Convection -- Forest Fires on Exoplanets ??

[Widdekind]

According to Wikipedia, the Raleigh Convection Index, for the onset of natural convection in systems heated from below (as w/ air heated from below by burning materials), due to Thermal Expansion, is:

 

[math]Ra = \frac{\alpha}{\kappa \, \mu} \rho g \Delta T L^{3} = \frac{k}{\rho \, c_{p}} \frac{1}{\kappa \, \mu} \rho g \Delta T L^{3} = \frac{k \, \Delta T}{c_{p} \, \kappa \, \mu} \times g L^{3}[/math]

 

Now, for "forest fires" on an hypothetical exoplanet, would not the characteristic convection length scale (L) basically be that world's Atmospheric Scale Height (Wiki.) ??

 

[math]H = \frac{k_{B} \, T}{\bar{m} \, g}[/math]

 

If so, an exoplanet's convection score for "forest fires" would be:

 

[math]Ra = \left( \frac{k \, \Delta T}{c_{p} \, \kappa \, \mu} \right) \left( \frac{k_{B} \, T}{\bar{m}} \right) \times g^{-2}[/math]

 

Isolating the effects of the exoplanet's Surface Gravity (essentially assuming similar atmospheric conditions & chemical compositions), Ra decreases as g^-2.

 

QUESTION: Is it realistic, then, for Science Fiction (say), to claim that conflagrations are somewhat suppressed upon bigger planets, but increasingly enhanced, on smaller worlds ?? To wit, that fires affecting an outpost (say) upon a Terraformed Mars (g = 0.4) might be half-a-dozen times more prone to forming full-fledged firestorms, as opposed to those upon this particular planet ??

[steevey]

Convection can sometimes be impacted by gravity. The Earth is big enough where it doesn't need gravity to put pressure on the core and heat it up, but smaller planets don't have that. But, if they are close enough to a big planet like Jupiter (making them a moon) or a star, then the gravitational effects would cause plates to rub together and create massive amounts of friction and possibly cause convection if the core is heated enough and the outer layers heat up enough. These gravitational effects can be seen on Jupiter's moon Io.

Edited February 7, 2011 by steevey

[swansont]

Convection can sometimes be impacted by gravity. The Earth is big enough where it doesn't need gravity to put pressure on the core and heat it up, but smaller planets don't have that. But, if they are close enough to a big planet like Jupiter (making them a moon) or a star, then the gravitational effects would cause plates to rub together and create massive amounts of friction and possibly cause convection if the core is heated enough and the outer layers heat up enough. These gravitational effects can be seen on Jupiter's moon Io.

 

And the relevance of this to forest fires is … ? (and I'd ask if not gravity, what puts pressure on the core? But that's not for this thread)

[steevey]

And the relevance of this to forest fires is … ? (and I'd ask if not gravity, what puts pressure on the core? But that's not for this thread)

 

I was explaining the circumstances for convection for any chance of having any forest fire in the first place. I mean, there's a reason why people in sci-fi don't right about volcanically active planets the size of Pluto, and its because they aren't in real life. He's basically asking people to confirm that. Maybe he saw a movie where a forest fire occurred on a small isolated planet and the cause was convection. That wouldn't happen in real life. Not enough pressure on the core of the planet to cause massive heating up and moving plates which if you look closely at my first post, answers your gravity question. Smaller planets don't produce as much pressure on the core, so generally when smaller planets are isolated, they don't have an active core.

Edited February 7, 2011 by steevey

[swansont]

The topic is atmospheric convection, not geologic convection.

[steevey]

Well if the planet is too small, it will not be able to hold atmosphere. If it is small but has atmosphere, then highly heated atmosphere might escape from its gravity.

Edited February 8, 2011 by steevey