As Cassini’s tour of Saturn draws to a close, a look back at postcards from the probe
Take a bow, Cassini. It’s been a marathon performance: 20 years in space, more than 200 orbits around Saturn, and hundreds of thousands of images of the giant planet, its splashy rings and its many moons. On September 15, the veteran spacecraft will use its last burst of fuel to plunge into the sixth planet from the sun. Scientists and space enthusiasts around the world will watch it go with awe and nostalgia.
“It’s hard not to anthropomorphize the spacecraft,” says Matthew Tiscareno of the SETI Institute in Mountain View, Calif., who has been working on Cassini since it entered Saturn’s orbit in 2004. “We’ve been riding on its back for these 13 years, and it’s done everything we’ve asked. I think it’s the most spectacularly successful mission that NASA has ever run.”
Cassini was designed to train its 12 scientific instruments on the Saturn system for a short four years, but NASA extended the mission twice. Even with the extra time, Cassini’s 13-year run is less than half of a year on Saturn, where a year lasts 29 Earth years.
After all this time, we’ve witnessed only the transitions to Saturnian spring and summer, the equivalent of January to June on Earth. And yet we’ve seen so much.
Cassini has revealed massive churning storms that rage for decades, rings that may be the best laboratory for studying how planets form and details of some of Saturn’s more than 60 moons. Two of those satellites, Titan and Enceladus, surprised Cassini scientists by having many of the right ingredients for life (SN: 9/2/17, p. 12). The craft has revamped our picture of Saturn and its celestial family.
Saturn’s potentially habitable moons are the reason Cassini must meet a dramatic end. The Cassini mission team decided it was safer to crash the craft into Saturn itself than to risk the craft wandering off and brushing up against Enceladus or Titan, spreading its earthly germs to any nascent ecosystems there.
But the craft will be busy until the very end. Since April, Cassini has been making weekly dives into the possibly rubble-strewn region between Saturn and its rings, a zone the team hadn’t dared explore before. Plus, the craft will collect data during its last hurtle into the gas giant’s atmosphere. Those final measurements should help solve some of the most basic mysteries about the planet, including when it got its iconic rings.
“Cassini data,” says team member Ralph Lorenz of the Applied Physics Lab, “is going to keep us busy for decades.”
In the eye of Saturn’s hexagon swirl, cloud speeds can reach 150 meters per second. The storm, shown here in false color from 2012, has probably been there for decades, if not centuries. Saturn has no mountains or oceans to interrupt the storm.
Saturn’s tiny moon Daphnis orbits within the 42-kilometer-wide Keeler Gap in Saturn’s outer A ring. This sequence of 10 images, each taken about one minute and 30 seconds apart, shows Daphnis’ gravitational pull perturbing the particles at the gap’s edge. The moon is only 8 kilometers across, but its gravitational pull is enough to raise ripples in the rings around it. These waves were first noticed in 2009, around the time of Saturn’s spring equinox. Daphnis has a ridge around its equator, which is probably made of fine particles it has gathered from the rings.
Saturn’s rings are “arguably the flattest structure known to man,” says astronomer Matthew Tiscareno. Over a span of hundreds of thousands of kilometers, their vertical thickness typically varies by only about 10 meters. But Cassini snapped these structures, as tall as 2.5 kilometers, in 2009, when sunlight struck the rings at a perfect angle to cast long shadows.
Cassini caught a family portrait of five of Saturn’s moons in this image from July 2011. From left to right, they are Janus (179 km across), Pandora (81 km across, nestled in the rings), Enceladus (504 km across, appearing half lit above the rings in the center of the image), Mimas (396 km across), and Rhea (1528 km across, to the far right).
Cassini mission scientist Ralph Lorenz has this false color image of Ligeia Mare, a large sea on Saturn’s moon Titan, hanging in his office. Cassini’s radar peered through the moon’s thick orange haze to reveal an Earthlike surface with seas, rivers and clouds filled with liquid ethane and methane. The moon could possess the ingredients for life. “Titan has been doing prebiotic chemistry experiments for us for a huge amount of time,” says team member Elizabeth Turtle. She and Lorenz are working with others on a proposed mission called Dragonfly that would land drones on the moon to sample its surface.
Titan is the only place in the solar system other than Earth known to host long-lived liquid lakes and streams. But on Titan, the liquid is mostly methane and ethane, not water. The video below combines radar images of Titan from 2004 to 2013 as Cassini flies over its two largest seas, Kraken Mare and Legia Mare. Where the lakes look dark, the liquid is exceptionally still and flat as a mirror.
By guiding tiny particles around themselves, small moons embedded in Saturn’s rings create the propeller-like features seen here. Scientists have followed these objects for over a decade, naming the larger ones after pioneers of aviation. These images, taken February 21, 2017, show two views of Santos-Dumont, named for a Brazilian-French aviator. “This is the only time in the history of astronomy that we’ve tracked the orbit of an object that is orbiting in a disk,” says astronomer Matthew Tiscareno. Studying the propellers can help reveal how planets forming in the disk of gas and dust around a young star grow.
One of the biggest surprises of the Cassini mission was that the icy moon Enceladus is spewing its guts into Saturn’s rings. These jets from the moon’s south pole come from a subsurface ocean, which may have the right chemistry for life. The jets also supply icy material to one of Saturn’s rings.
This false color image from 2005 shows the reach of the spectacular plumes on the moon Enceladus. Later sampling by Cassini revealed that the plumes contain ammonia, a variety of organic compounds and molecular hydrogen — all signs that the moon might be habitable. NASA is considering a mission to go back and sample the plumes.
Southern lights
Cassini spotted Saturn’s shimmering aurora dancing near its south pole in July 2017. The bright spots shooting across this video from the bottom left are due to charged particles hitting the detector. Behind the spots, you can see the aurora’s ghostly glow. These light shows are created when charged particles from the sun strike the planet’s atmosphere and make gas glow.
Saturn’s outer ring, called the F ring, is sculpted by tiny moons passing by the ring. The ring’s dust and ice particles are tugged by the moons’ gravity. These images from Cassini, taken between 2006 and 2008, show various disturbances in the outer ring.
Mother Earth
This iconic Cassini image is known as “The Day the Earth Smiled.” On July 19, 2013, Cassini turned back toward its planet of origin and shot a picture with Saturn’s rings and Earth and its moon all in the same frame (SN: 8/24/13, p. 8). It was the third time Earth was imaged from the outer solar system but the first time humankind got a heads-up, so people could look up and smile or wave for the camera.