Saturn’s ‘ring rain’ quenches dark bands of the planet’s atmosphere

Astronomers have found that the icy rings of Saturn 'rain' water down onto the planet causing cool, dark bands in the planet's atmosphere.

Years ago, when the Voyager spacecraft passed by Saturn, returning amazing images of the planet and its extensive and intricate system of icy rings, astronomers discovered that there were dark bands in the planet's atmosphere that they couldn't immediately explain. One idea the scientists had was that charged particles of water could have been falling onto the planet from the its rings, cooling those bands of the atmosphere. It took until 2011, and using the Keck Observatory on Mauna Kea to observe the planet in the infrared, for astronomers to see these bands again, and their observations not only confirmed their idea, but showed some surprising results.

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"Saturn is the first planet to show significant interaction between its atmosphere and ring system," said James O’Donoghue, a postgrad researcher at the University of Leicester, UK, who is the lead author of the study, according to a NASA press release.

"It turns out that a major driver of Saturn's ionospheric environment and climate across vast reaches of the planet are ring particles located some 36,000 miles [60,000 kilometers] overhead," said study co-author Kevin Baines, from NASA's Jet Propulsion Laboratory, according to the statement. "The ring particles affect both what species of particles are in this part of the atmosphere and where it is warm or cool."

Rather than drops of rain, like those that fall on Earth, the steady stream of 'rain' that falls onto Saturn is in the form of charged water particles that flow from the rings, along the lines of the planet's magnetic field, and down into the planet's atmosphere. This explains why the patterns of the cool bands on the planet match the structure of the rings (at least when taking into account the curvature of the magnetic field lines). It also explains a peculiar pattern astronomers have seen in what they call the 'electron densities' in the planet's upper atmosphere. The water particles streaming out of the rings are positively charged, which is why they are 'attracted' to flow along the field lines in the first place, and they lower the electron density in the planetary bands as they pick up electrons and balance out.

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"Where Jupiter is glowing evenly across its equatorial regions, Saturn has dark bands where the water is falling in, darkening the ionosphere," said Tom Stallard, a paper co-author who is also at the University of Leicester, according to NASA. "We're now also trying to investigate these features with an instrument on NASA's Cassini spacecraft. If we're successful, Cassini may allow us to view in more detail the way that water is removing ionized particles, such as any changes in the altitude or effects that come with the time of day."

(Image courtesy: NASA/JPL-Caltech/Space Science)

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