The Cassini mission to Saturn ended a year and a half ago, but scientific results are still coming from all of the data it collected. When Cassini moved in closer to Saturn in its final months, it took a very detailed look at the gas giant’s rings, travelling between them and the planet itself. That detailed inspection raised quite a few questions about all the interactions shaping those rings.
Tiny moonlets abound inside Saturn’s ring system, and those bodies interact with the icy particles that make up the bulk of the rings. This results in a variety of shapes and textures in the ring structure, much like how planets are formed in the protoplanetary disk around a star. That means the rings are like a laboratory that scientist can peer into to learn about the formation of planets.
During the Cassini spacecraft’s final look at the rings, it discovered textures and patterns that ranged from clumpy to straw-like. This variety raises questions about how, exactly, they were shaped, and what the history of the interactions is that shaped them. Adding to the questions is how color, temperature, and chemistry changes across the ring structure.
But like a lot of science, this study both begs new questions, and sheds some light on existing questions.
The F ring is the outermost definite ring in Saturn’s ring structure, and probably the most active. It contains features that can vary in timescales as small as a few hours. A 2008 study showed that there are several moonlets at work in the F ring, and their collisions and gravitational effects are responsible for some of the clumps and longitudinal channels in that ring.
In this new paper, researchers zeroed in on a series of impact-generated streaks in the F ring that have the same length and orientation. They conclude that a flock of impactors that struck the rings at the same time are responsible. This is important because it rules out cometary debris as a cause, and highlights the role that streams of material orbiting Saturn play in shaping the structure of the rings.
“These new details of how the moons are sculpting the rings in various ways provide a window into solar system formation, where you also have disks evolving under the influence of masses embedded within them,” said lead author and Cassini scientist Matt Tiscareno of the SETI Institute in Mountain View, California.
The excellent close-up views of the rings that Cassini captured during its Ring Grazing Orbits between December 2016 and April 2017 showed three distinct types of structures: clumpy, smooth and streaky. They also clearly show that the three types exist in belts with sharp boundaries between each other. The reason for this is unclear, since scientists can’t identify any ring characteristics connected to the belts.
“This tells us the way the rings look is not just a function of how much material there is,” Tiscareno said. “There has to be something different about the characteristics of the particles, perhaps affecting what happens when two ring particles collide and bounce off each other. And we don’t yet know what it is.”
Cassini had even more mysterious findings, which were uncovered by its Visible and Infrared Mapping Spectrometer (VIMS). As the name says, it imaged the rings in both visible and Infrared. VIMS found unusually weak water-ice bands in the outermost part of the A ring. That is an unexpected finding, since that area is known to be highly reflective, and high reflectivity usually means less-contaminated ice and thus stronger water ice bands.
VIMS had more surprises in store. While scientists already knew that water ice is the main component in Saturn’s rings, they were pretty sure other ices like ammonia ice and methane ice were present. But VIMS couldn’t find either of those two ices, nor could it find organic compounds, which should be there given the organic material that Cassini previously found flowing from the D ring into Saturn’s atmosphere.
“If organics were there in large amounts — at least in the main A, B and C rings — we’d see them,” said Phil Nicholson, Cassini VIMS scientist of Cornell University in Ithaca, New York. “I’m not convinced yet that they are a major component of the main rings.”
These are interesting new findings for Saturn scientists, and are puzzles that require new models to explain. But none of them can top Cassini’s most surprising discovery: that Saturn’s rings are much younger than previously thought.
According to NASA Ames Research Center’s Jeff Cuzzi, we’re entering a new phase of studying Saturn that will see new models developed to explain these new findings.
“We see so much more, and closer up, and we’re getting new and more interesting puzzles,” Cuzzi said. “We are just settling into the next phase, which is building new, detailed models of ring evolution — including the new revelation from Cassini data that the rings are much younger than Saturn.”
Science is both exciting and frustrating because each new advance in understanding brings its own questions. This is certainly true of the new research based on the data from Cassini’s closest, best look at Saturn and its rings.
“It’s like turning the power up one more notch on what we could see in the rings. Everyone just got a clearer view of what’s going on,” said Linda Spilker, Cassini Project Scientist at NASA’s Jet Propulsion Laboratory. “Getting that extra resolution answered many questions, but so many tantalizing ones remain.”
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