Physical question

[Thilo]

I have heard several times that Io was only a crust of ice, but down under io there was water, because of the gravitation of Jupiter the moon is pounded, thus warmth is being generated.

But energy in any form, even as warmth does not come out of the blue - somewhere energy has to be used up. Won't Io crash soon on Jupiter then?

[julesstoop]

Yes, it will lose energy and one day crash into Jupiter's atmosphere. But it might still take some score of millions of years to do so.

Actually, something strange just pops into my mind; the distance from moon to earth - I heard - is actually increasing because of these same tidal forces?

Would someone care to explain this. Does it have to do with the fact that in the earth-moon case the planet (earth) is actually losing most of the energy, in stead of the moon, as in the Jupiter-Io case?

[Guest]

I have heard several times that Io was only a crust of ice, but down under io there was water, because of the gravitation of Jupiter the moon is pounded, thus warmth is being generated.

Nah, that's Europa... Io is the one with nasty 500 degree sulphur volcanoes.

Actually, something strange just pops into my mind; the distance from moon to earth - I heard - is actually increasing because of these same tidal forces?

Yes, the Earth-Moon distance IS increasing - in a few million years we won't get decent solar eclipses anymore. Don't think it's do with tidal forces though but I could be wrong.

Would someone care to explain this. Does it have to do with the fact that in the earth-moon case the planet (earth) is actually losing most of the energy, in stead of the moon, as in the Jupiter-Io case?

Not sure - do you mean the orbit of Earth (i.e. its velocity) is being altered, as is the case with gravitationally-slingshotting probes away from it ? If so, I wonder how much of an orbit change the Earth would experience when the moon finally leaves....

Perhaps someone more knowledgeable could give a better answer...

[High Dark Templar]

I hope this will enlighten the common mistakes about orbits evolution.

First one: the energy is ever constant but this means the energy of all COMPLETE system, in the case of a moon and planet It means:

Moon Rotational E. + Planet Rot. E. + Moon-Planet system rotation E. = Constant

(E. = Energy)

If one sumand is getting smaller then other must be bigger.

Second one: Tides slow down or fast up the rotation of a moon or planet depending of wich one is faster than the other. If the planet is faster then the tides slow the rotation and viceversa.

This means that in the case most common: Io, Europa, Earth moon, etc. The distance of this moon to its planets is getting bigger because the tides they generates in the planet overtake the rotation of the moon "stealing" the rotational Energy of his parent planet and increasing the potential E. of the moon and its own orbital distance. The planet does the same on the moon but more quickly (because it has more mass) slowing the moon "day" and increasing the moon planet distance.

This will happend until the moon day and moon orbital period are the same, locking one moon side to the planet ( case of all near moons: earth moon, Io, europa, ganymede.... ) But the moon continues reducing rotational speed of the planet and increasing its own orbit until the total lock of the system, the lock of the planet too ( case of Pluto and Charon) at this point there is no more evolution. (Earth day is getting longer; 900 millions years ago the day only had 18 hours and the moon was much more near of the Earth: Today the moon orbit is getting bigger by 3cms/year )

The other possible case is Triton and Phobos. Triton rotates arond Neptune in the "wrong" way and Phobos rotates faster than Mars. This produces that they overtake the tides in its planet, so the planet rotates fast and the moon losts potential energy reducing its orbit and eventually hitting the planet (case of small phobos) or breaking into parts in the Roche's limit (case of big Triton).

So the energy to moltening interior of Io and Europa are from the lost rotational energy of Jupiter this only happend when the tidal force are extreme because the proximity to the planet and the huge planet mass (don't worry Jupiter is very massive so don't sense this lost of rotational speed very much).

Almost all increase their distances of his parent planet except the counter rotation moons and the very close moons. (There are complex resonancies between the diferents moons of a planet too but this is another task to explain in other post)

I hope this could be usseful for all of you. I am working hard in the planet generator add-on for Celestia and the Tidal lock is a hard point to take into account in the planets and moons generation.

If anybody have any doubt about this or want all the formulae ask me here or directly by messenger.

Sorry for my bad english.

[selden]

Of course, the planet+moon system isn't really closed. There are also solar tides to mess things up, too.

[julesstoop]

Thanx High Dark Templar, but still I oftenly heard that Io is going to 'lose it', and will be devoured by Jupiter. Could that have anything to do with other kinds of friction? Like Jupiter's upper atmosphere, or the ionized gas bridge between Io and Jupiter, or any other electric or magnetic force?

[HankR]

My understanding is that in the case of Io, orbital resonances with Ganymede and Europa are the critical factor, as they maintain the eccentricity of Io's orbit, which is necessary for tidal heating.

- Hank

[Mikotaj]

I hope this will enlighten the common mistakes about orbits evolution.

First one: the energy is ever constant but this means the energy of all COMPLETE system, in the case of a moon and planet It means:

Moon Rotational E. + Planet Rot. E. + Moon-Planet system rotation E. = Constant

(E. = Energy)

...

Is it correct for the "star - planet" system (Sun - Earth)? If yes, what forecast for the future?

[High Dark Templar]

The last question is the first one I must answer


Is it correct for the "star - planet" system (Sun - Earth)? If yes, what forecast for the future?

Yes it is correct for all kind of systems in fact mercury is tidaly locked to the sun but its high excentricity makes a complex tidal lock with a resonance of 3/2. That is with a excentricity of 0 (circular orbit) we obtain a tidal factor of 1 (complete locked one side ever facing the sun) but when the excentricity is high we have a resonance between orbital an rotational period that means:

excentricity day/tear ratio

0.0 1/1
0.21 3/2
0.39 2/1
0.57 5/2
0.72 3/1
0.87 7/2

In case of earth is more complex we have here the tidal force of the sun and the tidal force of the moon the combination makes the moon the predominant factor so... we will lock with moon ( in 15000 mill years ) too late the sun will be a red giant in 5000 mill of years. mars is too far from the sun to be locked but Venus is partially locked with a very long day and a complex resonance with the earth this persuade the total lock.


Second One about the resonance in Jupiter, the three firsts big Moon: Io Europa and Ganymede are locked together by resonance

L io + 3 L europa + 2 L ganymede = 180?

L.- enlongation

This means that these trhee monns can't be in the same face of Jupiter at the same time, and what more? the most afected is Io its orbit can't never be perfectly circular to be total lock with Jupiter and it excentricity varies making in a long future a probably encounter with Jupiter this means the Jupiter moon system is intrinsically inestable but in very long time periods. Sorry none of us are going to see the crash of Io in Jupiter

I hope this will aid more to the understanding of this complex phenomenon.

Sorry for my english

[Thilo]

High Dark Templar: Thanks for you explanation.
How do you know all this .. are you studying astrophysics? :)

[High Dark Templar]

I am not an astrophysics student, but I like very much astrophysics. I am Naval Architect and Oceanic engineer also renewable energy systems technician. I had learned lot of physics in my years studying so it is easy to apply the same knowledge to astrophysics... The physic involved to build ships are the same that rules the skys and planetary motion.

I hope this will answered your question Thilo. I usually read astrophysics books too and I make a bit of research for my own (specially now with the real planet generator for Celestia).