The Sacrifice of Cassini | Cosmos: Possible Worlds

[Ethereal music] Why do some worlds have rings and others don't? Why no rings for Earth or Mars? We wouldn't recognize Saturn without them. He looks naked without his rings. But how did he get them in the first place?

This is exactly what the French astronomer Édouard Roche asked himself when he looked at Saturn through his telescope in 1848. Roche speculated that Saturn's rings were the debris of a moon or moons that had ventured too close and were pulled apart by the massive planet.

[Orchestral music] Roche was able to devise an equation that applies to all worlds. It tells you how closely a body can come to a planet before it's pulled apart by the planet's tidal forces of gravity and is turned into a ring. That's the Roche limit.

But until NASA's Cassini spacecraft executed a series of daredevil maneuvers in the Saturn system, there was a vigorous scientific debate about when his rings formed. Some astronomers suggested they were nearly as old as the planet itself. More than 4 billion years ago, when the planet coalesced out of the disk of gas and dust that surrounded the newborn sun, a moon or moons likely violated Saturn's Roche limit.

Others thought the rings to be fairly recent, perhaps only 100 million years old or so, and the Cassini spacecraft proved them right. What is Earth's own Roche limit? If the moon were ever to come closer than 12,000 miles, which, by the way, it's absolutely in no danger of doing.

[Orchestral music] And it's a good thing too because I like our moon right where it is. There's only one other moon in the solar system that moves me like ours does. Maybe it's because it's the only one with a thick atmosphere like Earth's and the kind of surface features, lakes and mountains, that remind me of home.

All of this was hidden from view by a dense layer of orange smog until the European Space Agency collaborated with NASA to send a spacecraft to land on his mysterious surface. Yes, that would be the one named after you, Christiaan Huygens, first to see that world through your telescope.

[Dramatic music] After an interplanetary voyage of seven years, the Cassini-Huygens spacecraft arrived in the Saturn system, the fourth of our ships to venture there but the first to send a probe to explore the surface of Saturn's moon, Titan, and to reveal a moon of far greater complexity and splendor than our own rather dull and lifeless moon.

[Soothing music] As Carl Sagan had predicted more than two decades before, there were seas of methane and ethane, and there was water ice. When Cassini first arrived in 2004 at Saturn's Northern Hemisphere, it was in the depths of winter, and the sun didn't come out until five years later when Saturn's northern spring began.

Is it just me, or is this whirling hexagon at Saturn's north pole every bit as exotic as the fantasies our ancient ancestors had of these worlds? The geometrically regular hexagonal shape of this feature brings to mind the handiwork of intelligence, terraforming, reworking the surface for some unknown purpose.

But it's actually the result of the sudden change in wind speeds as vast upwellings of ammonia rise near the poles. It's the mother of all hurricanes, a frenzy of thunder and lightning, containing countless hurricanes within it.

Spring can be a violent, stormy season on Earth too. But it was during Saturn's seven-year-long summer that Cassini was commanded to take her own life.

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