Quirks & Quarks

Meet the odd little moons that interact with Saturn's spectacular rings

The last orbits of NASA's Cassini spacecraft got up close with 5 unusual moons within or just outside Saturn's main rings

They're embedded in or near the rings, and help shape them

NASA's Cassini spacecraft got up close to 5 moons inside Saturn's rings (NASA/JPL)
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Saturn is famous for its spectacular rings, which make it one of the most beautiful planets in our solar system. And with those rings and more than 60 moons it's also the most complex planetary system. But many of those moons are odd ducks that are embedded within or close to the rings, that until recently, had not been well studied. 

Between December of 2016 and April of 2017, the properties of those small moons were illuminated, thanks to data from the final orbits of NASA's Cassini spacecraft and it's looking like these moons and the rings are, according to researchers, "one and the same." 

Bonnie Buratti, a planetary astronomer at NASA's Jet Propulsion Laboratory in Pasadena, California, who led this part of the Cassini mission, told Bob McDonald in an interview with Quirks & Quarks, that these moons are slowly sweeping up Saturn's rings.  

The five ice moons 

The five moons observed by Cassini in order of proximity to Saturn are: Pan, Daphnis, Atlas, Pandora and Epimetheus. The moons — all made of ice — each have an interesting role to play within the rings.

Meet Saturn’s odd little moons that interact with its spectacular rings. The rings and the moons depicted in this illustration are not to scale. (NASA-JPL/Caltech)

Epimetheus actually shares an orbit with another moon, which is not one of the five. Those two moons, called "co-orbitals," were likely one body in the past before breaking apart. Mysteriously, they switch places every four years.

Pandora and Atlas skirt the main rings system of Saturn. In this position they help sculpt the rings.

Pan and Daphnis are found within the rings. Daphnis was actually discovered by Cassini, and Pan was found in images taken by the Voyager spacecraft. 

Cassini's final orbits and parting gift

Cassini observed these five moons in the final six orbits of its mission.

In its final stages, Cassini was supposed to be looking at the rings, the planet and the magnetic field of Saturn, but when scientists noticed that the spacecraft would be so close to these five moons, they decided to include them as their focus of Cassini's final flybys.

This montage of views from NASA's Cassini spacecraft shows three of the small, ring moons of Saturn and their tutu-like "skirts" inspected during close flybys: Atlas, Daphnis and Pan. (NASA/JPL-Caltech/Space Science Institute)

Images and data that were collected in this part of the Cassini mission were, as Buratti describes it, a parting gift from Cassini. 

"This was kind of the final thing that — those of us that are interested in moons got — we got our present with a great big bow on the top." 

Moons of mysterious shape and colour may sweep the rings clean

With the exception of Epimetheus, the other moons all have a mysterious "skirt" around their equator, which resembles a ballerina's tutu. The tutu is comprised of particles that have accumulated onto the moons from the rings. The moons are in effect sweeping their particular orbit clean of particles.

The moons, especially those that are closest to Saturn, take on a red hue — the same colour as the rings. The source of the reddish colour is not clear, although the researchers suspect it is organic material.

As the moons continue to collect material from the rings, that process as well as another process — the ring particles themselves are also decaying and falling into Saturn, Buratti believes says best estimates suggest the rings disappearing completely over the next 100 to 200 million years.  

The wavemaker moon, Daphnis, is featured in this view, taken as NASA's Cassini spacecraft made one of its ring-grazing passes over the outer edges of Saturn's rings in 2017. (NASA/JPL-Caltech/Space Science Institute)

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