July 22, 2018

Saturn’s clouds run deep, rings may rain organics


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Artist’s illustration of Saturn’s internal structure. Credit: NASA/JPL-Caltech

Saturn’s clouds have roots deeper inside the planet’s atmosphere than scientists previously thought, and Saturn’s rings — now believed to have formed in the last 200 million years — appear to be raining organic molecules down on the planet, according to observations made by NASA’s Cassini spacecraft last year in the final weeks of its mission.

The discoveries from Cassini’s grand finale, when the long-lived plutonium-powered space probe passed through a gap between Saturn and its rings, continue to keep scientists on their toes.

“The weather and what you’re seeing on Saturn is not just in the very thin atmosphere that you’re seeing, it’s deeper,” said Linda Spilker, the Cassini project scientist at NASA’s Jet Propulsion Laboratory.

During Cassini’s 22 passages inside Saturn’s rings, the spacecraft measured the gas giant’s gravity directly, allowing scientists to differentiate effects from the rings and the planet itself. Running low on fuel, Cassini plunged into Saturn’s atmosphere Sept. 15, 2017, as intended.

The measurements gave Cassini scientists a better idea of Saturn’s internal structure, showing how mass is distributed inside the planet. The data also help scientists improve calculations of the mass of Saturn’s rings, a figure that yields an estimate of their age.

Weather systems on Saturn are not as visually spectacular as those on Jupiter, but Spilker said scientists now see evidence that Saturn’s clouds and jet streams extend much deeper into the planet than they expected.

“The initial thought was that thickness was maybe only a few hundreds of kilometers, or something like that, and it’s turning out to be thousands of kilometers instead,” Spilker said last month at the 49th Lunar and Planetary Science Conference near Houston.

NASA’s Juno spacecraft, currently exploring the internal structure of Jupiter, has found that planet’s jet streams also extend well beneath the cloud tops, perhaps to a depth of 1,900 miles (3,000 kilometers).

The Juno team made that announcement in early March.

“Galileo viewed the stripes on Jupiter more than 400 years ago,” said Yohai Kaspi, Juno co-investigator from the Weizmann Institute of Science in Rehovot, Israel, and lead author of a Nature paper on Jupiter’s deep weather layer. “Until now, we only had a superficial understanding of them and have been able to relate these stripes to cloud features along Jupiter’s jets. Now, following the Juno gravity measurements, we know how deep the jets extend and what their structure is beneath the visible clouds. It’s like going from a 2-D picture to a 3-D version in high definition.”

In a stroke of fortune for planetary scientists, Cassini made similar measurements of Saturn’s deep interior at the same time as Juno was probing Jupiter.

“At Jupiter, they saw the atmospheric depth of 3,000 kilometers,” Spilker said. “That was pretty amazing, and now Saturn is much deeper. It will be interesting to see when they start comparing Jupiter and Saturn.”

Artist’s illustration of the Cassini spacecraft during one of its final orbits between Saturn and its rings. Credit: NASA/JPL-Caltech

Gravity data from Cassini’s final 22 orbits also point to a relatively recent formation of Saturn’s rings — some time in the last 200 million years, about the time dinosaurs began to flourish on Earth, and a fraction of the roughly 4.5 billion-year age of Saturn itself. The prevailing theory is that a comet, a moon, or some other cosmic interloper ventured too close Saturn. Saturn’s gravity ripped the object apart, and the leftover ice and dust formed the planet’s famous rings.

Scientists derived Saturn’s ring age from the rings’ mass.

“Prior to the grand finale orbits, that mass was uncertain by about 100 percent, which is a lot,” Spilker said.

There is still some uncertainty in the ring mass estimate after Cassini, but the error bars have narrowed, and the estimate centers on a number slightly less massive than earlier predictions.

Spilker said the results have been submitted for publication in Science magazine.

“This points to very young rings, rings that are probably on the order of 100 million years old or so, because of this very low mass for the rings,” Spilker said. “So this was really an astonishing result, a new result that we could get with Cassini.”

Jeff Cuzzi, an expert on Saturn’s rings at NASA’s Ames Research Center, said it is time for scientists to rethink their theories on how the rings formed.

The biggest objects in the solar system had settled into stable orbits around the sun by the time the rings appeared at Saturn, Cuzzi said, making the probability of a large chunk of rock or ice venturing close to the planet 200 million years ago “statistically unlikely.”

He said a moon the size of Titan, which is 50 percent larger than Earth’s moon, could have drifted too close to Saturn and been ripped apart. But that scenario was also much more likely to happen when the solar system was more chaotic billions of years ago.

“The only young scenario that has any plausibility … is whereby the Saturn moon system might have ben evolving very stably over almost the whole age of the solar system until a resonance was hit about 100 million years ago,” Cuzzi said.

The moons’ orbits would have become unstable in such a scenario, Cuzzi said, causing them to collide with one another and shed icy debris.

Scientists still have to resolve some lingering questions in such a scenario, such as how the debris could have migrated to the rings’ current positions, Cuzzi said. Research has shown that a recent resonance between moons could have only occurred at Saturn, and that may be why fresh, bright rings are seen there but not around other planets in the solar system.

“All of the giant planets have these little wimpy rings of dark primordial material,” Cuzzi said. “Only Saturn has these massive icy rings.”

“They’re not going to go away, they’re just going to keep getting darker,” Cuzzi said. “We’re just lucky to see them now.”

Cassini also made the first direct measurement of material raining down on Saturn from the planet’s innermost ring.

Several of Cassini’s instruments detected microscopic particles, most of which were smaller than a thousandth of a millimeter in size, as the probe dove between the visible rings and Saturn’s cloud tops. Previous studies suggested the rings may deposit material into Saturn’s atmosphere.

File photo of a backlit Saturn and its rings taken by the Cassini spacecraft in 2006. Credit: NASA/JPL-Caltech

The particles — or “nano-grains” as some scientists call them — were too small to pose a hazard to Cassini as the spacecraft flew through the ring gap at more than 60,000 mph.

The material rains down on Saturn’s atmosphere near the planet’s equator. Scientists have identified much of the material as water ice — no surprise because water makes up more than 90 percent of the rings.

But initial results show there are heavier particles, including organic molecules like methane, embedded in the material raining down from the D ring. And the ratio of water ice in the “ring rain” is lower than the percentage of water in the rings themselves, suggesting the water has been lost.

That discovery was unexpected.

Researchers are now on the hunt for the source of the carbon-bearing organic molecules. They could be brought in from external sources, such as Saturn’s moons or comets, scientists said.

Saturn’s rings have a muted reddish hue when analysts exaggerate their color in imagery.

“Are they red because of good, old-fashioned rust like Mars, or are they red because of the same kinds of organic materials … that make carrots, tomatoes and watermelon red?” Cuzzi said.

“To me, this answers the question of what makes the rings red. It’s organics.”

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Follow Stephen Clark on Twitter: @StephenClark1.


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