Cross the “snow line” to Saturn and relish its ancient ice

NASA’s Cassini spacecraft on July 29, 2011, shows Saturn’s A and F rings and five of its moons. From left, the moons are Janus, Pandora, Enceladus, Mimas and Rhea. Saturn is hidden at right behind Rhea. Credit: NASA/JPL-Caltech/Space Science Institute

Wipe the dust from an antique and you can begin to appreciate its vintage and workmanship. See beneath the space-worn colors of Saturn’s rings and moons and you might just get a glimpse of the primeval solar system. A new analysis of data from NASA’s Cassini spacecraft suggests that the Saturn system is tinted by “pollutants”.

Geysers of water ice crystals erupt from fissures in Enceladus’ south polar region in this photo taken by Cassini. Credit: NASA/JPL-Space Science Institute

The inner moons get whitewashed as they pass through water sprayed by geysers from the moon Enceladus. More distant moons wear a pale coating of red from particles of dust shed by Phoebe; the farther out they are from Saturn, the redder they appear.

Phoebe is an outer moon that may have once resided in the far-off Kuiper Belt beyond the planet Neptune before it was captured by the ringed planet.

Parts of the Saturn’s B-ring also appear faintly red perhaps from meteoroids that pepper the icy ring particles. Iron is a very common constituent of meteorites. Scientists think the reddish color could be either oxidized iron – better known as rust – formed when oxygen and iron combine in the presence of water –  or polycyclic aromatic hydrocarbons (PAHs), organic molecules common in tars and oil.

PAHs form in the atmospheres of aging, expanding stars when carbon and oxygen atoms are spat out into space. As they cool, the atoms bind together to form simple organic molecules and PAHs.

The precursors to the planets, called planetesimals, were mostly rocky stuff in the inner solar system within the snow or frost line. The outer planets formed from planetesimals composed of a mixture of rock and ice. Credit: Univ. of Colorado

Despite their diverse colors, data from Cassini’s visual and infrared mapping spectrometer (VIMS), which penetrates below the chemical veneer, found deep similarities. VIMS detected lots of water ice in both moons and rings, too much to have been deposited by recent comet collisions. The authors concluded that the ice must have formed around the birth of the solar system. How do you keep ice around that long? You either live in Duluth, Minn. or form it beyond the “snow line”, where temperatures are cold enough to keep things frozen almost forever.

Our solar system’s snow line starts at around 5 times the Earth’s distance from the sun or roughly at Jupiter’s distance. Indeed it’s the snow line which separates the inner terrestrial planets from the giant gas planets of the outer. Drop this side of the line and water either melts or vaporizes.

Saturn’s small 84-mile-long moon Prometheus creates a knot in the F-ring through its gravitational interaction with the icy ring particles. Prometheus may have formed from ring material. Credit: NASA/JPL-Space Science Institute

Cassini turned up another interesting fact. Saturn’s moon Prometheus has a similar reddish tint as the rings, hinting that it may have formed from ring material:

“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, Pasadena, Calif. “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.”

Saturn’s icy ring particles range in size from less than a millimeter to around 10 feet across. Credit: NASA/JPL-Space Science Institute

With Prometheus, the process may have worked the other way around, testing our assumptions once again.

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