PASADENA, California—Saturn, when glimpsed from afar, seems like Earth’s polar opposite.
It’s huge, gaseous, and carts around an entourage of 62 moons. Saturn’s rings alone give it a distinguished aura, and its polar jet stream is hexagonal, as though stamped on the planet with geometric intent. In short, Saturn is a refined, complicated creature. It’s unlike anything we see on Earth.
And yet, NASA’s Cassini spacecraft—which ends its life tomorrow morning by plunging deep into Saturn’s dense atmosphere—has helped us see the planet as a proxy for some of the universal patterns in our solar system and beyond. It has also challenged some of our most ingrained assumptions.
“I love the idea of Cassini destroying most of our models and theories,” says Athena Coustenis, an international co-investigator for Cassini’s Composite Infrared Spectrometer (CIRS). “None of us had imagined what we would see on the surface.”
Now, as Cassini braces itself for Friday’s crash landing into Saturn, NASA scientists are reflecting on the lessons Cassini has imparted and what those lessons could mean for future research. It may not be hyperbole to say that Cassini is responsible for bringing us much closer to finding life beyond Earth—or at least, the first intimations of it—than ever before.
A Swiftly Tilting System
Saturn’s tilt sets the tone for the rest of its more conspicuous comparisons to Earth. With respect to its orbit around the Sun, Saturn is tilted 27˚, just slightly more than Earth’s 23.5˚. Titan, Saturn’s largest moon, has a very similar incline, giving it weather patterns that mimic Earth’s.
“Somewhere on Titan right now it is raining, and it’s the only other place in the solar system where I could say that,” says Alexander Hayes, assistant professor of astronomy at Cornell University and member of the Cassini team. “But instead of raining water, it’s raining methane. Those methane raindrops are collecting on the surface, they’re channelizing in rivers and gullies, and they’re eroding the surface just like they do on Earth. So you have this environment that has all the same processes acting on it as we have here on Earth, but doing it in an extreme environment with vastly different materials—which makes Titan a natural laboratory for studying the same processes that affect Earth.”
Earlier this year, scientists from several universities in collaboration with NASA’s Jet Propulsion Laboratory discovered that Earth spent about a million years in a heavy methane haze, which ushered the planet into a “great oxidation event” 2.4 billion years ago. This suggests that Titan, with its liquid seas of methane and ethane, could eventually go down the same geochemical path. “Perhaps this atmosphere is the Earth’s past that we’re looking at in real time,” Coustenis says.
Titan’s chemistry could also be the key to understanding how life emerges. This summer, Cassini data detected the presence of negatively charged molecules on Titan that, in lieu of amino acids, could bridge the divide between simple molecules and complex organic compounds. The discovery downgrades the theory that the germs of life on Earth came riding on the tails of a comet. Instead, maybe Earth already had all the right tools—and interstellar molecules have them, too.
“It just goes to show you that we have the same physical processes everywhere in the universe,” Coustenis says.
Slingshot Toward Science
Titan isn’t just a scientific goldmine in its own right—it’s the lever arm that diverted Cassini’s course, giving it the chance to see Saturn from many different angles. In some ways, it’s the birthplace of Cassini’s legacy.
“Cassini’s enabled us to create new questions and then go after them,” says NASA’s Planetary Science Division Director Jim Green. “How do you do that? You have to take a different view, a different orientation. We had little idea of the importance Titan would play in its our ability to give us that different look.”
For one, Titan provided Cassini with enough juice to zoom past Saturn’s rings from a multitude of perspectives.
“All of us are most excited by how unique the rings are,” says Jeff Cuzzi, a member of the Cassini team studying Saturn’s rings. “They’re almost pure water ice, which is very unusual in the outer solar system.” The 22 Grand Finale orbits that’ve been running in succession since April gave the NASA team rich results on Saturn’s ring composition, the mass of the rings, and the flux of meteoroids through the rings.
“The rings are changing before our eyes. We’re seeing things coming and going, structures changing,” Cuzzi says. “Eventually, we will be able to say something about how old the rings actually are.”
One new finding that’s revising what we know about Saturn is the interaction between the planet’s interior mass distribution and structures that we see in the rings themselves. Gravitational perturbations—usually from the pull of moons—generate shapes called spiral density waves. “It turns out that [spiral density waves in Saturn’s rings] are pretty well explained by structures sloshing around inside the planet,” Cuzzi says. This gives scientists another way to explore the planet’s internal organs. It also reinforces the idea that what we see in Saturn echoes other places throughout the universe, as spiral density waves show up in galaxies, too.
Both the rings and Saturn’s atmosphere, which inherits material from the rings as it succumbs to the planet’s gravitational pull, bear signatures of our solar system’s infancy. “We’re on the hunt to determine what the original material [of the solar nebula] is,” Green says. “And Saturn’s atmosphere, just like Jupiter’s atmosphere, must have retained an enormous amount of it.”
Although Cassini’s life will soon be over, its spirit will live on in the data—and the data will help scientists answer these and other questions for years to come.
“The mission doesn’t end because the spacecraft will stop transmitting,” Coustenis says. “The mission will only end when we’re finished analyzing all of the data. We’re going to be working on this forever.”
Images courtesy NASA