This is the most complete mapping to date of the temperature and density of the upper atmosphere of a gas giant.

On Saturn, the auroras (in green) would contribute to considerably warming the air at the poles. This air would then be redistributed at the equator by the planet’s wind system, doubling the temperatures that could be expected from simple warming by the Sun – © NASA / JPL

Like the Earth, the upper layers of the atmospheres of gas giants like Saturn and Jupiter have high temperatures. But unlike the Blue Planet, these are too far from the Sun to be the reason for this. As a result, their heat source has long been a mystery to scientists.



A new analysis of data from NASA’s Cassini probe has enabled researchers to identify the likely phenomena that make Saturn’s upper atmosphere so hot: these would be the auroras at the poles of the planet, triggered by electrical currents, themselves generated by interactions between the solar winds and charged particles from the planet’s moons. Recently published in Nature Astronomy, this study is the most complete mapping to date of the temperature and density of the upper atmosphere of a gas giant.

«To understand this dynamic, you really need to have a global vision. This dataset allowed us to observe the upper atmosphere from pole to pole for the first time while seeing how temperature changes with altitude,» said Zarah Brown, lead author of the study.

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« These results are vital to our general understanding of the upper planetary atmosphere and are an important part of the Cassini legacy,» said Tommi Koskinen, co-author of the study. « They help us understand why Saturn’s upper atmosphere is so warm, while the rest, due to the great distance between the planet and the Sun, is cold. »


The Cassini spacecraft observed Saturn for more than 13 years before depleting its fuel reserves and finishing its run in the planet’s atmosphere in September 2017. But before this great plunge, it was able to make 22 ultra-close-up orbits of the gas giant: its “final bouquet” (see animation above). It was during this final stage that the crucial data that enabled the temperature map of Saturn’s atmosphere to be established were collected.

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Cassini / Nasa

For six weeks, Cassini observed changes in the brightness of different stars as they appear and disappear behind the horizon, allowing scientists to measure the density of Saturn’s atmosphere, which is conditioned by altitude and temperature. From the different density variations they observed, they were able to deduce that temperatures were higher at the poles of the planet.

« Even though thousands of exoplanets have been discovered, only the planets of our Solar System can be studied with such precision. Thanks to Cassini, we now have a more detailed picture of Saturn’s upper atmosphere than for any other giant planet in the Universe,» the study’s authors concluded.

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