The most feasible explanation is that icy ring particles raining down onto Saturn’s atmosphere cause this heating, according to a team of astronomers who analyzed several datasets obtained by the NASA/ESA Hubble Space Telescope, NASA’s Cassini spacecraft, NASA’s Voyager 1 and 2 spacecraft, and the NASA / ESA / SERC International Ultraviolet Explorer mission. This could be due to the impact of micrometeorites, solar wind particle bombardment, solar ultraviolet radiation, or electromagnetic forces picking up electrically charged dust.
This composite image shows the Saturn Lyman-alpha bulge, an emission from hydrogen which is a persistent and unexpected excess detected by NASA’s Voyager 1 spacecraft, NASA’s Cassini probe, and the NASA/ESA Hubble Space Telescope between 1980 and 2017. Image credit: NASA / ESA / Lotfi Ben-Jaffel, IAP & LPL.
“Though the slow disintegration of the rings is well known, its influence on the atomic hydrogen of the planet is a surprise,” said Dr. Lotfi Ben-Jaffel, a researcher at the Institute of Astrophysics in Paris and the Lunar & Planetary Laboratory at the University of Arizona.
“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 gasses that are probably heating the atmosphere at a specific altitude.”
The team’s conclusion required pulling together archival ultraviolet-light (UV) observations from four space missions that studied Saturn.
This includes observations from NASA’s Voyager probes that flew by Saturn in the 1980s and measured the UV excess.
At the time, astronomers dismissed the measurements as noise in the detectors.
NASA’s Cassini mission, which arrived at Saturn in 2004, also collected UV data on the atmosphere (over several years).
Additional data came from Hubble and the International Ultraviolet Explorer, which launched in 1978.
But the lingering question was whether all the data could be illusory, or instead reflected a true phenomenon on Saturn.
The key to assembling the jigsaw puzzle came in the team’s decision to use measurements from Hubble’s Space Telescope Imaging Spectrograph (STIS).
Its precision observations of Saturn were used to calibrate the archival UV data from all four other space missions that have observed Saturn.
The astronomers compared the STIS UV observations of Saturn to the distribution of light from multiple space missions and instruments.
“When everything was calibrated, we saw clearly that the spectra are consistent across all the missions,” Dr. 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 realized, wow — it’s the same.”
Four decades of UV data cover multiple solar cycles and help astronomers study the Sun’s seasonal effects on Saturn.
By bringing all the diverse data together and calibrating it, Dr. Ben-Jaffel and colleagues found that there is no difference to the level of UV radiation.
“At any time, at any position on the planet, we can follow the UV level of radiation. This points to the steady ‘ice rain’ from Saturn’s rings as the best explanation,” Dr. Ben-Jaffel said.
“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.”
A paper on the findings was published in the Planetary Science Journal.
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Lotfi Ben-Jaffel et al. 2023. The Enigmatic Abundance of Atomic Hydrogen in Saturn’s Upper Atmosphere. Planet. Sci. J 4, 54; doi: 10.3847/PSJ/acaf78
