On its final orbits, Cassini will thread the perilous gap between Saturn’s rings and its cloudtops.


In mission's last hurrah, Cassini aims to solve riddle of Saturn's rings

Are Saturn’s rings ancient or young? It’s time to place your bets, for NASA’s Cassini spacecraft, which has spent the past 13 years exploring the planet and its moons, is finally swooping close enough to take a look.

On 26 April, it will thread the gap between the gas giant and its rings in the first of 22 week-long orbits that will culminate in September with a fiery and fatal plunge into the atmosphere. The Grand Finale, as NASA is calling the forays, should help answer questions about Saturn’s atmosphere and interior. But most of all, it should end a debate about the mass of the water ice rings, which is a big clue to their age—whether they are 4.5 billion years old, nearly as old as the planet, or much younger. The Cassini team has a betting pool on the mass: $5 a wager, due before the first orbit.

The route will be dangerous. The gap contains ice particles finer than soot, which ought not to harm the spacecraft. But if the models are wrong, and Cassini collides at 122,000 kilometers per hour with something even as large as a BB, the mission will end early. “This is something we wouldn’t want to try at another time,” says Earl Maize, the Cassini program manager at NASA’s Jet Propulsion Lab (JPL) in Pasadena, California. To be cautious, on its first pass, Cassini will hide behind its large antenna, using it as a shield to protect its instruments.

As the spacecraft plunges toward the rings in subsequent orbits, a radio telescope in Argentina will be listening for tiny Doppler shifts in Cassini’s signal. Those will allow the team to tease out the gravitational pull of the rings from that of Saturn—a distinction that could not be made when Cassini was orbiting outside the rings, says Luciano Iess, a planetary scientist at the Sapienza University of Rome. The measurement could provide a verdict on the rings’ mass—and their age.

Traditionally, many scientists have assumed that the rings are nearly as old as Saturn itself—massive structures formed of icy planetary building blocks. Early on, Cassini showed the B ring, one of four main rings, to be bright and nearly opaque, bolstering the view that the ice particles are dense and that the rings have a total mass greater than Saturn’s 400-kilometer-wide moon, Mimas.

But an alternate view is gaining credence: that the rings are more delicate structures formed within the last 100 million years from the smashed ice of a lost moon. An age of 4.5 billion years is hard to reconcile with their pristine color, because a steady stream of sooty micrometeoroids from interplanetary space should have darkened them by now, like drops of black ink spreading in a pool of white paint. Moreover, the spacing between rippling, spiral waves within the rings, created by gravitational resonances with Saturn’s moons, can offer another clue to mass, and judging from a handful of wavelets recently spied within the bright B ring, it is low. Iess believes the total mass of the rings to be just 20% to 80% of Mimas.

Cassini may reveal another clue when its close passage gives it the best view yet of the interior C and D rings. Amid the C ring’s icy particles, there are hints of larger chunks of rubble, says Jeff Cuzzi, a ring specialist at NASA’s Ames Research Center in Mountain View, California, who believes the rings are young. “This could even be the core of the object that formed the rings.”

By catching stray ring particles, the spacecraft could also learn more about the makeup of the rings themselves, which are at least 90% water ice. Their distinctive shade of taupe could come from reddish iron oxide or carotene, the orange of carrots.

As the spacecraft’s orbits take it ever closer to Saturn itself, it will shift its attention from the rings to the planet’s roiling ammonia clouds. Its cameras should see the structure of storms, and shadows could reveal the architecture of sharp lines of clouds that may extend for several thousand kilometers. Close-up measurements of the planet’s magnetic field could finally reveal the length of a day on Saturn. With no solid surface to define a day, scientists hope to use periodic patterns in Saturn’s magnetic field to get at the rotation of its interior.

Finally, in the last five orbits, “we’re going to dip our toe into the atmosphere of Saturn,” says Linda Spilker, the Cassini project scientist at JPL. It will be the first time humanity has sampled the atmosphere of a gas giant since the Galileo spacecraft dropped a probe into Jupiter in 1995. If they get lucky, researchers could estimate the abundances of helium and hydrogen in Saturn’s outer layers, which, in turn, could help illuminate its interior structure and how gas giants form.

On its final dive into Saturn, on 15 September, Cassini will point its antenna to Earth, but the spacecraft should go silent quickly, vaporizing within 3 minutes. Planners deemed it necessary to destroy Cassini rather than leave it wandering near Saturn because it may carry stowaway terrestrial microbes that could contaminate two Saturn moons, Titan and Enceladus, that have potentially habitable conditions.

There’s much to do before then. Cuzzi, though he favors youthful, low-mass rings, hasn’t gotten his bet into the pool yet. He’s waiting until the last minute. “I know time is getting short.”