'Ring Rain' Quenches Saturn's Atmosphere


Saturn’s rings rain charged water particles down onto the gas giant’s atmosphere, causing measurable changes in the planet’s ionosphere. This intriguing conclusion comes from astronomers using the W. M. Keck Observatory atop Mauna Kea in Hawaii that observed dark bands forming in Saturn’s upper atmosphere.

“Saturn is the first planet to show significant interaction between its atmosphere and ring system,” said James O’Donoghue, postgraduate researcher at the University of Leicester and lead author of a paper to appear this week in the journal Nature. “The main effect of ring rain is that it acts to ‘quench’ the ionosphere of Saturn, severely reducing the electron densities in regions in which it falls.”

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Although highly charged ice particles are known to be transported from Saturn’s rings to the planet below via its powerful magnetosphere, this is the first time that global changes to the composition and temperature of the Saturnian ionosphere have been realized.

Like Earth, Saturn’s ionosphere is composed of highly charged particles. The solar wind continually slams into this region, igniting space weather phenomena such as aurorae.

In 1980, during the Voyager 1 flyby of Saturn, dark bands were detected in Saturn’s upper atmosphere — it was surmised that there may be some interaction between Saturn’s rings and its ionosphere. However, in follow-up studies, little further evidence for these bands was found. That was until the Keck II telescope’s NIRSPEC instrument was trained on the planet in 2011.

The high-resolution near-infrared spectrograph detected the variations in the emission of a hydrogen molecule in the Saturnian ionosphere. This particular molecule — composed of three hydrogen atoms, known as trihydrogen cation (H3+) — is commonly found in space environments and has a specific emission spectrum. Normally, one would expect a uniform distribution of H3+ (as is seen in the atmospheres of Earth and Jupiter), but planet-wide bands in its distribution were discovered on Saturn.

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NASA/JPL-Caltech/Space Science Institute/University of Leicester

The most intriguing thing was that the NIRSPEC data appeared to show a “mapping” of Saturn’s rings down onto the planet’s ionosphere — the magnetosphere is guiding the charged water particles down onto the upper atmosphere, creating a ring pattern at varying latitudes (see diagram, right). The water particles from the rings are raining down and neutralizing (or quenching) the H3+ at discrete locations covering 30 to 43 percent of the planet’s upper atmosphere from 25 to 55 degrees latitude.

“It turns out a major driver of Saturn’s ionospheric environment and climate across vast reaches of the planet are ring particles located 120,000 miles overhead,” said Kevin Baines, of NASA’s Jet Propulsion Laboratory, Pasadena, Calif., and research co-investigator. “The ring particles affect which species of particles are in this part of the atmospheric temperature.”

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The team now want to utilize an instrument on NASA’s Cassini probe currently in orbit around Saturn to observe further evidence for Saturn’s “ring rain” and it’s impact on the planet’s ionosphere.

Image credit: NASA/JPL-Caltech/SSI, edit by Ian O’Neill

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