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That's why the Saturn rotation is so hard to measure



For a rocky planet, it can be easy to determine the length of a day. Just pick a reference point and watch how long it takes for the camera to disappear. But for planets like Saturn it's not that easy. There are no surface features to track.

Scientists have spent decades trying to determine the Saturn rotation period. But the gas giant did not want to reveal its secrets. A new study in AGU's Journal of Geophysical Research: Space Physics could finally give the answer. The study is titled "Saturn's Multiple, Variable Periodicities: A Dual Flywheel Model of Thermosphere-Ionosphere-Magnetosphere Coupling."

For a planet like Earth, we know what we measure when we measure the rotation period. We measure the surface of the planet. But for a gas giant, things are more complex. Which layer of the planet are scientists talking about?

Saturn is a multi-layered gas giant, probably with a rocky core. This core is surrounded by a layer of ice, then by metallic hydrogen and helium. Then an area of ​​helium rain, further surrounded by a region of liquid hydrogen. Then comes a large area of ​​gaseous hydrogen. The upper atmosphere of Saturn consists of three layers: At the top are ammonia clouds, including ammonium hydrogen sulfide, and below them water vapor clouds.

Diagram of Saturn with some details on the planet structure. Picture credits: By Kelvinsong – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=32219154[19659006lightboxesWhenscientiststalkabouttherotationperiodofSaturnspeakingabouttheupperatmospheresomeoftheplanetaryrealitycanbemeasured

. Scientists examine the radio frequency patterns that a gas giant sends out to its audience Day length to determine. The difficulty with Saturn is that it only emits low-frequency radio patterns that block the Earth's atmosphere. This is in contrast to Jupiter, which emits higher frequency patterns that flow through the Earth's atmosphere. For this reason, scientists were able to calculate the rotation period of Jupiter before the advent of the spacecraft.

Saturn had to wait until 1980 and 1981 when Voyager 1 and Voyager 2 visited and rallied data. At this time, the rotation period was measured at 10 hours and 40 minutes. That was the best measurement available at the time and it got stuck. For two decades.

Then Cassini visited Saturn and spent 13 years studying him and his moons. The astronomers were astonished to discover that the rotation period of Saturn had changed. Cassini data showed that over the twenty years between Voyagers and Cassini – an insignificant period in the life of a planet – the length of the day had changed.

"About 2004, we saw that the period had changed by 6 minutes, about 1 percent.

Duane Pontius of Birmingham-Southern College, Alabama, study co-author.

Cassini showed that the rotation period had changed by 6 minutes or about 1 percent.

"Around 2004, we saw the period change by 6 minutes, about 1 percent," said Duane Pontius of Birmingham-Southern College, Alabama, co-author of the new study. "For a long time, I assumed there was something wrong with the data interpretation," recalls Pontius. "It's just not possible."

How does an entire planet change its rotation period in such a short time? A change of this magnitude should take hundreds of millions of years. But there was more: Cassini also measured electromagnetic patterns that showed that the northern and southern hemispheres had different rotation periods.

Saturn's Changing Seasons

Pontius and the other authors wanted to understand what had happened and why there was a discrepancy in the measurements. Assuming that the Cassini data were understood correctly, there had to be a reason for the change and for the difference between the hemispheres. They decided to compare Saturn with his next sibling, Jupiter.

One thing that Saturn has is seasons. Saturn has an axial inclination of almost 27 degrees, similar to the Earth's 23 degree slope. Jupiter has only a slope of three degrees. Like Earth, the northern and southern hemispheres of Saturn receive different amounts of energy as they circle the sun.

The tilt and rotation of the 8 planets of the solar system. NASA / James O & # 39; Donoghue (JAXA).

At the outer edge of Saturn's atmosphere is a region of plasma. Pontius and the other authors believe that the varying amount of UV energy that reaches the hemispheres during the seasons interacts with this plasma. In the model they have developed, the UV fluctuations affect the plasma and cause more or less resistance at the intersection of the plasma and the outside atmosphere.

Air resistance determines the rotation of the atmosphere as shown by radio wave emissions, and this rotation changes according to the observed season.

   A mechanical analogue model of what could happen to the northern and southern hemispheres of the Saturnian atmosphere and the magnetospheric plasma to produce misleading signals of how fast the planet is spinning. The
A mechanical analogue model of what could happen to the northern and southern hemispheres of the Saturnian atmosphere and the magnetospheric plasma to produce misleading signals of how fast the planet is spinning. The "brake" is the slowing of the plasma as it flies farther away from the planet, just as the arms of a spinning dancer move slower when outstretched than when they are held close to the body. Picture credits: E.L. Brooks et al., 2019, JGR: Space Physics.

The resistance of the plasma slows down the rotation and gives us the rotation period indicated by the radio emissions. As the season changes, the plasma resistance and also the radio emissions change. Again, it is the radio emissions that scientists use to measure the Saturn rotation period because there are no solid surface features.

This model, developed by Pontius and his colleagues, provides an explanation for the change in rotation over the 20 years between the Voyagers and Cassini. However, this measurement only applies to Saturn's surface layers. The rocky core, which lies between 9-22 times the mass of the earth, is hidden under tens of thousands of kilometers of atmosphere and unfathomable.

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