Saturn’s rings have been slowly heating up its atmosphere

A new study brings together 40 years of planet-warming data.
A blue UV image of the planet Saturn.
This composite image shows the Saturn Lyman-alpha bulge, an emission from hydrogen which is a persistent and unexpected excess detected by three distinct NASA missions, namely Voyager 1, Cassini, and the Hubble Space Telescope between 1980 and 2017. NASA, ESA, Lotfi Ben-Jaffel (IAP & LPL)

Nothing can really stay a secret forever, and this otherworldly mystery has evaded astronomers for four decades. Saturn’s signature ring system is heating the planet’s upper atmosphere. According to NASA, this phenomenon has never been seen in the solar system, and the unexpected interaction between Saturn and its vast rings could provide a tool for predicting if the planets around other stars have ring systems like Saturn’s.

The findings were published March 30 in the Planetary Science Journal.

The evidence that caused Saturn to spill its secrets is an excess of ultraviolet radiation that is seen as a spectral line of hot hydrogen in Saturn’s atmosphere. This bump in radiation indicates that something is heating and contaminating the planet’s upper atmosphere from the outside. 

[Related: Hubble telescope spies Saturn’s rings in ‘spoke season.’]

According to the paper, the most feasible explanation is that icy ring particles raining down onto Saturn’s atmosphere cause this heating. A few things could be driving this shower of particles, including the impact of micrometeorites, bombardments with particles from solar wind, solar ultraviolet radiation, or electromagnetic forces picking up electrically charged dust. Additionally, Saturn’s gravitational field is pulling particles into the planet while this is all occurring.

In 2017, NASA’s Cassini probe plunged into Saturn’s atmosphere and measured the atmospheric constituents, confirming that many particles are indeed falling in from the rings. This new discovery used that Cassini data in addition to observations from NASA’s Hubble Space Telescope, the Voyager 1 and 2 spacecraft, and the retired International Ultraviolet Explorer mission.

“Though the slow disintegration of the rings is well known, its influence on the atomic hydrogen of the planet is a surprise. From the Cassini probe, we already knew about the rings’ influence. However, we knew nothing about the atomic hydrogen content,” astronomer and co-author Lotfi Ben-Jaffel of the Institute of Astrophysics in Paris and the Lunar & Planetary Laboratory, said in a statement

“Everything is driven by ring particles cascading into the atmosphere at specific latitudes. They modify the upper atmosphere, changing the composition,” said Ben-Jaffel. “And then you also have collisional processes with atmospheric gasses that are probably heating the atmosphere at a specific altitude.”

To come to this conclusion, Ben-Jaffel pulled together archival ultraviolet-light (UV) observations from four different space missions that studied the ringed planet. During these missions spaced out over 40 years, astronomers dismissed the measurements as noise in the detectors. By 2004, when the Cassini mission arrived on Saturn, it also collected UV data on the atmosphere over a period of several years. Some of the additional secret-cracking data came from Hubble and the International Ultraviolet Explorer, an international collaboration between NASA, the European Space Agency, and the United Kingdom’s Science and Engineering Research Council that launched in 1978.

[Related: The origin of Saturn’s slanted rings may link back to a lost, ancient moon.]

The lingering question among decades of data was whether all of it could be illusory or actually reflect a true phenomenon on Saturn.

The key turned out to be Ben-Jaffel’s decision to use measurements taken by the Hubble’s Space Telescope Imaging Spectrograph (STIS). These precision observations of Saturn helped calibrate the archival UV data from all four of the other space missions that have observed Saturn. He 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. 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,” said Ben-Jaffel. “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.”

Forty years of UV data covers multiple solar cycles and helps astronomers study the sun’s seasonal effects on Saturn. Bringing this data together and calibrating it helped Ben-Jaffel find that there was no difference in the level of UV radiation. The UV level of radiation can be followed at “at any time, any position on the planet,” which points to the steady ice rain coming from Saturn’s rings as the best explanation.

Some of the next goals for this research include seeing how it can be applied to planets that orbit other stars