NASA's Hubble Space Telescope has helped unravel a secret that had been hiding in plain sight for 40 years. Hubble, the retired Cassini probe, the Voyager 1 and 2 spacecraft, and the retired International Ultraviolet Explorer mission have discovered a phenomenon that was never seen before in the solar system.
Observations from these spacecraft have helped scientists find that Saturn's vast ring system is heating the gas giant's upper atmosphere, an unexpected interaction between Saturn and its rings. The discovery could provide clues for predicting if planets around other stars have glorious Saturn-like ring systems as well, according to NASA.
What made scientists realise that Saturn’s rings and upper atmosphere interact with each other?
Scientists understood that Saturn's rings and the planet's upper atmosphere interact with each other when they saw evidence of excess ultraviolet radiation. The excess in ultraviolet radiation was seen as a spectral line of hot hydrogen in Saturn's atmosphere, which indicates that something is contaminating and heating the upper atmosphere from outside.
The study describing the findings was published March 30 in the Planetary Science Journal.
Why does Saturn’s ring system cause atmospheric heating?
The above is a composite image showing the Saturn Lyman-alpha bulge, which is an emission from hydrogen. Three distinct NASA missions — Voyager 1, Cassini and Hubble — detected this persistent and unexpected excess of hydrogen emission between 1980 and 2017.
NASA sketched the Lyman-alpha emission of Saturn by using as reference a Hubble near-ultraviolet image obtained in 2017 during the Saturn summer in the northern hemisphere of the planet.
The Lyman-alpha bulge, located above the rings and the dark equatorial region, appears as an extended latitudinal band that is 30 per cent brighter than the surrounding regions.
Between the rings and the equatorial region, one can see a small portion of Saturn's southern hemisphere which is dimmer than the northern hemisphere.
The upper-right portion of the image shows the region north of the bulge. In the upper-right region, the disk brightness declines gradually with latitude until a certain portion, where a bright aurora region can be seen. There is a dark spot inside the aurora region. The dark spot represents the footprint of Saturn's spin axis.
According to NASA, icy ring particles rain down on Saturn's atmosphere at specific latitudes and, together with seasonal effects, result in the atmosphere's heating. This probably happens because of the impact of micrometeorites, solar wind particle bombardment, solar ultraviolet radiation or electromagnetic forces picking up electrically charged dust. All this takes place under the influence of Saturn's gravitational field pulling particles into the planet.
Scientists are investigating this unexpected interaction with Saturn's rings and the upper atmosphere in depth to develop new tools for estimating if distant exoplanets have extended Saturn-like ring systems.
Observations which helped astronomers make this discovery
NASA's Cassini probe, which plunged into Saturn's atmosphere at the end of its mission in 2017, measured the planet's atmospheric constituents and confirmed that many particles are falling in from the rings.
Lotfi Ben-Jaffel, one of the study authors, said in a NASA statement that astronomers already knew about the slow disintegration of Saturn's rings, but did not have any clue about the influence of the disintegration on the atomic hydrogen of the planet. Ben-Jaffel explained that the ring particles cascading into the atmosphere at specific latitudes modify the upper atmosphere and change the composition, and collide with atmospheric gases, resulting in the heating of the atmosphere at a specific latitude.
Ben-Jaffel decided to use measurements from Hubble's Space Telescope Imaging Spectrograph (STIS), and compared its UV observations of Saturn to the distribution of light obtained by the two Voyager missions, the Cassini mission, and the International Ultraviolet Explorer. Surprisingly, the spectra are consistent across all the missions.
In short, the icy ring particles interact with gaseous hydrogen atoms, and cause atmospheric heating and make the hydrogen release UV radiation.