Re: Terraforming Mars piecemeal

On 2009-10-04, Greg Goss <gossg@xxxxxxxxx> wrote:
The pressure fades out pretty quickly.

Not on Mars with its 0.38 gee gravity. Scale height is inversely
proportional to gravity, so breathable air of similar composition and
temperature would extend 2.6 times higher. Earth's scale height for
dry air is about 8 km, so on Mars it would be more like 20 km altitude
for the pressure to drop to 1/e of the surface pressure.

The wall itself needs to be many (at least 10) scale heights high,
else there will be massive rates of loss over the top into the
comparative vacuum outside. Even at 10 scale heights (~200 km), there
will be significant outflows of air. The density would only be on the
order of 50 mg/m^3, but near-sonic flow adds up to a rate on the order
of a billion tonnes per year for each kilometre of wall.

A 3000 km diameter region would have a circumference of 10000 km, so
at 10 scale heights something like 10 trillion tonnes/year would be
lost out of a 200 trillion tonne reservoir. So in that case it would
have to be completely replenished over a timescale of about 20 years.
It would probably be much cheaper to build the wall taller than 200 km
than to replace the lost air.


So you have an angled set of anchors a mile up, and another set five
miles up and then support the rest of the wall with tension. You
wouldn't brace from the top of the wall, would you?

The upper portions probably do not need cables, maybe only from the
10% pressure mark downward. That's still about 50 km up.

5 miles up on Mars, the pressure is still about 2/3 the surface
pressure. If you stop the cables at that height, you may as well not
bother with them at all.


If the wall was built to take tension, would it collapse like a
balloon or a Mercury rocket if the air leaked, or during construction?

It would be comparatively cheap to give it at least a little bracing
against loss of pressure. If it had no such bracing then yes it would
probably collapse. A wall 200 km high is unlikely to be stable as a
free-standing structure.


- Tim
.

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