Cassini Turns 15

The Jet Propulsion Lab's Cassini spacecraft roared toward Saturn in a spectacular nighttime launch on October 15, 1997. After two gravity assists from Venus and one each from Earth and Jupiter, the school-bus-sized orbiter arrived at Saturn on June 30, 2004 Pasadena time, putting itself on a wildly looping path designed to give its Earthbound handlers repeated close-up looks at Saturn's rings, all of Saturn's big moons, a good few of the small ones—and, of course, Saturn itself. Saturn and its retinue have not disappointed; new wonders have been revealed with every orbit.

Cassini carried a second spacecraft along for the ride—a probe named Huygens, built by the European Space Agency and designed to land on Titan, Saturn's largest moon. Titan could have been a planet itself, if it hadn't grown up in the wrong neighborhood. Titan is bigger than Mercury (but smaller than Mars) and has an atmosphere that's four times denser at the surface than Earth's is at sea level. The surface, however, had never been seen  before—or at least not clearly. There's so much methane in Titan's atmosphere that the entire moon is wrapped in an impenetrable, orange pall of photochemical smog.

Cassini dropped off its hitchhiker six months after arriving, and on January 14, 2005, Huygens took a two-hour, 28-minute parachute ride through Titan's atmosphere, smelling, tasting, and feeling its winds all the way down. At an altitude of about 40 kilometers, the hydrocarbon haze became transparent, revealing a vista much like the mountain ranges and dry lakes of the Mojave Desert. But because of Titan's average surface temperature of 95 kelvins (nearly –290° F) the gullies on these mountainsides had been carved by methane rain; the mountains themselves proved to be made of water ice frozen as hard as any rock on Earth.

Hyugens made a soft landing on material with the consistency of wet sand and was still broadcasting 72 minutes later when Cassini dropped below Titan's horizon and radio contact was lost. The pictures Huygens beamed back showed that it had come to rest on a sepia-tinted plain strewn with cantaloupe-sized cobblestones of ice that had clearly been rounded by a flowing liquid. It didn't take long to identify this fluid as methane, since Huygens's mass spectrometer detected a puff of the stuff vaporized from the greasy mudflat by the lander's body heat.

Cassini's radar mapper has since confirmed what earlier observations had suggested—that lakes of liquid methane and ethane exist, primarily in Titan's northern hemisphere. One, near the north pole, is the size of Lake Superior. Titan has taken this low-temperature parody of Earth, where methane is water and ice is rock, even further. Cassini's radar has spotted areas where there may be "cryovolcanoes" that ooze slush instead of lava.

Titan, big as it is, is far too small to have retained the warmth that volcanoes—even of ice—demand. However, Cassini found that it flexes like a stress ball in the grip of Saturn's gravity, which induces a 10-meter-tall "solid tide" in Titan's crust. This periodic deformation not only generates heat, but implies the presence of liquid water under the ice. A Titan made of solid rock could only bulge by about a meter, but a liquid would be more susceptible to Saturn's pull. "We have long suspected that Titan, like Jupiter's moons Europa, Ganymede, and Callisto, has a subsurface ocean," says David Stevenson, Caltech's Goldberger Professor of Planetary Science. "These results show that this is very likely the case."

Titan isn't the only satellite of Saturn to join the subsurface saltwater society. The biggest surprise of the Cassini mission has come from Enceladus, a small, undistinguished moon that turns out to be spewing curtains of icy mist from the vicinity of its south pole. These geysers issue from fissures called tiger stripes—dark parallel gashes on Enceladus's otherwise bright surface. These intriguing stripes were discovered in May 2005, and the trajectory of Cassini's next flyby in July was altered to take a closer look. Cassini spotted the geysers on its way in, shortly before whizzing through them at an altitude of about 170 kilometers. A thermal map of the surface below linked the plumes to hot spots with temperatures of some 145 K—on a moon whose average surface temperature does not exceed 70 K. (See "A Nice Place to Visit?" in E&S No. 1, 2006.)

Life is found wherever liquid water exists on Earth. Could this hold for the outer solar system as well? Perhaps, says Professor of Planetary Science Andrew Ingersoll. "The idea of a 'habitable zone' has existed for a hundred years. All the water is frozen out on Mars, and it's all vaporized on Venus. Earth is just right—we have oceans, and we have life. But now, the habitable zone might also include an archipelago of these isolated moons."

Enceladus probably gets its heat by flexing, like Titan, Ingersoll says. "But this would dissipate orbital energy, and eventually Enceladus wouldn't be where it is any more. And neither would the other nearby moons, because they all influence each other." This could mean that the geysering is a relatively recent phenomenon, says Ingersoll. "Or is it cyclic, and we just got lucky enough to see it?"

Getting lucky may be putting it mildly. "You have to get the energy from somewhere," Stevenson observes. The proximate source, he says, could be a moon named Dione, with which Enceladus is in an orbital resonance. Enceladus makes two trips around Saturn for every one that Dione makes, and each time Enceladus catches up, Dione gives it a gravitational tug. Like a child being pushed ever higher on a swing, these gravity assists could nudge Enceladus into an increasingly eccentric orbit, Stevenson says. Eventually enough orbital energy might accumulate to trigger the geysers, which would then erupt for a while as Enceladus's orbit became more circular again. "This cycle might well take a million years or more," Stevenson says. "But Io, Jupiter's volcanic moon, is also overly active. And now you start to get scared, because this means you're looking at a very special time for both of them." If the odds of this coincidence are 1 in 10, it's not so bad; but if they work out to 1 in 100, it's time to find another theory.

Saturn's rings have also held surprises. Cassini arrived just after the northern hemisphere's winter solstice, when the rings were at their maximum tilt relative to the sun. This allowed the spacecraft to measure the sunlight passing through the rings, revealing them to be incredibly thin—a mere 10 to 20 meters thick, on average. During Saturn's equinox in August 2009, Cassini got a never-before-seen view of the rings aimed edge-on to the sun. As the light swept across them, giant shadows were cast by anything sticking up from the surface. Suddenly, towering waves of ice as much as four kilometers tall were thrown into sharp relief. They proved to be disturbances set off by the passage of the nearby moon Daphnis, whose orbit is slightly tilted relative to the ring plane. (Many more discoveries are described in "Cassini's Ringside Seat" in the Spring 2010 issue of E&S.)

And what of Saturn itself? Unlike Jupiter's psychedelic swirls of candy-cane colors bedecked with semipermanent storms such as the Great Red Spot, the bands in Saturn's atmosphere are muted shades of butterscotch. Not that there's nothing going on down there—Cassini has witnessed 10 thunderstorms in its eight-plus years over Saturn, all in the summer skies of the southern hemisphere. But on December 5, 2010, Cassini caught a storm 500 times the size of the others—in the northern hemisphere. The spacecraft heard the storm before it saw it, says Ingersoll. "So everything is very quiet, and then this giant thunderstorm goes off. We were getting copious noise on the radio receiver, and then we see this." What showed up as a tiny white spot in a picture shot that day grew to the diameter of Earth in three weeks, spawning a trail of white clouds that wrapped all the way around the planet. Such storms have been seen once every 30-odd years since 1876. They usually appear late in the northern hemisphere's summer, but this one arrived 10 years early at the beginning of spring.

"Saturn is normally very bland," Ingersoll says, "which makes this giant storm just weird. It's just crying out to be explained." Like thunderstorms on Earth, these storms are driven by convection, as air masses of different temperatures redistribute their heat. But rather than supporting a bunch of small storms all the time, Saturn stores up these heat differences until it unleashes a doozy. In some ways, Ingersoll says, Saturn behaves less like a terrestrial weather system and more like a volcano, in which the pressure builds up for many years before an eruption. Volcanic pressure is contained by layers of rock, but what keeps the lid on Saturn's atmospheric energy?

"To some extent, that same question applies to both Titan's and Saturn's meteorology," Ingersoll says. "There is more methane in Titan's atmosphere waiting to rain out than there is water vapor in the atmosphere on Earth. There's more latent heat stored, too, but only 1 percent as much sunlight to evaporate it. So there are two ways this could work: tremendously violent but infrequent rainstorms, because there isn't much energy to resupply them; or a steady, continuous drizzle. And the choice seems to be the same on both Saturn and Titan. We've seen the storms on Saturn, and we see erosion on Titan. A drizzle won't carve those channels. So that seems to be how weather works. You turn down the energy, and you still get storms. Who knew? That's why we study this stuff."

There's still time to figure it out. Cassini's mission is now planned to run through the northern hemisphere's summer solstice, in May 2017, allowing us to observe one complete change of Saturn's seasons, or a little less than half of a Saturnian year.