How and why the upper layers of the atmospheres of Saturn, Jupiter, Uranus and Neptune are hot has been one of the mysteries of planetary science. The Sun is too far away, so what is the heat source?
In the case of Saturn, it may be auroras at its north and south poles.
NASA says new analysis of data from the Cassini spacecraft suggests that electric currents triggered by interactions between solar winds and charged particles from the planet’s moons spark the auroras and heat the upper atmosphere.
The global wind system can distribute the 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, according to a paper in the journal Nature Astronomy.
“The results are vital to our general understanding of planetary upper atmospheres and are an important part of Cassini’s legacy,” says co-author Tommi Koskinen, a member of Cassini’s Ultraviolet Imaging Spectrograph (UVIS) team.
Cassini performed 22 ultra-close orbits of Saturn during a 13-year mission that ended in September 2017.
NASA says the data for the new temperature map of the atmosphere was collected during its final tour, known as the Grand Finale.
For six weeks, it targeted several bright stars in the constellations of Orion and Canis Major as they passed behind Saturn. As the stars rose and set, scientists analysed how the starlight changed as it passed through the atmosphere.
Measuring the density of the atmosphere provided the information they needed to find the temperatures; density decreases with altitude, and the rate of decrease depends on temperature.
They found that temperatures peak near the auroras, indicating that auroral electric currents heat the upper atmosphere.
Putting density and temperature measurements together helped them figure out wind speeds.
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