Explosive atmospheres.

[Cropduster23]

The atmosphere of Neptune is 80% hydrogen. How would one go about setting the whole planet on fire? More to the point, would it not just explode? That is, assuming that Neptune is just gas all the way down...which seems reasonable.

 

Also, would not a flaming Neptune have more kinetic energy then a not-on-fire Neptune? If so it would definitely change its orbit.

 

Clearly this is an absurd and mostly humorous thought experiment about the violent thermodynamic destruction of an entire gas giant (no Death Star required, although it seems to me that the best way to do such a thing is indeed with a giant laser), and as such I've posted it in speculations.

[insane_alien]

The atmosphere of Neptune is 80% hydrogen. How would one go about setting the whole planet on fire? More to the point, would it not just explode? That is, assuming that Neptune is just gas all the way down...which seems reasonable.

 

you need an oxidizer to burn it. neptune does not have an atmosphere rich in both reducing and oxidizing gases, just reducing gases.

 

hydrogen by itself is not explosive at all. remember the fire triangle, you need oxidiser, fuel and heat. it applies to explosives as well.

 

on neptune you have plenty of fuel and it has lightning so you have an ignition source for heat but little to no oxidiser. so the triangle is broken.

 

Also, would not a flaming Neptune have more kinetic energy then a not-on-fire Neptune? If so it would definitely change its orbit.

 

nope, no change to the kinetic energy, but it'd have a lot more thermal energy. this doesn't change the orbit though.

 

Clearly this is an absurd and mostly humorous thought experiment about the violent thermodynamic destruction of an entire gas giant (no Death Star required, although it seems to me that the best way to do such a thing is indeed with a giant laser), and as such I've posted it in speculations.

 

i don't think the planet would be destroyed as much as the death star would destroy it. a lot of the material would remain (although, a lot will also be blown off).

 

i think its safe to say that neptune would drasticly reduce in size at anyrate. could be fun to simulate it. i imagine it'd go similar to simulations of supernova.

[Cropduster23]

no change to the kinetic energy, but it'd have a lot more thermal energy

 

but I thought that radiation conveys inertia...and that two objects of exactly equal mass would begin to differ in their mass if one of them was suddenly increased in temperature?

 

Such a mass increase would imply an acceleration of some sort in another reference frame? Help me out here.

[insane_alien]

well, thats only if the heat is from an external source, but in the case of a burning planet, the energy was already there in the form of chemical potential energy, it has merely underwent a conversion to thermal energy.

 

and when this energy is lost to thermal radiation, the ratiation will be essentially the same in all directions resulting in no net force on the planet to accelerate it.

[Sisyphus]

I don't think you'd even lose much mass, actually. The temperature would increase, so I guess the radius of the atmosphere would increase a bit. And then what? The combustion products would still be there. You would lose a relatively small amount of mass as the extra heat radiated away, but couldn't that take millions if not billions of years?

[insane_alien]

well, the velocity of expansion could be quite high, throwing of a shell of gas, it won't be much but it could definitely happen

[Cropduster23]

Well it seems to me that the gravitational potential of the planet will decrease, because the mass of the planet will have decreased, as some of it's energy has been lost by the thermal radiation.

 

But I guess my confusion comes from my understanding that gravity and the potentials it forms in space-time is a fictitious force, wholly dependent upon relative acceleration between reference frames, as is elucidated in the Einstein equivalence principle. From this I reasoned that when an object changes mass through emission and absorption, it still must "accelerate" into some dimension, and in the case of a body like Neptune where it's radiation is emitted into all 3 space-like dimensions (and thus doesn't move at all in those three) then perhaps that "acceleration" is into a complex space-like one, where the relationship is something like

 

[math]\frac{\delta S}{\delta p}=\frac{\delta v}{\delta T}[/math]

 

where S is the total entropy of Neptune,

p is the average momentum of the thermal photons emitted (deBroglie relationship),

v is the "velocity" of Neptune through such an extra space-like dimension,

T is the average temperature of Neptune associated with it's entropy,

and the derivatives are all with respect to time.

 

I suppose that any hypothetical analysis such as this would not necessarily translate to real velocity differences in 3-space, so the orbit of Neptune would not be effected by the homogeneous burning of it's atmosphere. But, given that [math]e=mc^{2}[/math], the release of chemical potential energy as thermal energy, which then leaves the system, would in fact effect the relativistic mass. It would seem like a contradiction.

 

Does an exothermic reaction on a system affect it's rest mass? If it doesn't then that would mean the resulting black body radiation would carry no energy and so you have a sort of inverse of ultraviolet catastrophe featuring "ghost" photons. I guess this whole line of reasoning is sort of like asking if Unruh Radiation is real...but the last time a checked, Feynman's bead on a stick still heats up.

 

Am I wrong here? What am I missing?

 

EDIT: I just realized that the equation I threw out above is also directly related to this:

 

well, the velocity of expansion could be quite high, throwing of a shell of gas, it won't be much but it could definitely happen

 

where p is the average momentum of the particles of hydrogen (or whatever else is on fire), such that v is the average velocity of the thermal expansion of the atmosphere.

Edited July 8, 2010 by Cropduster23
mathematical epiphany