Re: Dirty Snowball Worlds

Logan Kearsley wrote:
> On Feb 18, 1:48 am, Bryan Derksen <bryan.derk...@[EMAIL PROTECTED]
> wrote:
>> A while back I read the article "Simulations of the Atmospheres of
>> Synchronously Rotating Terrestrial Planets Orbiting M Dwarfs:
Conditions
>> for Atmospheric Collapse and the Implications for Habitability" by M.
M.
>> Joshi, R. M. Haberle and R. T. Reynolds
>> (<http://dx.doi.org/10.1006/icar.1997.5793>)
and IIRC the simulations
>> they used suggested that as soon as the antistellar pole of a
>> tide-locked world got cold enough for carbon dioxide to freeze out the
>> atmosphere would permanently "collapse" into a frozen state because all
>> the greenhouse gas that could potentially warm it back up again would
>> get trapped there.
> 
> This does not make sense to me. If CO2 were the only component in the
> atmosphere, then it makes sense, but if there's anything else around
> with a lower freezing point, it will still be possible for the
> remaining atmosphere to move heat from the lightside to the darkside.

If it is cold enough for CO2 to start freezing out on the antistellar 
pole, then the atmosphere is already demonstrably not capable of 
sufficient heat transport thaw frozen CO2 back there.

The point the paper was making is that once the antistellar pole's 
temperature drops that far for whatever reason it's a runaway process. 
All of the CO2 freezes out, the planet loses its greenhouse gases, and 
barring something weird like a big increase in the remaining 
atmosphere's thickness the temperature stays down permanently. In order 
to have any CO2 in the atmosphere you need to make sure the planet's 
antistellar pole remains above the freezing point of CO2.