Effect of axial tilt on a planet

[Desdichado]

2)The Moon is tidally locked. You don't get friction heating out of tidal forces unless the forces change. The heating comes from repeated/continuous flexing of the body. The forces on a tidally locked body are basically static, and cause no new flexing, and thus no heating.

I don't see what that has to do with anything. Io is tidally locked too.

 

[Umbran]

I don't see what that has to do with anything. Io is tidally locked too.

Ah, but there Jupiter isn't the only body to consider in that instance. Ganymede and Europa put massive forces on Io. Io's surface is regularly moved up or down by up to 100 meters by the tides it is subjected to, and that will generate huge amounts of heat.

 

[Desdichado]

Hmmm... that's true. Still, there is evidence of past (and relatively recent) volcanic activity on other worlds that are not subject to tidal forces or plate tectonics, notably Venus and Mars. It's debatable how geologically "active" those worlds are today, but clearly they have been in the past.

Also, Io apparently radiates more heat than tidal forces would generate -- one of the genuine open issues and mysteries surrounding it. I don't know what that means for your claim, but clearly it's a complication.

 

[Desdichado]

actually, on the albedo scale... (highest to lowest)


clouds
ice
un-vegitated land
vegitated land
open water

...geology 101.

Then why does Enceladus have a higher albedo than Venus?

 

[lgburton]

Then why does Enceladus have a higher albedo than Venus?

i should clarify:
earth albedo scale. and note that the clouds i'm talking about are water, rather than the co2/hydrogen sulfide and other hydrocarbons that make up venus' atmosphere.

 

[Umbran]

I don't know what that means for your claim, but clearly it's a complication.

I'm a physicist, so I'm quite aware that nothing is "spherical horse in a vacuum" simple. One can usually find a work-around for any one thing that is "required". However, that doesn't change the general trend.

A sphere of molten rock floating alone at a life-sustaining distance from a star, without a huge excess of radioactives in it (or some other mechanic for heating), is going to cool surprisingly quickly. That is simple thermodynamics. It will not stay tectonically active forever - Mars being a pretty darned good example. Barring external effects, it becomes a simple rock. Simple rocks eventually lose their atmospheres.

 

[lgburton]

I'm a physicist, so I'm quite aware that nothing is "spherical horse in a vacuum" simple. One can usually find a work-around for any one thing that is "required". However, that doesn't change the general trend.

A sphere of molten rock floating alone at a life-sustaining distance from a star, without a huge excess of radioactives in it (or some other mechanic for heating), is going to cool surprisingly quickly. That is simple thermodynamics. It will not stay tectonically active forever - Mars being a pretty darned good example. Barring external effects, it becomes a simple rock. Simple rocks eventually lose their atmospheres.

however, if we're going to play with the laws of thermodynamics, then EVERYTHING gets cold and becomes a "simple rock" eventually. entropy is fun, isn't it?


now, is your thought experiment wrong? nope, dead on. but how is it relavent to the point? consider that the moon is not the only thing keeping the earth hot, and the tidal stresses that it does exert are not enough to cause excessive stress (such as the above mentioned case of io). you say "a huge excess of radioactives," but what does that really mean? how much is a huge excess?

so, another thought experiment might be worthwhile:

consider two spheres, one larger, one smaller, of similar composition.

how does a moon effect each body? if each body were given a moon that were exactly proportional (say 1/10 of the mass of the body in question), how would that effect the rate of cooling? would they be similarly proportional?


so, i guess my question for you is: what is the point you are trying to prove? having gone through your posts, i'm not sure what hypothesis you are forwarding. are you saying that a moon or other large tidal-stress causing mass must be affecting a planet for it to have volcanos? olympus mons not withstanding, of course... (or pavonious, or any of the other volcanos of that particular bulge, since they were most likely caused by the great martian northern hemisphere impact.)

 

[S'mon]

actually, on the albedo scale... (highest to lowest)


clouds
ice
un-vegitated land
vegitated land
open water

...geology 101.

As I understand it from Geography class, clouds reflect much visible light, so a cloud-covered planet is bright. But lots of visible & ultraviolet light still passes through the cloud layer.

Land then absorbs the high wavelength light and radiates it at lower wavelengths (heat), so if the sky is cloudy a lot of heat is reflected back off the clouds and trapped in the system. Ergo a dry cloudless planet is cold (Mars), but a dry cloudy planet gets very very hot (Venus).

Is that right?

 

[Umbran]

however, if we're going to play with the laws of thermodynamics, then EVERYTHING gets cold and becomes a "simple rock" eventually. entropy is fun, isn't it?

Quite correct. Luckily for us, there are types of stars for whom "eventually" is a long way off yet.

you say "a huge excess of radioactives," but what does that really mean? how much is a huge excess?

so, another thought experiment might be worthwhile:

Both of these are calculable, at least as a rough approximation. I can perhaps take a stab at them later this evening.

so, i guess my question for you is: what is the point you are trying to prove? having gone through your posts, i'm not sure what hypothesis you are forwarding.

Have you considered the possibility that I am not forwarding a particular hypothesis so much as fostering discussion and correcting the occasional inaccuracy or incomplete picture?

While there's been a bit of topic drift, the point seems to have been to do world design with at least some nods to current scientific thought. In order to do that, some of that needs to be available, no? Doesn't discussion of the individual facets tend to help reveal the factors the designer may wish to consider?

 

[lgburton]

Quite correct. Luckily for us, there are types of stars for whom "eventually" is a long way off yet.

viva la red dwarf main-sequence stars!

Both of these are calculable, at least as a rough approximation. I can perhaps take a stab at them later this evening.

neet - i'm actually very interested in seeing the difference. the entire idea of an orbital body keeping a planet tectonicly active interests me... what % of the larger body's mass is needed for a moon to keep a planet viable? how would this relate to the pluto/charon system, since charon is a signifigant % of pluto's mass? hmm...

Have you considered the possibility that I am not forwarding a particular hypothesis so much as fostering discussion and correcting the occasional inaccuracy or incomplete picture?

well, yes, but there has been a logical consistancy with all of the points you've made, so i'm wondering what theoretical model you are working with, if at all? if not, hey, it's a good question to get you started making one!

a lot of my reaction is in the very question of tectonics: you're calling plate tectonics a symptom, rather than a cause - but no really good theoretical models have managed to explain why, of all the rocky worlds, earth seems to be the only one to exhibit plate tectonics. (though there is one other example in the solar system - europa's ice sheets seem to work similarly). so, could it be lunar tidal stress that causes earth's surface and venus' surface to work so very differently? sure.. but how?

While there's been a bit of topic drift, the point seems to have been to do world design with at least some nods to current scientific thought. In order to do that, some of that needs to be available, no? Doesn't discussion of the individual facets tend to help reveal the factors the designer may wish to consider?

totally, and that's one of the reasons i got involved in the discussion . mostly i'm replying to you with questions i don't know the answers to, and seeing if you can come up with something that makes sense.

mostly, your answers make sense, and when they don't i'm more prodding you with my questions trying to get a different kind of explanation, so that I can understand where you are coming from, rather than trying to goad you into a flame war. (despite my overall obnoxious and combative tones... sorry).