Re: Probability of Earthlike conditions:

"Aaron Denney" <wnoise@xxxxxxx> wrote in message
news:slrnf5vjtu.ke3.wnoise@xxxxxxxxxx
["Followup-To:" header set to rec.arts.sf.written.]
On 2007-06-01, David Johnston <david@xxxxxxxxx> wrote:
On Fri, 01 Jun 2007 12:49:14 +1000, James A. Donald
<jamesd@xxxxxxxxxxx> wrote:

.Despite the absence of significant plant life, the atmosphere of
mars is mildly oxidizing. Presumably if was a bit warmer, it would
have liquid water on the surface. If it had liquid water, could
support plants, if plants, would have decent amounts of oxygen and an
ozone layer.

I doubt it. That's not much gravity with which to retain your air.

Gravity matters less than gravity gradient, and escape velocity.
Mars is 5 km/s vs Earth's 11.1 km/s.

IIRC, the Maxwell-Boltzmann momentum distribution drops off as
exp(-p^2), so cutting where they can escape in half actually makes a big
difference. Nevertheless, it should still be tiny, except over geologic
time scales. But enough outgassing might be possible to make up for it.

You also get great gains by lowering the exosphere temperature. Reduce the
percentage of oxygen and increase the percentage of CO2 by a large amount
relative to Earth (and, how convenient, we want to warm Mars anyway, for
which CO2 is well suited) so as to make the upper atmosphere more emissive
in infrared, and the atmosphere ought to stick around against Jeans escape
just fine. Unmodified humans would want to wear breathing masks on the
surface to avoid CO2 poisoning, though.

I think the big effect is actually the lack of a large Martian
magnetosphere, so the solar wind can directly affect the atmosphere, and
remove it.

There's also some question of sequestration in the polar caps.

-l
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