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Astronomers (re)discover never-before-seen phenomenon on Saturn

Rain of icy and rocky debris from rings was initially dismissed as bad data

The infall of icy debris from Saturn's rings is heating the planet's upper atmosphere, according to astronomers, who discovered the phenomenon among old data that was once considered noise.

Saturn is distinctive in our Solar System with its large, complex ring system, composed of chunks of ice and rock that have been captured by the planet's gravitational field. The rings orbit the planet at different speeds, and aren't as stable as they appear. Fragments of debris from the rings rain down on Saturn's atmosphere, heating it up in the process.

Astronomers observed the phenomenon by analyzing UV radiation emitted from hydrogen atoms. Data collected from the Hubble Space Telescope was compared with archival data taken from four historical spacecraft – the Cassini probe, Voyagers 1 and 2, and the long-retired International Ultraviolet Explorer – and revealed telltale signs of excess Lyman-alpha radiation, a form of UV light produced by excited hydrogen atoms.

Some of the data was mistaken as noise when first collected up to 40 years ago, and researchers failed to recognize its significance until now. "When everything was calibrated, we saw clearly that the spectra are consistent across all the missions," announced Lotfi Ben-Jaffel, lead author of the new research published in Planetary Science Journal on Thursday, and a researcher at the Institute of Astrophysics in Paris and the Lunar & Planetary Laboratory, University of Arizona, in a statement. 

"This was possible because we have the same reference point, from Hubble, on the rate of transfer of energy from the atmosphere as measured over decades. It was really a surprise for me. I just plotted the different light distribution data together, and then I realized, wow – it's the same."

The researchers traced the increased levels of Lyman-alpha UV radiation to Saturn's atmosphere, and concluded some external process must be exciting the hydrogen atoms. The most plausible explanation is that the icy particles in Saturn's rings are crashing down onto Saturn's atmosphere, causing it to heat up. 

These bits and pieces get dislodged by colliding with micrometeorites, or by solar wind particle bombardment, solar ultraviolet radiation, or electromagnetic forces picking up electrically charged dust. Once they are knocked out of place, Saturn's gravity pulls them in. 

"Though the slow disintegration of the rings is well known, its influence on the atomic hydrogen of the planet is a surprise," Ben-Jaffel said. "From the Cassini probe, we already knew about the rings' influence. However, we knew nothing about the atomic hydrogen content."

"Everything is driven by ring particles cascading into the atmosphere at specific latitudes. They modify the upper atmosphere, changing the composition. And then you also have collisional processes with atmospheric gases that are probably heating the atmosphere at a specific altitude," he explained.

The team believe this phenomenon could provide astronomers with a new way to look for ring systems on exoplanets. If a spacecraft detects similar excess UV radiation bands in the upper atmosphere of a faraway planet, it could mean it might be supporting a ring system like Saturn's.

"We are just at the beginning of this ring characterization effect on the upper atmosphere of a planet. We eventually want to have a global approach that would yield a real signature about the atmospheres on distant worlds. One of the goals of this study is to see how we can apply it to planets orbiting other stars. Call it the search for 'exo-rings'," Ben-Jaffel concluded. ®

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