Saturn's rotation period
I was surprised to discover the rotation period of Saturn is not certain.
Data obtained by the Cassini probe were not consistent with previous measurements.
"[From both Voyager probes around 1981 to ] about 2004 we saw the period had changed by 6 minutes, about 1 percent,"
But how does an entire planet change the speed of its rotation in 20 years? That's the sort of change that takes hundreds of millions of years. Even more mysterious was Cassini's detection of electromagnetic patterns that suggested the planet's rotation is different in the northern and southern hemispheres.
Wikipedia has a detailed description of the interior; I assume this is the accepted view:
Examination of Saturn's gravitational moment, in combination with physical models of the interior, has allowed [estimates] on the mass of Saturn's core. In 2004, scientists estimated that the core must be 9–22 times the mass of Earth, which corresponds to a diameter of about 25,000 km.This is surrounded by a thicker liquid metallic hydrogen layer, followed by a liquid layer of helium-saturated molecular hydrogen that gradually transitions to a gas with increasing altitude. The outermost layer spans 1,000 km and consists of gas.
After reading the wikipedia text, this article's figure with a mechanical brake (!) is worthless other than pointing out there are different rates of rotation.
I honestly did not know radio emissions from the planet are used to determine the rotation.
If the LMH layer is emitting in radio with a periodic fluctuation that period might not be due to rotation.
These observations indicate there is not a clear understanding of Saturn including its internal structure and heat transfers within it, to even its rate of rotation of the body or its two hemispheres.
I assume this blog from AGU is reputable as a source.
However, detail from Cassini is lacking observations from the hemispheres suggesting which rotation rates occur where.
Saturn data for reference:
Equatorial radius = 60,268 km (37,449 mi)
Sidereal rotation period = 10h 33m 38s
Equatorial rotation velocity = 9.87 km/s (6.13 mi/s; 35,500 km/h)
I wondered how Saturn's rate of rotation was calculated before this recent discovery by Cassini.
From an unrelated January 2019 story titled 'Waves in Saturn's rings give precise measurement of planet's rotation rate' (nothing to do with a brake) I found:
The researchers studied wave patterns created within Saturn's rings by the planet's internal vibrations. In effect, the rings act as an extremely sensitive seismograph by responding to vibrations within the planet itself.
Similar to Earth's vibrations from an earthquake, Saturn responds to perturbations by vibrating at frequencies determined by its internal structure. Heat-driven convection in the interior is the most likely source of the vibrations. These internal oscillations cause the density at any particular place within the planet to fluctuate, which makes the gravitational field outside the planet oscillate at the same frequencies.
"Particles in the rings feel this oscillation in the gravitational field. At places where this oscillation resonates with ring orbits, energy builds up and gets carried away as a wave," explained a student in astronomy and astrophysics at UC Santa Cruz.
In my opinion I have trouble directly relating waves in the thin rings (where energy builds up before being carried away) to changes in the gravitational field from the planet. Saturn is thought to have a solid core surrounded by layers of liquid hydrogen, with density decreasing with radius. Can a rotating liquid within a sphere have a varying gravitational field based on its rate? This sounds like a resonance of sloshing to vary the field but it also sounds unconvincing. I doubt varying the density of a liquid but not its mass will vary the gravitational field.
Earth has large oceans which rotate with the planet but probably cause no daily change in its gravitational field. Saturn's layers are certainly different. Our oceans are not affected by heat convection cycles from the sea floor, which is part of the Saturn story. Our oceans are heated by the Sun. The solar energy is not mentioned with Saturn where it would first affect its gaseous atmosphere; its affect on the outer liquid layer is not described.
In this description Saturn is heated from within not by the very distant Sun.
'Heat-driven convection in the interior as the source of vibrations' almost implies a brown dwarf.
Perhaps this post including different references to compare different observations might cause confusion.
I find these observations are interesting, though astronomers are still working on it; the attached is from this week.
Perhaps EUT can offer constructive comments, and 'make sense of this.'
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