Re: Dirty Snowball Worlds

On Feb 18, 6:12 pm, Bryan Derksen <bryan.derk...@[EMAIL PROTECTED]
> wrote:
> 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.

It's demonstrably not capable of sufficient heat transport to thaw
frozen CO2 given not-necessarily-permanent current conditions. That
doesn't mean it can't warm up again.

> 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

Or something not so weird, like sunspot activity decreasing, thus
increasing insolation. Or dust condensing out of the atmosphere- a
particularly large volcanic eruption might temporarily freeze the
planet. And I don't know how stable it would be, but I can imagine a
cyclic situation where an evaporating ice cap could induce planet-wide
dust storms such as are seen on Mars, which then cool the planet and
cause the cap to re-condense, after which the dust settles and it
starts to warm up again.

> to have any CO2 in the atmosphere you need to make sure the planet's
> antistellar pole remains above the freezing point of CO2.

Even if there's a dry ice cap at the antistellar pole, there will
still be CO2 in the atmosphere- just not very much, with the pressure
set by the vapor pressure of CO2 over the ice cap. And other gases can
still remain.

-l.