How Cassini Changed Our View of ... Saturn
Overview: Before Cassini, scientists viewed Saturn’s unique features only from Earth and from a few spacecraft flybys. But over more than a decade orbiting the gas giant, Cassini studied the composition and temperature of Saturn’s upper atmosphere as the seasons changed there. Cassini also provided up-close observations of Saturn's exotic storms and jet streams, and Saturn’s radio and plasma waves, which can’t be detected from Earth.
Saturn may be known for its rings, but the planet itself offers irresistible and inexhaustible subjects for study. With more than 700 times the volume of Earth, Saturn is a behemoth. But its core is thought to be only a bit larger than Earth, leaving the vast bulk of Saturn’s volume filled by a churning cauldron of gas and compressed metallic hydrogen, mixed with helium and other trace molecules, so deep, dense and hot that the crushing depths of Earth’s deepest ocean trenches seem a comparatively tame environment.
Saturn’s global and inhospitable abyss might never completely reveal itself to humans, but during Cassini’s lengthy stay, Saturn shared some of its strange secrets and magnificent spectacles.
A Royal Crown for a Regal Planet
As different as Saturn is from Earth, the two planets have some similarities, one of which is the presence of jet streams. Saturn’s most famous and most visible jet stream, however, is far different from those of Earth. It's called the hexagon.
The hexagonal jet stream around Saturn’s north pole was first observed by the Voyager mission in the early 1980s. Cassini began studying it during Saturn’s northern winter, when the hexagon was in shadow. At the time, Cassini observed the great, six-sided jet stream in the infrared, sensing the heat coming from within the planet. Over time, as Saturn’s north entered spring, Cassini imaged the hexagon in visible wavelengths of light, revealing a stunningly symmetric shape 20,000 miles (30,000 kilometers) across with winds at around 200 miles per hour (322 kilometers per hour).
“No one really knows what drives it,” said Scott Edgington, deputy project scientist for the Cassini mission at NASA’s Jet Propulsion Laboratory. Though decades had passed since Voyager first spotted it, the hexagon was still there, racing around Saturn’s north pole. “With Cassini, we saw that it was a long-lived phenomenon,” Edgington said.
On Earth, mountain ranges and other features of the planet’s surface cause jet streams to bend and kink, but Saturn has no such obstacles jutting into its atmosphere, so the jet stream just keeps flowing, orderly and hexagonal. But questions still remain, Edgington said. “Why isn’t it five-sided? Why isn’t there another one in the south? What drives it?”
That's No Hurricane
Saturn’s north and south poles are also each beautifully (and violently) decorated by a colossal swirling storm. “Folks were surprised that they were so organized,” Edgington said. “You can see the eye walls.”
Cassini got an up-close look at the north polar storm and scientists found that the storm’s eye was about 50 times wider than an Earth hurricane’s eye. Unlike the Earth hurricanes that are driven by warm ocean waters, Saturn’s polar vortexes aren’t exactly hurricanes, Edgington said. “They’re hurricane-like, though.”
Lightning is lightning, however, no matter what planet it’s on. Cassini’s instruments ‘heard’ lightning ever since entering Saturn orbit in 2004, in the form of radio waves. But it wasn’t until 2009 that Cassini’s cameras captured images of Saturnian lightning for the first time, and Cassini scientists even assembled a short video of it, the first video of lightning discharging on a planet other than Earth.
Observing Saturn's lightning from space was more difficult than doing the same task at Earth. This is partly because Cassini observed from hundreds of times farther from Saturn than most Earth-observing spacecraft are from our planet. Lightning is also easier to see in the dark, and Saturn’s night side isn’t very dark. Its rings reflect far more light onto Saturn’s night side than a full moon does at Earth. But Cassini eventually caught lightning, even on Saturn’s day side -- something Cassini scientists didn’t think they’d be able to see. And the storm that produced the lightning was a doozy.
Every 28 to 30 Earth years, Saturn hosts a megastorm. Birthed in December 2010, the storm was as vast as any ever observed at the ringed planet. Raging for close to two-thirds of an Earth year, it eventually wrapped all the way around Saturn and spanned 190,000 miles (300,000 kilometers). In 2011, Cassini observed flashes in the storm that scientists determined were produced by lightning in the range of the most powerful lightning on Earth. It was the first time lightning was seen on Saturn’s day side.
Saturn’s so-called 30-year storms appear seasonally, like Earth’s hurricanes and typhoons but on a longer timescale. Saturn is nearly 10 times farther from the sun than Earth and takes far longer to orbit the sun – about 29 Earth years. Saturn’s axis of rotation is tilted like Earth’s, and so it too has seasons, but the longer year produces seasons that last more than seven Earth years each. Likewise, what we call Saturn’s 30-year storm is, for Saturn, an annual occurance. “The remarkable thing about this storm is that it arrived 10 years earlier, placing Cassini at the right moment in time to observe this fascinating weather feature,”Edgington said.
“Another interesting thing is that all these storms have been seen from either at the equator or in the northern hemisphere,” Edgington said. “But maybe that’s just in our lifetime.” Many generations from now, maybe the unseen mechanism that produces the storms will flip, and these giant tempests will form in the south. “Who knows,” Edgington said. “We haven’t yet dropped any probes into Saturn.”
Cassini scientists found that Saturn’s seasonal storm, also known as the Great White Spot, kicks up water vapor and other materials up from as deep as 100 miles (160 kilometers) below the cloud tops. The vapor freezes on its way up. Hence the whiteness of the storm. It takes roughly another 30 years to generate the conditions for the next great storm.
Cassini also studied Saturn’s auroras, atmospheric composition, the swelling of its outer atmosphere (the thermosphere) and the planet’s rate of rotation (a Saturn day is about 10.5 Earth hours). By the time Cassini dove into Saturn at the end of its mission, it had observed the planet for less than half of a Saturn year. But it had also orbited the gas giant 294 times, forever changing our understanding of the Saturn system and yielding insight for understanding the entire solar system.