“Rain” from Saturn’s rings heats the planet’s upper atmosphere

A composite image of Saturn shows excess ultraviolet radiation from atomic hydrogen in the planet’s upper atmosphere, most evident in a bright equatorial band just above the much darker rings. It is believed that icy ring particles falling into the atmosphere at specific latitudes, along with seasonal effects, cause unexpected atmospheric heating. Image:NASA, ESA, Lotfi Ben-Jaffel (IAP & LPL)

Using archival data and more recent observations of Saturn from a variety of current and retired spacecraft, a researcher has found a surprise hiding in plain view for more than 40 years: icy particles raining down from the vast ring system help heat the planet’s upper atmosphere.

“Everything is driven by ring particles cascading into the atmosphere at specific latitudes. They modify the upper atmosphere, changing the composition,” said Lotfi Ben-Jaffel of the Institute of Astrophysics in Paris and the Lunar & Planetary Laboratory, University of Arizona.

“And then you also have collisional processes with atmospheric gasses that are probably heating the atmosphere at a specific altitude.”

Ben-Jaffel’s conclusion, in a paper published in the Planetary Science Journal, is based on archival ultraviolet observations from the Voyager 1 spacecraft that flew by Saturn in the 1980s, the Cassini orbiter, which arrived at the planet in 2004, the Hubble Space Telescope and the International Ultraviolet Explorer.

The Voyager data were dismissed at the time as the result of “noisy” detectors. But all of the spacecraft appeared to detect a persistent excess of ultraviolet radiation from atomic hydrogen in the upper atmosphere, a so-called Lyman-alpha bulge. The data hinted that something was contaminating and heating Saturn’s upper atmosphere from the outside.

And so the question remained: were the data illusory, or might they reflect a previously unknown phenomenon?

To find out, Ben-Jaffel used Hubble’s Space Telescope Imaging Spectrograph, or STIS, to make precise observations that allowed him to calibrate the data from the other space missions.

“When everything was calibrated, we saw clearly that the spectra are consistent across all the missions,” Ben-Jaffel said. “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 realised, wow – it’s the same.

The most feasible explanation is that ring particles raining down into the atmosphere cause the observed heating. The “rain” could be caused by micrometeoroid impacts, solar wind bombardment, solar ultraviolet radiation or electromagnetic interactions.

“We are just at the beginning of this ring characterisation 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.'”