It cannot be kept secret forever. This otherworldly mystery has eluded astronomers for 40 years. Saturn’s characteristic ring system heats the planet’s upper atmosphere. The phenomenon has never been seen in the solar system, according to NASA, and the unexpected interaction between Saturn and its giant rings could have led to planets around other stars having ring systems like Saturn. may provide tools for predicting whether
Findings were published on March 30. Journal of Planetary Science.
The evidence that caused Saturn to leak its secrets is the excess ultraviolet light seen in the spectral lines of hot hydrogen in Saturn’s atmosphere. This radiation bump indicates that something is heating and polluting the planet’s upper atmosphere from the outside.
[Related: Hubble telescope spies Saturn’s rings in ‘spoke season.’]
The most plausible explanation, according to the paper, is that ice ring particles raining down on Saturn’s atmosphere cause this heating. Several things can cause particle showers, such as impacts of micrometeorites, bombardment by particles from the solar wind, ultraviolet radiation from the sun, or electromagnetic forces picking up electrically charged dust. Additionally, Saturn’s gravitational field is pulling particles toward the planet while all this is happening.
In 2017, NASA’s Cassini spacecraft plunged into Saturn’s atmosphere, measured the composition of the atmosphere, and confirmed that many particles were indeed falling out of the rings. The new discovery used Cassini data, along with observations from NASA’s Hubble Space Telescope, Voyager 1 and 2 spacecraft, and the retired International Ultraviolet probe mission.
“It’s well known that rings decay slowly, but their effects on planetary hydrogen atoms are astonishing. We already knew about ring effects from the Cassini spacecraft. I didn’t know anything about the content,” said astronomer and co-author Lotfi Ben-Jaffel of the Institute for Astrophysics and the Institute for Lunar and Planetary Studies in Paris.
“Everything is driven by ring particles that cascade into the atmosphere at specific latitudes. They change the upper atmosphere, changing its composition,” Ben Jaffel said. “And there’s probably also collision processes with atmospheric gases that are heating the atmosphere at certain altitudes.”
To reach this conclusion, Ben-Jaffel compiled archived ultraviolet (UV) observations from four different space missions that studied ringed planets. While these missions have been spaced apart by more than 40 years, the astronomer dismissed the measurements as detector noise. His UV data of his atmosphere were also collected over several years by his 2004 when the Cassini mission arrived at Saturn. Some of the additional covert decryption data came from Hubble and the International Ultraviolet Spacecraft, an international collaboration between NASA, the European Space Agency, and the UK’s Science and Technology Research Council started in 1978.
[Related: The origin of Saturn’s slanted rings may link back to a lost, ancient moon.]
A lingering question among the decades of data was whether all of it was an illusion, or whether it actually reflected Saturn’s true phenomenon.
The key turned out to be Ben Jaffel’s decision to use measurements taken by the Hubble Space Telescope’s Imaging Spectrograph (STIS). These precise observations of Saturn helped coordinate archival UV data from all four of her other space missions that observed Saturn. He compared his STIS UV observations of Saturn to light distributions from multiple space missions and instruments.
“When everything was aligned, we could clearly see that the spectra were consistent across all missions. For rates, we have the same reference point from Hubble,” Ben Jaffel said. “It was a real surprise to me. I plotted the various light distribution data together and it turned out that they were wow the same.”
The 40-year UV data covers multiple solar cycles and helps astronomers study the Sun’s seasonal effects on Saturn. Adjusting this data together, Ben-Jaffel found no difference in his level of UV radiation. UV levels can be tracked “anytime, anywhere on earth”. This is the best explanation for the steady ice rain that falls from Saturn’s rings.
Next goals of this work include seeing how it can be applied to planets orbiting other stars.