Expecting the unexpected on Saturn’s moons
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In deep space exploration, as in life, planning gets you only so far. Preparation is essential, but you have to be ready to improvise when you encounter surprises — such as a world constantly erupting with ice volcanoes, or a system of rivers and lakes made from liquid methane, or giant dunes made from plastic.
The dunes and methane lakes are on Titan and the ice volcanoes are on Enceladus — both moons of the giant ringed planet Saturn, and part of the frontier that Nasa is exploring with its epic Cassini mission. Cassini’s scientists have had to think on their feet many times over the nearly 20-year endeavor, which will end this fall when the spacecraft runs out of fuel.
“We had to change plans … to make observations we didn’t know we wanted to make ’til we saw things we didn’t expect to see,” said Jeffrey Moore, planetary scientist at Nasa’s Ames Research Centre in California. And at 900 million miles from home, you can’t exactly go back to get a different camera.
In the early 1980s, Nasa’s twin Voyager spacecraft had flown by and photographed Saturn and its largest moons, but these were brief encounters and with mid-20th-century technology. They left only hints at what a sustained mission like Cassini might observe, said Moore.
So Cassini was built with some flexibility. It bristles with instruments, he said, “like a flying Swiss army knife.” You may start out thinking the blade or screws or scissors are meant for one thing, but once you’re out hiking, you realise they might be used for something else.
On Cassini, he said, instruments designed to study particles trapped in the magnetic field of Saturn were re-purposed to measure the composition of the material erupting from fields of active volcanoes on Enceladus.
That turned out to reveal something important. Enceladus receives meagre sunlight, but it gets internal energy from friction generated from the motion of massive tides. So while the crust of Enceladus is icy, this internal heat warms a sea of liquid water underneath. Cassini’s measurements showed that the water erupting from the ice volcanoes is mixed with materials from the moon’s rocky interior — organic matter and minerals. Thanks to Cassini, Enceladus now joins Jupiter’s moon Europa as a possible abode for extraterrestrial life.
Flybys of Titan, Saturn’s biggest moon, have revealed something even more surprising. It’s the only other body in the solar system with earth-like surface features — lakes, seas, rivers, rain, wind and sand dunes.
“Titan is an explorer’s utopia,” says Alex Hayes, a planetary scientist at Cornell University. People can see Titan from ground-based telescopes, he said, and in the early 20th century astronomers saw features they attributed to a thick atmosphere. Pioneer 2 and the Voyager flew by Titan and took pictures, which revealed nothing more than a sphere of orange haze.
But what was underneath? This was tantalizing, said Hayes, because remote sensing techniques showed the atmosphere contained a lot of methane, and at the temperature and pressure expected on Titan, methane should act a lot like water does on Earth — existing in liquid, solid and gas forms. “There could be this complete analogy to the way water works on Earth,” he said.
Back in 2005, in the early part of the mission, Cassini dropped a probe called Huygens into a suicide plunge toward Titan’s surface. The scientists took bets on whether it would crash land or splash down into a methane ocean. It crashed, but on the way down, the camera captures a 2,400-foot-high ice cliff.
After that, Cassini visited Titan for 126 short flybys. A device called synthetic aperture radar revealed the features of the surface — mostly land, but dotted with methane lakes and rivers shaped by methane rain. During the mission, Hayes said, scientists learned how to use system to explore the lakes — using them as a ruler to measure the roughness of the surface, and as depth sounders.
Oddly, they found the lakes were glassy, where they expected them to be ruffled by the same winds that carved 400-foot-high dunes on the moon’s surface. Later, however, the radar observations showed features on the water, which the scientists dubbed “magic islands” because they kept appearing and disappearing. Hayes said they aren’t islands but features on the water, possibly shaped by winds, which appear to come and go with the Titanian seasons.
The spacecraft is in the news this year because it’s almost out of fuel and will die by September. But how Nasa would use the craft’s dying weeks wasn’t decided until last summer. Moore, of Nasa, said the researchers were influenced by a more recent spacecraft, Juno, which arrived last summer at Jupiter and is making unprecedented close flybys to learn about the solar system’s largest planet.
And so Cassini is now death spiraling into Saturn. Over the summer it should get some unprecedented close-ups of those rings — beautiful in a way no computer-generated drawing can equal. On 15 September, it will plunge into Saturn’s stormy atmosphere.
It’s too bad the adventure has to end so soon, but the craft finished not just its primary mission but two extended ones, and it gets to die in an interesting way. It’s all a good reminder that exploration is as much in humanity’s blood as squabbling and tribal hostility. Goodbye Cassini. You did us proud.