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From: (Henry Spencer)
Subject: Re: Mars: More practical, Water or CO2 for fuel?
Date: Mon, 3 Jul 2000 05:17:02 GMT

In article <>,
Michael J Wise  <> wrote:
>And some time after that, with all the CH4 and H2O in the upper-upper
>atmosphere, on a planet with less than half the gravity of Earth, a lot
>of those gasses will be acted upon by the solar wind, and blown gently
>away into oblivion, leaving Mars deader than it was when we got there.

Uh, molecular weight is not everything.  Water is lighter than the other
gases in Earth's atmosphere too.  Random motion is not so deterministic as
a naive look might suggest, turbulence provides very effective mixing, and
changes in temperature make a big difference too.  (The reason why Earth
has held onto its water is *not* that it doesn't tend to float up, but
rather that the stratosphere is extremely cold and the water condenses
out.  The water-vapor content of the atmosphere drops by, literally,
*several orders of magnitude* over a vertical distance of only a few
kilometers at the tropopause.)

Also, you need to think about the time scales involved.  If memory serves,
the latest assessment is that Mars could hold a relatively thick
atmosphere for geological lengths of time.  Its gravity is not *that* much
weaker than Earth's, and solar-wind erosion is not *that* rapid.  Water
loss by that route should be slow enough that if we care about it at all,
we can import the occasional comet to make up for it.
Microsoft shouldn't be broken up.       |  Henry Spencer
It should be shut down.  -- Phil Agre   |      (aka

Newsgroups: alt.sci.planetary
From: (Henry Spencer)
Subject: Re: Living on Mars
Date: Fri, 9 Apr 1999 04:31:49 GMT

In article <7ei7k7$>,
Joseph Szadai <cw819@FreeNet.Carleton.CA> wrote:
>Mars has a small mass and only relatively heavy gases like
>carbon dioxide (CO2 and Argon Ar) can be kept by its low
>gravity field.

This is less of an issue than was once thought.  It can hang onto the
lighter gases well *enough* to make it habitable for geological ages.
Long before gas loss becomes an issue, human capabilities will have
increased to the point where fixing such problems will be simple.

>It is impossible to increase the vapour (H2O) content of Mars for
>long term period if temperature is high, say 10-15 Celsius degrees..
>But H2O will be decomposed because of the radiation of the space and
>there is no way that H2 will hold...

That can actually happen on much larger planets -- that's how Venus lost
its water.  The key issue is not gravity, but atmospheric structure.
Earth does *not* lose significant amounts of water that way because its
stratosphere is extremely cold and dry:  the water content of Earth's
atmosphere drops by, literally, several orders of magnitude in a few
kilometers of altitude at the tropopause.  The water is trapped down in
the troposphere, where the solar UV mostly can't get at it, and where
any released hydrogen promptly recombines.

Can this be duplicated on Mars?  Maybe.  But again, the time scale is in
your favor.  You don't *have* to make Mars retain its water for billions
of years.
The good old days                   |  Henry Spencer
weren't.                            |      (aka

Date: Sun, 09 Jul 2000 22:16:20 -0700
From: Doug Jones <>
Subject: Re: Terraforming Venus

David Sander wrote:
> Suppose an atmosphere of Earthlike pressure and composition
> is achieved on Mars. Whoopdidoo. What I want to know is what's
> going to keep it there? I'm not talking about gravity, I'm
> talking about magnetic fields. Earth has strong magnetic
> fields, strong enough to largely keep the atmosphere from being
> blown away by the Sun. These fields protect the surface from
> violent CMEs, the ever-present solar wind, and even some
> extra-solar system particles.

There's an old saying to the effect of, "it ain't what you don't know
that hurts you, it what you do know that ain't so."  The solar "wind"
doesn't scrub gases away from planets- the primary loss mechanism is
photodissociation by UV light.  However, even highly energized oxygen or
nitrogen atoms are very unlikely to be lost from a terraformed Mars
atmosphere.  On the other hand, if water vapor reaches the top of an
atmosphere (such as on Venus) the hydrogen *can* be lost to
photodissociation because it's low molecular weight allows it to reach
escape velocity.  On the gripping hand, Earth's ozone layer (by
preventing convection in the stratosphere) preserves the planet's water
supply by trapping all the water vapor down low and keeping the
ionosphere very dry.  A properly terraformed Mars would have such an
ozone layer.

The largest loss mechanism for a terraformed Mars would probably be
ablation from large asteroid impacts, with time scales of tens of
megayears. Such impacts would be prevented by any responsible
civilization anyway.

A terraformed Mars' atmosphere would not be lost to solar wind erosion,
and a planetary magnetic field is not necessary to protect the

Doug Jones
Rocket Plumber, XCOR Aerospace

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