[quote=neufer post_id=301824 time=1588258904 user_id=124483]
[quote=Ann post_id=301818 time=1588224972 user_id=129702]
You said that a 2:1 Kirkwood resonance could be useful. (Or rather, you said that an orbit [b][color=#0040FF]just inside[/color][/b] a 2:1 Kirkwood resonance could be useful.) Useful in what way? Is it useful because the Galilean moons stay in their orbits thanks to their Kirkwood resonances? Or is it useful because Europa is subjected to the kind of tidal forces that might cause it to have an underground ocean?[/quote]
My own hypothesis is that just as moon induced Earth tides have caused the Moon to "social distance" from Earth over time
Galilean moon induced Jupiter tides have caused the Galilean moons to "social distance" from Jupiter over time [list][b][u][color=#0000FF]...starting with the closest Galilean moons.[/color][/u][/b][/list]
Ergo: [list]1) Io moved out until it started to approach the dreaded Europa 2:1 resonant "Kirkwood Gap" and
1) Europa moved out until it started to approach the dreaded Ganymede 2:1 resonant "Kirkwood Gap"
And Ganymede is [b][u]currently[/u][/b] in the process of moving out until
it starts to approach the dreaded Callisto 2:1 resonant "Kirkwood Gap."[/list]
Anyway, that's my take on it.
[code] The Galilean moon system
...............................................................................
Orbital radius (Gm) Orbital period (days) Mass (10 Yg)
...............................................................................
3:1 1.184 day Europa resonant "Kirkwood Gap"
...............................................................................
Io 0.4218 1.769 day 89.3
...............................................................................
2:1 1.776 day Europa resonant "Kirkwood Gap"
3:1 2.385 day Ganymede resonant "Kirkwood Gap"
...............................................................................
Europa 0.6711 3.551 day 48.0
...............................................................................
2:1 3.577 day Ganymede resonant "Kirkwood Gap"
3:1 5.563 day Callisto resonant "Kirkwood Gap"
...............................................................................
Ganymede 1.070 7.155 day 148.
...............................................................................
2:1 8.345 day Callisto resonant "Kirkwood Gap"
...............................................................................
Callisto 1.883 16.69 day 108. [/code][/quote]
[quote=https://www.syfy.com/syfywire/titan-has-had-enough-of-saturn-leaving-the-planet-100x-faster-than-expected]
Titan has had *enough* of Saturn, leaving the planet 100X faster than expected
Contributed by Phil Plait@BadAstronomer
SyfyWire, Jun 9, 2020
<<Personally, if I were orbiting Saturn, I'd want to stick around as long as possible. But Titan, however, is not me, and is hightailing it away from the planet — [b][u][color=#0000FF]receding from Saturn 100 times faster than expected[/color][/u][/b]. Not that it's exactly zipping away; it's moving at 11 centimeters per year from Saturn.
The reason it's moving away at all is due to tides. Titan is a gigantic moon, the second largest in the solar system and almost as big as Mercury (if Saturn weren't there, we'd be tempted to call it a planet in its own right). It raises a pretty decent tidal bulge in Saturn… but in some cases this whole process depends very strongly on the distance of the moon and the planet (double the distance and things slow by a factor of nearly 50). At Titan's distance of 1.2 Gm, it was thought that it would barely recede at all, only 0.1 cm per year.
For Titan they used data from the Cassini mission, which orbited the ringed planet for 13 years and passed very close to Titan well over a hundred times. Using radio data from the spacecraft, they could measure its exact position to very high accuracy, and from that determine just where Titan was.
They also used good old-fashioned astrometry; that is, Earth-based telescopic observations of the satellites (dating back to 1886!) as well as Cassini observations to measure their positions over time as well. Using these measurements for the moons, and combining the radio data for Titan, they were able to measure how strongly Saturn affects the moons through tides, and found that the distance is not nearly as strong a factor as thought in this case.
The very cool thing about this is that the two methods both independently agree with a hypothesis proposed in that earlier paper a few years ago. There, the scientists proposed that tidal forces from the moons act differently than thought for Saturn. In most cases, the tides create friction inside of a planet, which generates heat, and this gets dissipated throughout the planet, lessening the effect. But the new idea is that Titan creates a resonance inside Saturn, pumping energy into it more efficiently. In this case, the orbital motion of Titan synchs up with motions inside Saturn, increasing the efficiency of the tidal process.>> [/quote]