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Electric Currents power Saturn's Atmosphere
What makes Saturn's atmosphere so hot
by Mikayla MacE, University of Arizona
The upper layers in the atmospheres of gas giants—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.
New analysis of data from NASA's Cassini spacecraft finds a viable explanation for what's keeping the upper layers of Saturn, and possibly the other gas giants, so hot: auroras at the planet's north and south poles. Electric currents, triggered by interactions between solar winds and charged particles from Saturn's moons, spark the auroras and heat the upper atmosphere. (As with Earth's northern lights, studying auroras tells scientists what's going on in the planet's atmosphere.)
The work, published today in Nature Astronomy, is the most complete mapping yet of both temperature and density of a gas giant's upper atmosphere—a region that has been poorly understood.
"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 and a graduate student in the University of Arizona Lunar and Planetary Laboratory.
By building a complete picture of how heat circulates in the atmosphere, scientists are better able to understand how auroral electric currents heat the upper layers of Saturn's atmosphere 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 study co-author Tommi Koskinen, a member of Cassini's Ultraviolet Imaging Spectograph team. "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."
Managed by NASA's Jet Propulsion Laboratory in Southern California, Cassini was an orbiter that observed Saturn for more than 13 years before exhausting its fuel supply. The mission plunged it into the planet's atmosphere in September 2017, in part to protect its moon Enceladus, which Cassini 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 scientists 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.
Density and temperature measurements together helped scientists figure out wind speeds. Understanding Saturn's upper atmosphere, where planet meets space, 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.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA's Jet Propulsion Laboratory, or JPL, a division of Caltech in Pasadena, manages the mission for NASA's Science Mission Directorate in Washington. JPL designed, developed and assembled the Cassini orbiter.
https://phys.org/news/2020-04-saturn-at ... e-hot.html
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https://phys.org/news/2020-06-alternati ... aturn.html
https://www.pnas.org/content/early/2020 ... 2000317117
A pair of researchers at Harvard University has developed a computer simulation that may explain Saturn's mysterious polar hexagon. In their paper published in Proceedings of the National Academy of Sciences, Rakesh Yadav and Jeremy Bloxham describe the factors that went into developing their simulation and what it showed.
Back in 1981, the Voyager 2 space probe passed by Saturn and captured images. One of the things that stood out was a very large hexagon-shaped entity (approximately 30 thousand kilometers across) near the planet's North Pole. Further study suggested the hexagon was an atmospheric phenomenon that was likely similar in nature to a hurricane on Earth—but its hexagonal shape was a mystery. Subsequent research showed that the hexagon shape persisted to this day, almost 40 years later—but the reason for its shape and persistence remains mystery. Space scientists have debated the nature of the hexagon, and over the past several years have divided into two camps: those who believe it is a shallow phenomenon, and those who think it is very deep. In this new effort, the researchers sought to solve the mystery of the hexagon by building a 3-D computer simulation to emulate its behavior.
To build their simulation, the researchers studied and used data regarding the planet from multiple resources, most specifically from the Cassini spacecraft, which generated massive amounts of data over its 13-year mission.
The simulation showed deep thermal convection moving in the outer layers of the planet's atmosphere, which led to the formation of three large cyclones near the poles—and an eastward moving jet that moved in a polygonal pattern. The simulation also showed one of the giant vortices pinching the jet. In the simulation, the forces of the cyclones and the eastward-moving jet combined to create the hexagonal shape of the central vortex, which spins in an opposite direction of the smaller vortices. The simulation also showed the hexagon as very deep, perhaps thousands of kilometers.
The researchers suggest that the reason the smaller adjacent cyclones are not visible in photographs of the planet is because they are covered by turbulent gasses.
It may only be a computer model but at least we have an acknowledgement of counter-rotation; a requirement for the formation of Saturn's north polar Hexagon.
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