And so Cassini has met its end. One of the most successful space missions ever launched, it revealed Saturn and its moons in glorious detail. Images beamed home from the probe showed raging hurricanes that enveloped the planet, and millions of rings that surround it. The spacecraft dropped a lander on Titan, the largest of Saturn’s 62 known moons, marking the first touchdown on a heavenly body on the other side of the asteroid belt. But it was observations of the tiny, icy moon Enceladus that stunned astronomers most, and transformed their views on the potential for life elsewhere in the solar system.
From the moment Cassini arrived at Saturn in 2004, its cameras and other instruments radically altered scientists’ understanding of the most distant planet visible to the naked eye. Before the mission astronomers thought Saturn, the second largest planet in the solar system and the sixth from the sun, was circled by thousands of rings. Cassini showed it was millions. And rather than static bands of dust, the rings have a complex life of their own. Cassini saw ring matter clumping into snowballs, and being cleared by tiny moonlets, leaving propeller-shaped gaps in the rings thousands of kilometers long. In places the rings are incredibly thin, only hundreds of meters from to top to bottom, and when nearby moons pass by, they send giant waves rippling through them.
Cassini carried a European lander named Huygens. Its destination was Titan, a world so mysterious that engineers designed the lander to touch down on both liquid and solid surfaces. The moment came in January 2005 and, at first, Huygens appeared to have settled on a crème brûlée landscape, a brittle layer coating softer ground. The conclusion turned out to be premature: scientists later worked out Huygens had set down on a pebble before finally coming to rest.
John Zarnecki, president of the Royal Astronomical Society, worked on the instrument Huygens used to measure Titan’s resistance to the landing. “It was the first part of Huygens to make contact and for all of 15 milliseconds we were the only thing touching Titan,” he said. “It was the most memorable day of my professional life. Fifteen years of blood, sweat and tears and it all came down to those minutes. It was incredibly emotional.” For Zarnecki, one consolation is that while Cassini has burned up, the defunct Huygens could remain on Titan for generations to come.
Titan turned out to be an extraordinary world. Bright, feathery methane clouds float in the hazy orange sky and pour methane rain onto a surface carved by rivers, lakes and seas, producing a landscape resembling the early Earth’s. In the seas, scientists noticed what they called “magic islands” that come and go with time. The features are not well understood, but may be massive upwellings of nitrogen bubbles. Meanwhile, in the upper atmosphere, Cassini found molecules known as “carbon chain anions”, precursors of complex organic molecules that ultimately form the building blocks of life.
Cassini explored 18 moons beyond Titan, but none was more revelatory than Enceladus, an icy ball only 500km wide. In 2005, Cassini spotted plumes of salty water containing simple organics blasting into space from the moon’s southern pole. The spacecraft later found convincing evidence for a subsurface ocean that was feeding the geysers, most likely with help from hydrothermal vents on the ocean floor. Scientists consider such vents on Earth as promising places for life to have started.
“We thought Enceladus was a run-of-the-mill icy satellite and yet we found it is active and nobody expected that. It is game-changing for how we look at the solar system,” Zarnecki said. “Many of these icy moons, we think, have large reservoirs of water beneath the surface. It means most of the icy satellites of Saturn and Jupiter could in theory be harbors for life.” NASA is now considering a mission that would fly back to Enceladus, to look for alien microbes in the moon’s hidden ocean. But even if the mission is selected, it would be more than a decade before launch.
Late in 2010, Cassini watched a monstrous thunder and lightning storm form on Saturn. Months of observations showed the hurricane growing until it eventually wrapped around the planet. Geography ensures the same cannot happen on Earth, but without land to intervene, Saturn’s storm stretched over 300,000km. It lasted for most of the year and only petered out when its head caught up with its tail and disrupted the gigantic weather system.
Saturn is so distant that a year on the planet lasts 30 years on Earth. But for all scientists have learned about the ringed world, they are still unsure how fast it spins, and so the length of its day. With no landmasses to observe, clocking the spin of gas giant is far from straightforward. Jupiter, another gas giant and the largest planet in the solar system, emits radio waves that reveal its day to last a mere 10 Earth hours. Similar radio waves come from Saturn, but they have not been so helpful. The waves’ patterns, which are controlled by the planet’s spin, turned out to be different at the northern and southern hemispheres, and even changed with the seasons. For now, the length of the day is one secret Saturn has not given up.
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