spaceSpace and Physics

Cassini Spotted Something Unexpected In Saturn's Rings Before It Plunged To Its Doom


Jonathan O'Callaghan

Senior Staff Writer

A previous image of the shadow of Saturn's rings from Cassini. NASA/JPL-Caltech/ESA/Cassini Imaging Team

Scientists using data from the Cassini spacecraft have discovered something unusual about the shadow cast on Saturn by its rings.

Cassini was sent crashing into Saturn in September as its mission came to an end. However, in its final months, it dipped to between 2,600 and 4,000 kilometers (1,600 and 2,500 miles) above the planet to gather invaluable science.


Research conducted by scientists in Sweden and the US, and published in Science, looked at data from this period. And they found that the shadow of the ring, which Cassini flew through, was having a direct effect on the ionosphere of Saturn – its upper atmosphere.

“The ionosphere is surprisingly variable and structured on small scales,” a short statement from the team read. “One reason for that is shadows cast by the rings, which block ultraviolet radiation from the Sun, thereby reducing the ionization in those regions.”

On these passes, the Radio and Plasma Wave Science instrument on the spacecraft revealed that the ionosphere of the planet is relatively cold and dense. But of most interest from this study is that the ionosphere appears to be strongly affected by the rings. That is, the shadow of the rings has a dramatic effect on the weather on Saturn as it blocks out the Sun’s light.

It’s thought that the planet’s B-ring, and most of its A-ring, are likely opaque to most extreme ultraviolet radiation, something not seen in the other rings. The rings appear to affect the way that the ionosphere is charged with plasma. When Cassini passed in the shadow of the rings, the amount of ionized plasma dropped by a huge amount.


The cause may be due to something called “ring rain”. This is where water ions from the planet’s rings rain down on the planet through magnetic field lines and interact with free electrons in the ionosphere.

Saturn is largely made of hydrogen and helium, with its ionosphere located at an altitude of between 300 and 5,000 kilometers (190 and 31,00 miles). This is the region where radiation from the Sun causes an electrically charged layer, through which Cassini flew.

Cassini first flew through the ionosphere on April 26, 2017, and then continued to do so every 6.5 days for the rest of its life. This data comes from just the first 11 of those orbits, going up to late July, meaning there’s plenty more science to come.


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