Unraveling the mysteries of the solar system
Rain of ice particles affects the weather of a giant planet
The planet Saturn is easily recognized by its rich ring system, which can be easily seen with a small telescope. Astronomers have discovered that the rings are not as serene as they seem. Particles of ice rings fall into Saturn’s atmosphere. This is the heating of the upper layers of the atmosphere. It took a collection of 40 years of observations of Saturn from four NASA planetary missions to come to this conclusion. Hubble Space Telescope observations were used to tie together all the evidence collected in ultraviolet light. These results can be applied to determine whether similar ring systems surround planets orbiting other stars. Their rings would be too far away to be seen, but ultraviolet light spectroscopy of the planets may provide clues.
The Hubble Space Telescope has discovered the rings of Saturn heating its atmosphere
The secret has been hiding in plain sight for 40 years. But it took the insight of a skilled astronomer to piece it all together within a year, using observations of Saturn from NASA’s Hubble Space Telescope and the decommissioned Cassini probe, in addition to the Voyager 1 and Voyager 2 spacecraft and the retired mission “International Ultraviolet Investigator”.
Discovery: Saturn’s vast ring system heats the giant planet’s upper atmosphere. This phenomenon has never before been observed in the solar system. This is an unexpected interaction between Saturn and its rings that could potentially provide a tool to predict whether planets around other stars have magnificent Saturn-like ring systems.
The telltale sign is an excess of ultraviolet radiation observed as a spectral line of hot hydrogen in Saturn’s atmosphere. A burst of radiation means that something is polluting and heating the upper atmosphere from outside.
The most plausible explanation is that this heating is caused by particles of ice rings raining down on Saturn’s atmosphere. This may be due to the impact of micrometeorites, bombardment by solar wind particles, solar ultraviolet radiation, or electromagnetic forces that collect electrically charged dust. All this happens under the influence of the gravitational field of Saturn, which draws particles into 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.
“Although the slow decay of rings is well known, its effect on the planet’s atomic hydrogen is surprising. From the Cassini probe, we already knew about the influence of the rings. However, we did not know anything about the content of atomic hydrogen,” said Lotfi Ben-Jaffel of the Institute of Astrophysics in Paris and the Lunar and Planetary Laboratory of the University of Arizona, author of an article published March 30 in the Planetary Science Journal.
“Everything is controlled by ring particles cascading 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 collisional processes with atmospheric gases that probably heat up the atmosphere at a certain height.”
Ben Jaffel’s finding required combining archival ultraviolet (UV) light observations from four space missions that had studied Saturn. This includes observations by two NASA Voyager probes that flew past Saturn in the 1980s and measured excess UV radiation. At the time, astronomers dismissed the measurements as noise in detectors. The 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, launched in 1978 as a result of an international collaboration between NASA, ESA (European Space Agency) and the United Kingdom’s Research and Engineering Council.
But the unresolved question was whether all of the data could be illusory, or instead reflect a true phenomenon on Saturn.
The key to assembling the puzzle was Ben-Jaffel’s decision to use Hubble Space Telescope Imaging Spectrograph (STIS) measurements. His precise observations of Saturn have been used to calibrate archival UV data from all four other space missions that have observed Saturn. He compared STIS UV observations of Saturn with the distribution of light from several space missions and instruments.
“When everything was calibrated, we clearly saw that the spectra were the same in all missions. This is possible because we have the same reference point, obtained from Hubble, in terms of the rate of energy transfer from the atmosphere, measured over decades,” said Ben-Jaffel. “It really came as a surprise to me. I just put together different data about the distribution of light, and then I realized: wow, it’s the same thing.
Four decades of UV data span several solar cycles and help astronomers study the Sun’s seasonal effects on Saturn. By pulling together all the various data and calibrating it, 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 the constant “freezing rain” from Saturn’s rings as the best explanation.
“We are only at the beginning of this circular impact on the planet’s upper atmosphere. Ultimately, we want to have a global approach that will give a real characterization of the atmospheres of 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.”
Reference: “The Mysterious Abundance of Atomic Hydrogen in Saturn’s Upper Atmosphere” by Lotfi Ben-Jaffel, Julianne I. Moses, Robert A. West, Klaus-Michael Aye, Eric T. Bradley, John T. Clarke, Jay B. Hallberg, and Gild E Ballester, March 30, 2023, Planetary Science Journal.
The Hubble Space Telescope is an international collaboration between NASA and ESA. The NASA Goddard Space Flight Center in Greenbelt, Maryland operates the telescope. The Space Telescope Science Institute (STScI) in Baltimore conducts Hubble science operations. The STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington DC.