On Feb 18, 1:48 am, Bryan Derksen <bryan.derk...@[EMAIL PROTECTED]
> wrote:
> 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.
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.
> 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.
If it's cold enough that CO2 precipitates, it must also be cold enough
for water to permanently freeze out on the darkside (unless glaciers
creep over the terminator and the lightside is still warm enough to
melt them back into a sea), thus stopping sequestration. What I'm
hoping in this case is that dry-ice precipitation would provide a new
sequestration mechanism to keep the CO2 content of the atmosphere
right around the vapor pressure of CO2 at darkside temperatures.
> 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.
I've got a copy. It's not quite ideal, because all of the simulations
are of planets with orbital periods on the order of days (being colder
and farther out, I expect my alien world to rotate much more slowly),
and they're looking mainly for situations that would produce a zone of
human-comfortable temperatures, but still a useful reference for
guessing at temperature gradients and whatnot.
-l.
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