Contradiction to the model: According to current theory, Saturn’s strong polar lights heat up its atmosphere. But the gas envelope of the planet is clearly colder in the Aurora zone than the models predict, as evaluations of data from the Cassini spacecraft now reveal. Researchers are now explaining where the aurora energy disappears and what this has to do with the already puzzling warmth of the ring planet in the journal “Nature Astronomy”.
Planet researchers have long wondered where the two gas giants Jupiter and Saturn get their heat from. Because the outer atmospheric layers of both planets are several hundred degrees warmer than they should be. However, the weak sunlight or decay processes inside the planets cannot fully explain this heat.
“Heating” the gas giant puzzles
But where does the heat come from? A hypothesis According to huge hurricanes like the Great Red Spot of Jupiter act as a heater by emitting high-energy gravity waves. Another theory assumes that the strong polar lights heat up the outer atmosphere at the poles of the gas giants. The catch, however: How this thermal energy comes from the polar regions to the middle latitudes and the equator region is not clear.
Now data from NASA’s Cassini spacecraft could answer this question. Because it has just before it burns up in the Saturn atmosphere in autumn 2017 almost two dozen extremely close orbits of the ring planet – the “grand finale”. During these orbits, sensors recorded how much starlight falls through the upper layers of the Saturn gas envelope.
First global map of the Saturn thermal atmosphere
From this data, Zarah Brown from the University of Arizona and her colleagues have now created a first map of the temperature and pressure in the entire thermosphere of Saturn. This provides valuable information about how the heat energy is distributed in the gas envelope of the planet. In combination with a model, they also reveal the role that winds play in this.
“This data set allows us to look at the upper atmosphere of Saturn from pole to pole for the first time, while at the same time we see how the temperature changes with depth,” explains Brown. “Because in order to understand the dynamics correctly, you need a global view.” At the same time, the new data provide first insights into the previously unexplored thermal atmosphere of Saturn.
Colder than the model allowed
The result: The heat distribution in the outer gas envelope of Saturn is significantly different than expected. “The temperatures observed in the aurora zones are lower than the models predict,” Brown and her colleagues report. According to the measurements, the values increase from almost 30 to 65 degrees of latitude from almost 400 to 513 Kelvin. But then they drop to the 86th parallel to only 370 Kelvin in the south and 437 Kelvin in the north.
However, this means that precisely where the Aurors would have to generate the greatest heat, Saturn’s thermosphere is colder than in the middle latitudes. “These polar temperature minima are difficult to explain in the context of the existing models,” say the researchers. “Because heating by the Aurora currents is the main heating mechanism of the high latitudes.” In the middle latitudes, on the other hand, the measured temperatures roughly correspond to those of the models.
Winds as energy conductors?
But how can this be explained? For one thing, the transport of aurora energy from the aurora zone to the poles appears to be weaker than expected. “In addition, polar currents could provide additional adiabatic cooling,” the researchers speculate. However, the data are not sufficient to prove this.
But the wind bands that run around the gas planet also seem to play a role in the polar cold zones. The scientists conclude from the Cassini measurement data that the winds in the middle and higher latitudes blow westwards at around 500 to 800 meters per second. “These zonal winds are slower, wider and reach up to smaller latitudes than predicted in the models,” Brown and her team report. You could therefore derive the energy of the Northern Lights towards the equator.
“Models have to be revised”
According to the researchers, the new data provide the first clues as to where the energy and heat of Saturn’s northern lights stay. “Our results provide evidence of a redistribution of Auror energy in lower latitudes – and they underline the importance of atmospheric dynamics for the temperature of the thermosphere,” stated Brown and her team. It is therefore probably the westerly winds that dissipate heat generated by the polar lights from the polar regions and distribute them to temperate latitudes.
However, it also seems clear that the current models for Saturn’s gas envelope and its processes are incomplete and no longer applicable. “Our results require a revision of the existing models in order to explain the polar temperature structure in a reasonable way,” said the researchers. The geography of Saturn is therefore far from being aired. (Nature Astronomy, 2020; doi: 10.1038 / s41550-020-1060-0)
Source: University of Arizona, NASA
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