The planet is far from the sun. And yet the atmosphere is quite warm. How is that possible? The late Cassini figured it out!

Temperatures in the upper layers of Saturn’s atmosphere can get quite high. Just like the other gas giant in our solar system – Jupiter – and the ice giants Uranus and Neptune. Actually that is quite crazy. Because unlike our Earth, the sun is too far from these outer planets to be responsible for this. Where this heat comes from was therefore a great mystery. Until now. Because after analyzing data from the retired Cassini spacecraft, researchers are now coming up with a credible statement: auroras.

About Cassini
Cassini launched in 1997 and revolved around Saturn between 2004 and 2017. During that time, the probe discovered geological activity on the moon Enceladus and methane-filled lakes on Titan. The probe also repeatedly dived into the gap between Saturn and its rings – a first – to learn more about the gas giant and the rings. Among other things, it resulted in fantastic close-up photos of the small moons that Saturn is rich in. In mid-September, after nearly 20 years of faithful service, NASA ordered the probe to drill into Saturn’s atmosphere. A very conscious choice: Cassini’s fuel was running out and NASA wanted to prevent the probe from going out of control and crashing into one of Saturn’s potentially habitable moons.

As the end for Cassini approached, he performed one last assignment also known as the “Grand Finals”. This was perhaps the most dangerous part of Cassini’s nearly twenty-year mission. The probe entered an unexplored area: the space between Saturn and its rings. However, this “dangerous” maneuver provided crucial information. Because it was during this “grand finale” that key data was collected for the new temperature map of Saturn’s atmosphere.

Bright stars
For six weeks, Cassini studied several bright stars in the constellations Orion and Big Dog as they passed behind Saturn. As the spacecraft observed the stars that disappeared behind the giant planet, scientists analyzed how this star light changed as it trickled through the atmosphere. By subsequently measuring the density of the atmosphere, researchers got the information they needed to find out the temperature of Saturn’s atmosphere.

The researchers found that the temperature peaked near the auroras located at Saturn’s north and south poles. And so these auroras may be responsible for the mysterious perceived warmth. Auroras – also known as the Northern and Southern Lights – are beautiful and colorful light phenomena and are perhaps one of the most beautiful natural phenomena on Earth. Similar light phenomena also occur on Saturn. On the gas giant, the electric currents are caused by interactions between solar winds and the charged particles of Saturn’s moons. And this may be the reason why the top layer of Saturn’s atmosphere is getting so warm.

This photo taken by Cassini shows the aurora at Saturn’s south pole. The blue color represents this “southern light” and the orange color is reflected sunlight. Image: NASA / JPL / University of Colorado

The study provides the most complete map yet of both the temperature and density of the top layer of a gas giant’s atmosphere: a region that is generally poorly understood. Thus, the findings may hold true not only for Saturn, but also for the other gas and ice giants in our solar system. “The results are vital to our overall understanding of the upper echelons of planetary atmospheres and are an important part of Cassini’s legacy,” said researcher Tommi Koskinen. “They help answer the question of why the upper part of the atmosphere is so hot, while the rest of the atmosphere is cold because of the distance from the sun.”

But that’s not the only question researchers can answer now. A better understanding of the top layer of Saturn’s atmosphere – where the planet meets space – is also key to expanding our knowledge of space weather and its impact on other planets in our solar system. And similar phenomena may also play a role on exoplanets orbiting other distant stars.