Decoded: What makes Saturn's atmosphere hot

The outer layer of Saturn, Jupiter, Neptune and Uranus, are hot just like the Earth's likely due to electric current at the planets' poles, according to a study, which is the most complete mapping yet of both temperature and density of a gas giant's upper atmosphere.

Researchers, including those from the University of Arizona in the US, used data from the Cassini spacecraft which observed Saturn for more than 13 years before exhausting its fuel supply, and found that electric currents in the upper layers of these planets could be making them very hot.

According to the study, published in the journal Nature Astronomy, these electric currents, triggered by interactions between solar winds and charged particles from Saturn's moons, spark the auroras and heat the upper atmosphere.

It said the phenomenon is similar to the Earth's northern lights, or auroras, indicating that the process can shed light on what's going on in the atmosphere of other planets.

"Understanding the dynamics really requires a global view. This dataset is the first time we've been able to look at the upper atmosphere from pole to pole while also seeing how temperature changes with depth," said Zarah Brown, lead author of the study from the University of Arizona.

In the study, the scientists built a picture of how heat circulates in the atmosphere of Saturn, helping understand how auroral electric currents heat the upper layers of the planet's atmosphere and drive winds.

They said the planet's global wind system can distribute this energy, which is initially deposited near the poles toward the equatorial regions, heating them to twice the temperatures expected from the Sun's heating alone.

"The results are vital to our general understanding of planetary upper atmospheres, and are an important part of Cassini's legacy," said study co-author Tommi Koskinen from the University of Arizona.

"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," Koskinen said.

Measuring the density of Saturn's atmosphere using data from the Cassini mission gave the scientists the information they needed to find the temperatures, the study noted.

The scientists said the density decreased with altitude, with the rate of decrease depending on temperature.

They found that temperatures peak near the auroras, indicating that the electric currents near the poles heat the upper atmosphere.

Density and temperature measurements helped figure out wind speeds, the study noted.

Understanding Saturn's upper atmosphere, where planet meets space, according to the study, is key to understanding space weather and its impact on other planets in our solar system and exoplanets around other stars.

"Even though thousands of exoplanets have been found, only the planets in our solar system can be studied in this kind of detail. Thanks to Cassini, we have a more detailed picture of Saturn's upper atmosphere right now than any other giant planet in the universe," Brown said.