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Saturn’s Thermosphere

Pole-to-Pole Map of Saturn’s Thermosphere Reveals Likely Reason Why It’s So Hot

A team of planetary researchers from the United States and the United Kingdom has created a 2D map of densities and temperatures in Saturn’s thermosphere (upper atmosphere).

Composite of a true color image of Saturn, observed by Cassini in 2016, overlaid with a false color representation of the ultraviolet aurora in the northern hemisphere as observed on August 20, 2017. Image credit: NASA / JPL-Caltech / Space Science Institute / A. Bader, Lancaster University.

Composite of a true color image of Saturn, observed by Cassini in 2016, overlaid with a false color representation of the ultraviolet aurora in the northern hemisphere as observed on August 20, 2017. Image credit: NASA / JPL-Caltech / Space Science Institute / A. Bader, Lancaster University.

Thermospheres of gas-giant planets — Saturn, Jupiter, Uranus and Neptune — are hot, just like Earth’s.

But unlike Earth, the Sun is too far from these outer planets to account for the high temperatures. Their heat source has been one of the great mysteries of planetary science.

“Understanding the dynamics really requires a global view,” said lead author Zarah Brown, a graduate student in the Lunar and Planetary Laboratory at the University of Arizona.

“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.”

By building a complete picture of how heat circulates in the atmosphere, scientists are better able to understand how auroral electric currents heat Saturn’s thermosphere and drive winds.

The 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 co-author Dr. Tommi Koskinen, also from the Lunar and Planetary Laboratory at the University of Arizona.

“They help address the question of why the thermosphere is so hot, while the rest of the atmosphere — due to the large distance from the Sun — is cold.”

Saturn’s exospheric temperatures as a function of planetocentric latitude at the half-light point for the observations described in the study. GF - Grand Finale. Image credit: Brown et al, doi: 10.1038/s41550-020-1060-0.

Saturn’s exospheric temperatures as a function of planetocentric latitude at the half-light point for the observations described in the study. GF – Grand Finale. Image credit: Brown et al, doi: 10.1038/s41550-020-1060-0.

In September 2017, NASA’s Cassini spacecraft plunged it into the Saturnian atmosphere, in part to protect its moon Enceladus, which scientists discovered might hold conditions suitable for life.

But before its plunge, Cassini performed 22 ultra-close orbits of Saturn, a final tour called the Grand Finale.

It was during the Grand Finale that the key data was collected for the new temperature map of Saturn’s atmosphere.

For six weeks, Cassini targeted several bright stars in the constellations of Orion and Canis Major as they passed behind Saturn.

As the spacecraft observed the stars rise and set behind the giant planet, scientists analyzed how the starlight changed as it passed through the atmosphere.

Measuring how dense the atmosphere is gave researchers the information they needed to find the temperatures. Density decreases with altitude, and the rate of decrease depends on temperature.

The team found that temperatures peak near the auroras, indicating that auroral electric currents heat the thermosphere.

Density and temperature measurements together helped the scientists figure out wind speeds.

“Even though thousands of exoplanets have been found, only the planets in our Solar System can be studied in this kind of detail,” Brown said.

“Thanks to Cassini, we have a more detailed picture of Saturn’s upper atmosphere right now than any other giant planet in the Universe.”

The results appear in the journal Nature Astronomy.

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Z. Brown et al. A pole-to-pole pressure-temperature map of Saturn’s thermosphere from Cassini Grand Finale data. Nat Astron, published online April 6, 2020; doi: 10.1038/s41550-020-1060-0

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