Re: Dirty Snowball Worlds

Logan Kearsley wrote:
> sigidu...@[EMAIL PROTECTED]
 wrote:
>> On Feb 17, 11:03 am, Logan Kearsley <chronosur...@[EMAIL PROTECTED]
> wrote:
>>
>>
>>> Thoughts?
>> This sounds very similar to "Snowball Earth", a theory that involves
>> super-ice-age conditions in the late Precambrian period.
> [...]
>> One other problem is that Snowball Earth would have been a temporary
>> state, lasting only some tens of millions of years.  But that could
>> probably be handwaved too.
> 
> Similar, but not identical. It needs to be a permanent condition, so
> there's at least a good billion years with large areas illuminated but
> in deep freeze. If the planet is geologically active, carbon dioxide
> will build up until it causes the Snowball Earth to melt. As I said
> for idea #2, there needs to be some reason why that sort of thing
> can't happen. I think it can be handwaved for idea #1 by saying that
> higher CO2 pressure just causes more CO2 to precipitate out on the
> antistellar cap, but that's not an option for #2.

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.

There were some interesting speculations about how stable some planetary 
atmospheres might be. For example, M dwarfs often have extreme starspot 
activity, and large persistent starspots can reduce the insolation a 
planet receives by a consider amount over a timescale of months. In some 
cases this could be enough to cause enough CO2 to freeze out on the far 
side of the planet to permanently flip it over into an iceball state. In 
the less extreme case where a planet is merely cold enough for all the 
water to permanently freeze out into an icecap on the antisolar pole, 
the lack of liquid water puts a stop to carbonate sequestration and CO2 
starts building up in the atmosphere until the icecap melts again.

I can't seem to find a full version of the article available online for 
free any more, I think I may have the PDF stashed away somewhere though 
so if you want to get ahold of it let me know and I'll see if I can dig 
it up.