From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: NASA Experiment Lays Groundwork For 'Living Off The Land' On Mars
Date: Sun, 28 Mar 1999 04:45:12 GMT

In article <7dh9uc$mq9$2@nntp.ucs.ubc.ca>, pete <VINCENT@reg.Triumf.CA> wrote:
>...Why use the very low density atmosphere to generate
>the O2? Seems to me you'd get a lot more bang for the buck by
>cracking the oxides in the soil, probably requiring only the
>application of heat...

Several problems.

First, it is generally much easier to handle a fluid (gas or liquid) than
a solid.  The Martian air is thin, yes, but compressors are not complex or
hard to build -- they're much simpler than the sort of machinery that
tends to be needed to handle solid materials in bulk, and they're much
less prone to wearing out or jamming.  (Indeed, the solid content of the
air -- suspended dust -- is probably the biggest problem there.)

Second, it's actually quite difficult to get oxygen out of metal oxides
(which will be what the soil has).  Mere heat does not do the job unless
the temperature is very high indeed.  Chemical processes will work, but
tend to involve their own complications, especially when dealing with a
complex mixture of compounds.  (For example, one way is to add hydrogen
and heat, giving water, which you then electrolyze to give oxygen and get
back the hydrogen.  Trouble is, if there are any sulfides in the soil, you
get not water, but dilute sulfuric acid.)  People have looked at this
quite extensively for lunar oxygen production, and it's a hard problem.

And third, overwhelmingly important, is that we simply do not know very
much about the composition and physical structure of the Martian soil, so
it is very difficult to design processing machinery for it, especially
since it probably isn't the same everywhere.  The atmosphere *is* the same
everywhere, and its composition is definitely known in great detail.  So
a first-generation fuel-making system can be designed with much greater
confidence if it uses the atmosphere.

>I would think that to get a reasonable volume
>out of the atmosperic CO2, you'd either need a very large device,
>or a very long time.

It's not that bad.  You need to process about 100 cubic meters of the
atmosphere to get a kilogram of CO2.  That's not great, but it's not a big
problem either.  A compressor for a small experimental unit might pass a
liter of atmosphere per second (just guessing), which is a bit under a
kilogram a day.  That's probably plenty; electrolytic processes need lots
of power and that will be a limiting factor.  Scale that up by a factor of
ten and you've got something that can fuel a small sample-return vehicle
in a month or two.
--
The good old days                   |  Henry Spencer   henry@spsystems.net
weren't.                            |      (aka henry@zoo.toronto.edu)