Astronomers have used data from the Cassini mission's “Grand Finale” to produce a global map of one of the outer layers of Saturn’s atmosphere. The map shows pressure and temperatures from pole to pole in Saturn’s thermosphere, providing a better understanding of what’s happening up there and what scientific models are failing to predict.
Cassini was an extremely successful mission, a cooperative enterprise between NASA, the European Space Agency, and the Italian Space Agency, which ended in 2017 after 13 years studying Saturn. It has wildly expanded our knowledge of the ringed planet and its numerous and fascinating moons, even after its planned swan dive into Saturn's atmosphere ended the mission.
"The observing campaign that we designed for this Grand Finale sweep of occultations has led us for the first time to see the pole-to-pole trends in temperature with both latitude and depth for any giant planet," lead author Zarah Brown, a graduate researcher at the University of Arizona, told IFLScience. "And that's important because the planets in our Solar System are the only ones that we can study in this level of detail."
Saturn’s thermosphere has produced several head-scratchers for researchers. It has much higher temperatures than it should, based on the heat it receives from the Sun alone. The source of the extra energy is the aurorae, which deposit a lot of energy above the planet’s atmosphere where the magnetic field lines twist and catch the electrically charged particles produced by the Sun.
These aurorae are a lot more energetic than the kind we get here on Earth and so need to be studied in UV light. They certainly produce enough energy to heat up the whole thermosphere but models suggest due to the rotation of the planet, this heat shouldn’t spread all the way to the equator.
The latest data, reported in Nature Astronomy, paints a very different picture of Saturn’s higher atmosphere. The regions around the poles, further north and south from where the aurorae are forming, are actually cooler than expected. The regions where the aurorae are forming are instead very hot.
The highest temperatures detected, which is roughly between 240°C and over 315°C (460-600 °F) are actually at latitudes of about 70° both in the Northern and in the Southern hemisphere. Another surprising fact is that the highest temperatures are at higher altitudes than previously expected.
The researchers have also found that pressure across the thermosphere from the poles to the equator doesn’t change sharply, suggesting a flow of heated gas from the hottest regions towards the equator. The team has reasons to believe that waves are present in the thermosphere and these waves help to redistribute the heat around the planet as well.
“The results are vital to our general understanding of planetary upper atmospheres and are an important part of Cassini’s legacy,” co-author Tommi Koskinen, a member of Cassini’s Ultraviolet Imaging Spectrograph (UVIS) team explained in a statement. “They help address the question of why the uppermost part of the atmosphere is so hot while the rest of the atmosphere -- due to the large distance from the Sun -- is cold.”
The team will continue to study the atmosphere of Saturn focusing on other wavelengths that highlight the presence of different molecules in the higher parts of Saturn's atmosphere. Cassini might be gone but its data will continue to be analyzed for decades to come.
"It was a really great mission and I got emotional seeing it go but it has really, truly given us a wealth of data. And while we've had tons of wonderful studies, there's still a lot of data that we can work with," Brown continued. "We're lucky that we're still benefiting from this mission"