The secret was hidden in plain sight for 40 years. But it took the insight of an experienced astronomer to piece it all together over the course of a year, using observations of Saturn from NASA’s Hubble Space Telescope and the decommissioned Cassini probe, in addition to the Voyager 1 and 2 spacecraft and the defunct International Ultraviolet Explorer mission .
Discovery: Saturn’s large ring system heats the giant planet’s upper atmosphere. The phenomenon has never before been observed in the solar system. It’s an unexpected interaction between Saturn and its rings that could potentially be a tool for predicting whether planets around other stars also have remarkable Saturn-like ring systems.
A demonstrable proof is the excess of ultraviolet radiation, which is visible as a spectral line of hot hydrogen in Saturn’s atmosphere. Rising radiation means that something is contaminating and heating the upper atmosphere from outside.
The most likely explanation is that ice ring particles falling into Saturn’s atmosphere cause this heating. This can be caused by micrometeorite impacts, solar wind particle bombardment, solar ultraviolet radiation, or electromagnetic forces collecting electrically charged dust. All this happens under the influence of Saturn’s gravitational field, which attracts particles to the planet. When NASA’s Cassini probe plunged into Saturn’s atmosphere at the end of its mission in 2017, it measured the composition of the atmosphere and confirmed that many particles are falling from the rings.
“While the slow decay of the rings is well known, its effect on the planet’s atomic hydrogen is surprising. We already knew about the influence of the rings from the Cassini probe. However, we didn’t know anything about the atomic hydrogen content,” said Lotfi Ben-Jaffel of the Institute for Astrophysics in Paris and the Lunar Planetary Laboratory at the University of Arizona, an author of the paper. published on March 30 in Planetary Science Journal .
“Everything happens thanks to ring particles that cascade into the atmosphere at certain latitudes. They modify the upper layers of the atmosphere, changing the composition,” said Ben-Jaffel. “And then you also have collision processes with atmospheric gases that probably heat the atmosphere at a certain altitude.”
Ben-Jaffel’s conclusion required combining archival ultraviolet (UV) light data from four space missions that studied Saturn. This includes observations from NASA’s two Voyager probes, which flew past Saturn in the 1980s and measured the ultraviolet excess. At the time, astronomers dismissed the measurements as detector noise. The Cassini mission, which arrived at Saturn in 2004, also collected UV data on the atmosphere (for several years). Additional data came from Hubble and the International Ultraviolet Explorer, which was launched in 1978 and was the result of an international collaboration between NASA, ESA (European Space Agency) and the United Kingdom’s Science and Technology Research Council.
But the question was whether the data could all be illusory, or whether they instead reflected a real phenomenon on Saturn.
Ben-Jaffel’s decision to use Hubble Space Telescope Imaging Spectrograph (STIS) measurements was the key to the puzzle. His precise observations of Saturn were used to calibrate archival UV data from all four other space missions that observed Saturn. He compared the STIS UV observations of Saturn with light distributions from many space missions and instruments.
“Once everything was calibrated, we could clearly see that the spectra were consistent across the missions. This is possible because we have the same reference point, from Hubble, for the rate of energy transfer from the atmosphere measured over decades.” —Ben. – said Yaffel. “It was really a surprise for me. I just plotted different light distribution data together, and then I realized, oh well, it’s the same thing.”
Four decades of UV data span several solar cycles and help astronomers study the Sun’s seasonal influence on Saturn. By combining all the various data and calibrating them, Ben-Jaffel found that there was no difference in the level of UV radiation. “At any time, anywhere on the planet, we can monitor the level of UV radiation,” he said. This points to a constant “ice rain” from Saturn’s rings as the best explanation.
“We are only at the beginning of the study of the impact of ring characteristics on the upper layers of the planet’s atmosphere. Eventually, we want to have a global approach that will give real information about the atmospheres of distant worlds. One of the goals of this research is to see how we can apply this to planets orbiting other stars. Call it the search for “exo-rings.”
