiding just a few dozen feet below the ice in pockets of warm water, living off dissolved organic compounds and reproducing using some alien version of DNA—or an entirely different kind of genetic material. Future missions may sample the plumes of Enceladus or drift across the skies of Titan, seeking evidence of life on Saturn’s surprising worlds.

Jets of frozen water, backlit by the distant sun, hurtle into space from vents near the south pole of Enceladus.NASA

Jets of frozen water, backlit by the distant sun, hurtle into space from vents near the south pole of Enceladus. Researchers remain uncertain about how the tiny moon maintains liquid water beneath the ice.

 

NASA/JPL-Caltech/Space Science Institute

As Cassini’s epic mission concluded, the cameras took 80 images of the planet and ring system. Forty-two were montaged, creating this final somber mosaic on the cover of Science. “It is our goodbye to Saturn picture,” said Project Scientist Linda Spilker. Two days later, on 15 September 2017, Cassini’s engines fired one last time, slowing the spacecraft for a dramatic plunge into the Saturn’s atmosphere.

Bill Douthitt is the Photography Managing Editor at Science.