Re: Dirty Snowball Worlds

On Feb 18, 1:48 am, Bryan Derksen <bryan.derk...@xxxxxxx> wrote:
Logan Kearsley wrote:
sigidu...@xxxxxxxxx wrote:
On Feb 17, 11:03 am, Logan Kearsley <chronosur...@xxxxxxxxx> 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.
.

Relevant Pages

  • Re: Dirty Snowball Worlds
    ... in deep freeze. ... higher CO2 pressure just causes more CO2 to precipitate out on the ... 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 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. ...
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  • Re: Dirty Snowball Worlds
    ... atmosphere would permanently "collapse" into a frozen state because all ... If it is cold enough for CO2 to start freezing out on the antistellar ... All of the CO2 freezes out, the planet loses its greenhouse gases, and ... Even if there's a dry ice cap at the antistellar pole, ...
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