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The Cassini spacecraft observes three of Saturn s moons set against the darkened night side of the planet. Image Credit: NASA/JPL/Space Science Institute The Cassini spacecraft observes three of Saturn's moons set against the darkened night side of the planet. Image Credit: NASA/JPL/Space Science Institute

The antiquities of the Saturn system

Sen— Data from NASA's Cassini spacecraft suggests that the moons and rings of Saturn are antiquities from around the time of our Solar System's birth.

Though tinted on the surface from recent “pollution," these bodies date back to around the time that planetary bodies in our neighbourhood began to form out of the protoplanetary nebula, more than 4 billion years ago.

"Studying the Saturnian system helps us understand the chemical and physical evolution of our entire solar system," said Gianrico Filacchione, a Cassini participating scientist at Italy's National Institute for Astrophysics, Rome, who led the research. "We know now that understanding this evolution requires not just studying a single moon or ring, but piecing together the relationships intertwining these bodies."

Data from Cassini's visual and infrared mapping spectrometer (VIMS) revealed how water ice and colours, signs of non-water and organic materials, are distributed throughout the Saturnian system. The spectrometer’s data in the visible part of the light spectrum shows that colouring on the rings and moons generally is only skin-deep.

VIMS also detected abundant water ice, too much to have been deposited by comets or other recent means. So the authors deduce that the water ices must have formed around the time of the birth of the Solar System, because Saturn orbits the Sun beyond the so-called "snow line," where the environment is conducive to preserving water ice, like a deep freezer.

The coloured patina on the ring particles and moons roughly corresponds to their location in the Saturn system. For Saturn's inner ring particles and moons, water-ice spray from the geyser moon Enceladus has a whitewashing effect.

The scientists found that the moons orbiting Saturn at a greater distance were redder. Phoebe, one of Saturn's outer moons which is thought to have begun life in the far-off Kuiper Belt, seems to be shedding reddish dust that eventually rouges the surface of nearby moons, such as Hyperion and the 'death star' Iapetus. Scientists think the reddish colour could be oxidized iron (rust), or polycyclic aromatic hydrocarbons, which could be progenitors of more complex organic molecules.

Two global images of Iapetus show the brightness dichotomy on the surface. The left panel shows the moon

Two global images of Iapetus show the brightness dichotomy on the surface. The left panel shows the moon's leading hemisphere and the right shows the moon's trailing side. Image Credit: NASA/JPL/Space Science Institute

One of the big surprises from this research was the similar reddish colouring of the potato-shaped moon Prometheus and nearby ring particles. Other moons in the area were more whitish.

"The similar reddish tint suggests that Prometheus is constructed from material in Saturn's rings," said co-author Bonnie Buratti, a VIMS team member based at NASA's Jet Propulsion Laboratory. "Scientists had been wondering whether ring particles could have stuck together to form moons, since the dominant theory was that the rings basically came from satellites being broken up. The colouring gives us some solid proof that it can work the other way around, too."

"Observing the rings and moons with Cassini gives us an amazing bird's-eye view of the intricate processes at work in the Saturn system, and perhaps in the evolution of planetary systems as well," said Linda Spilker, Cassini project scientist, based at JPL. "What an object looks like and how it evolves depends a lot on location, location, location."

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