Science | September 16, 2024

The Seven Most Interesting Discoveries We’ve Made by Exploring Saturn

Scientists continue to learn new things about the planet, its sweeping rings and its many moons

Saturn, the second-largest planet in our solar system, is best known for its layered rings. Credits: NASA / JPL-Caltech/Space Science Institute

Shi En Kim

Reporter

Saturn comes second to Jupiter in many respects—in size, age, prominence of stripes—but in terms of scientific intrigue, it most certainly can hold its own. Saturn stokes scientific interest not simply as a planet, but a system. Among its appealing qualities, Saturn has a lush ring and tantalizing moons that potentially harbor life. Space scientists across different specializations, from geologists to atmospheric scientists to those studying the possibility of life on other worlds, can all find their niche studying a corner of the Saturnian system.

To date, only four spacecraft have visited the Saturn system. From 1979 to 1981, three flybys provided tantalizing glimpses. And beginning in 2004, Cassini-Huygens was the first mission to stay a while. For 13 years, it squirreled away data, before plunging into the gas giant to taste its atmosphere for the first and last time. No active missions are exploring Saturn today, although a new one, Dragonfly, is slated to rendezvous at one of its moons in 2034.

Until scientists learn more then, here are seven of the most fascinating discoveries they’ve made about Saturn so far.

Enceladus is a prime location for searching for extraterrestrial life

The tiny moon, small enough to fit within the state of Arizona or Colorado, has generated enormous buzz among scientists. The frozen speck hides a massive secret: Underneath its icy shell is an ocean of liquid water.

Kevin Baines, a NASA planetary scientist, remembers seeing the Voyager probe’s images of Enceladus in the 1980s. The blinding ball of white earned the moon the title of the most reflective planetary body in the solar system. He recalls remarking, “It’s as bright as freshly driven snow.” Until Cassini took a closer look in 2005, he didn’t know that he wasn’t that far off. “It turns out, it really is driven snow,” he says.

The “snow” comes from liquid water escaping Enceladus and falling back onto the moon’s surface as icy particles. Despite the surface seeing a chilly minus 330 degrees Fahrenheit, Enceladus’ interior luxuriates at around 200 degrees Fahrenheit, thanks to tidal heating from Saturn. The mother planet’s immense gravity pulls and pushes at the mini moon incessantly until its insides warm. As a result, the interior is cozy enough to hold liquid water.

Scientists might not have guessed Enceladus hides a subterranean sea, were it not for geysers that signaled abundant liquid water lurked there. In 2005, Cassini photographed plumes erupting from Enceladus’ South Pole into space. The images came as a major shock, but that was just one of the surprises that scientists would uncover in the coming years. Since then, researchers have found that the ejecta also contain organic compounds and silica, indicating ongoing chemical reactions that are only possible where liquid water meets rock. The chemical processes resemble what’s happening in hydrothermal vents on Earth’s seafloor, where life may have first emerged on our planet. The chemical energy source and balmy conditions in Enceladus’ oceans make the far-flung moon a prime target in the search for extraterrestrial life in the solar system.

“The habitable zone is not just where the planet is relative to its star,” Baines says. Scientists also should scrutinize moons, he adds. “This is one example where you have to open up your mind.”

Titan’s topography looks like Earth’s, with one crucial difference

Like little Enceladus, the largest moon of Saturn, Titan, is also a wet world and a candidate for habitability. An ocean of liquid water hides in the moon’s interior, and on the outside, rivers, lakes and seas ripple with liquid methane instead of water.

The moon has a methane cycle that in a lot of ways reflects the water cycle on Earth, says Kelly Miller, a planetary scientist at the Southwest Research Institute. She and other scientists think that the same weather phenomena that occur on our planet occur on Titan, except they’re driven by methane.

Like Earth, Titan has retained a substantial atmosphere, the only moon in our solar system to do so. The atmosphere consists of mostly nitrogen, like Earth’s, but it also contains complex organic molecules that paint the world in an orange haze. These bulky hydrocarbons, whiffed by the Huygens probe that Cassini dropped on the moon in 2005, eventually settle out of the atmosphere and coat the ground, like a veneer of soot. The carbon compounds on Titan’s surface amount to hundreds of times the fossil fuel reserves on Earth.

“It’s the best carbon-producing factory in the whole solar system,” says Hunter Waite, a retired planetary scientist formerly at the Southwest Research Institute.

This smog thwarted observations of Titan’s surface during the Pioneer 11 and Voyager flybys. Only when Cassini probed the atmosphere with radar in 2004 did scientists discover an eerily Earth-like landscape underneath the dusty shroud.

Titan’s atmosphere is four times more dense than Earth’s. For engineers, that’s good news, because flight on Titan—at least by robots—is possible. The upcoming Dragonfly mission that aims to make its way to the gassy satellite in 2034 plans to employ a rotorcraft-lander design to get around.

A major source of scientific interest when it comes to Titan is whether life exists there. The subsurface liquid-water ocean may host the kind of life that resembles Earth’s. More tantalizingly, life as we don’t know it might also exist on the surface, one whose biology runs on liquid hydrocarbons. Dragonfly seeks to hunt for the chemical clues that signal the possibility of life.

Crazy storms are powered by an unlikely fuel

Storm on Saturn — A huge storm churns through Saturn’s northern hemisphere. NASA / JPL-Caltech / Space Science Institute

Every 30 years or so, a planetary-scale hurricane brews in Saturn’s atmosphere. The storms can last for months, but their effects can persist for centuries.

Research led by University of Michigan planetary scientist Cheng Li discovered a commonplace culprit behind Saturn’s tempests: water. The molecule, heavy among the vast body of hydrogen and helium that make up most of Saturn’s heft, plugs heat from dissipating from the core, until enough energy accumulates to loft the molecule from the depths as steam. The mechanism is like a pressure cooker, Li says, with weighty water acting as the release valve. Eventually, the collective energy released is sufficient to fuel a globe-circling hurricane.

In 2010, the Cassini probe was fortunate enough to witness the solar system’s largest and most intense storm in modern history. The storm grew to nearly one and a half times that of Earth’s diameter in length, and its tail wrapped a third of the way around the planet.

Such storms leave tracks. In 2015, researchers trained ground-based telescopes toward the planet and found patches of ammonia gas in the planet’s atmosphere. The anomalous ammonia footprint is evidence of previous storms, which stirred up this trace gas as they passed as far back as 150 years. In essence, these ammonium tracks act as a fossilized record. “Atmospheres seem ephemeral,” says Li. “When we talk about fossils, we don’t really think about atoms.” Saturn’s atmosphere proves otherwise.

A hexagon crowns the North Pole

Hexagon Cloud Formation — Saturn’s northern hemisphere is capped with a hexagon cloud formation. NASA / JPL-Caltech / Space Science Institute

A long-lived jet stream caps Saturn’s North Pole, with the cloud edge tracing the latitude of 77 degrees. Scientists think that the edge, in the shape of a hexagon, has marked Saturn’s northern tip for centuries. A central cyclone pirouettes right on the pole, while smaller eddies dot the rest of the hexagonal space, many swirling in opposite directions.

The six-sided vortex was first imaged by the Voyager flybys in the early 1980s—almost by accident, as they captured bits and pieces of the hexagon in each shot. But it was only when scientists revisited the photos in 1988 that the hexagon’s presence came to light. Since then, Cassini has taken higher-resolution images that unmistakably showcase the geometric storm in all its glory. They reveal a hurricane that spans 20,000 miles in girth, towers 180 miles in height and billows at speeds of 300 miles per hour.

The most curious feature is the storm’s stunning symmetry. Scientists are befuddled by the storm’s hexagonal shape. In the lab, researchers have played with spinning basins of water and successfully conjured up hexagon-shaped waves as a simulation of Saturn’s gigantic crown, hinting that similar mechanisms are at play on the planet, but these experiments can’t fully explain Saturn’s wind patterns. For example, they can’t explain why the South Pole is missing a similarly sculpted storm.

The magnetic pole axis coincides with the spin axis

Aurora on Saturn — An aurora forms on the South Pole of Saturn caused in part by the planet’s magnetosphere. The magnetic pole axis and spin axis on Saturn are aligned. NASA, ESA, J. Clarke (Boston University), and Z. Levay

Among all the planets in the solar system, Saturn is the only one whose magnetic pole axis aligns with the planet’s spin axis. On other planets with a magnetic field, including Earth, the magnetic pole axis and the spin axis are tilted away from each other, crossing like an X. On Saturn, what you have is instead a perfect I. This lack of an offset foils attempts to calculate the planet’s spin speeds using conventional methods. Researchers usually use the magnetic field to tell the time on gas giants, but they haven’t figured out how to do so with Saturn’s unique configuration. Whereas length of day on Jupiter has been pinned down with millisecond precision, estimates for Saturn’s twirl varies by 20 minutes. “Here’s a planet that is the second biggest in the solar system, and we do not know the length of a day,” Baines says.

Saturn’s rings give a glimpse inside the planet

Saturn is best-known for its eye-popping rings, which sweep 175,000 miles from the planet. Scientists think that Saturn’s rings are the shredded remains of a moon that wandered too close. Icy fragments as big as mountains to as small as sand grains loop around the planet, bound by its immense gravity. That tight gravitational leash makes the icy particles in the ring sensitive to rumblings below.

Waves in the rings can reveal clues about the planet’s atmosphere and the interior of the planet. During its lifetime, Cassini collected observations of spirals in the ring that revealed two major findings. Firstly, Saturn is made up of stable gassy layers and potentially has a diffuse rocky core. Secondly, the layers are possibly spinning at different speeds.

The rings act as an intermediary between Saturn and Enceladus

Saturn’s rings are a relatively recent feature, with an age of less than a few hundred million years, scientists say. (Saturn itself is estimated at over four billion years old.) And the rings are still evolving till this day as they amass and jettison particulates.

The main source of fresh ring fodder is Enceladus’ plumes. Icy material escapes from the moon into space until it’s recaptured in Saturn’s sweeping lasso. Scientists have advanced their understanding of the watery moon by studying Saturn’s ring particles. In its last few years of operation, Cassini found large and complex organic molecules, the precursors of amino acids and thus the building blocks of life, stuck on ice shards floating in Saturn’s expansive halo. Trapped in the ice were also rare salts such as phosphates, the first to be detected originating from an ocean outside of Earth. These unlikely compounds paint a picture of Enceladus’ vibrant underground ocean.

However, Saturn’s wreath is likely shrinking faster than it’s growing. In Cassini’s final plunge into Saturn, it measured the shower of ice particles that was falling from the rings onto the planet. What Cassini saw was a deluge: As much as 22,000 pounds of rain hit Saturn per second. This rain neutralizes the energetic ions in Saturn’s atmosphere and alters the temperature where it falls, especially in the mid-latitude regions. This “ring rain” is the suspected culprit behind the dark bands on Saturn that Voyager observed in 1980, areas cleared of haze and lower in infrared emissions where the ring irrigated.

Given the sheer quantity of material loss, scientists think that the ring may also have an expiration date. At the current rate, the ring may only last for a few more hundreds of millions of years before it’s depleted.

Shi En Kim | | READ MORE

Shi En Kim is a Washington, D.C.-based freelance science journalist. Her work has appeared in National Geographic, Scientific American, the Atlantic, Popular Science and others. In 2021, she interned at Smithsonian magazine as an AAAS Mass Media Fellow.