Re: Just how plausible was the pulp-SF solar system?

"David Empey" <dempey@xxxxxxxxxx> wrote in message
news:Xns97AFAD5B2AF29dempeycruziocom@xxxxxxxxxxxxxxxxx
Kevin <ktn3654@xxxxxxxxxxxxxxxxxxxx> wrote in
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A habitable tide-locked Mercury sounds awfully implausible to
me.
The papers by Scalo and Joshi mentioned in an earlier post deal with
Earth- sized planets that are tidally locked to their star. Since
Mercury is quite a bit smaller than Earth, I doubt it could have
retained a thick atmosphere.

Wikipedia claims Mercury has about half the mass of Mars, and about
the same surface gravity. That's 1/20 the mass of Earth. Its escape
velocity is about 4.4 km/s, compared with Mars' 5 km/s. I have no
idea how to estimate how long such a planet could retain an atmosphere,
especially in a tide-locked planet.

The Scalo paper also assumed the planets would be getting the same
insolation (instellation?) as Earth, instead of more than 6 times
as much.

Depends on what the atmosphere is made of. Ability to hold onto certain
gasses depends on the upper atmosphere temperature and molecular weight
(thus, average particle velocity), and escape velocity. After playing with a
spread***, if you fiddle with the albedo a bit, holding on to a (thin) CO2
atmosphere oughtn't to be much of a problem (minimize absorption in the
upper atmosphere, and maximize the atmosphere's efficiency at radiating in
the infrared), although I'd worry about dissociation into monatomic oxygen
and CO. It looks like you could just barely hold onto a N2/O2 mix.
It becomes significantly easier to set up if you're just looking for a
habitable temperature band rather than atmosphere and water and everything,
and of course if you allow for human engineering you could always build a
worldhouse or something.

I don't have a cite on hand, but I agree that IF Venus had the
right
atmosphere, it could be more or less human-habitable in places.
Unfortunately, all the modelling I am familiar with indicates that it
could not have such an atmosphere. The problem is that it was too
close to the Sun to retain water, which in turn meant that carbon
dioxide built up in its atmosphere, which in turn caused a runaway
greenhouse effect.

Disappointing! Oh, well, I guess I can always invoke a vanished race
of aliens, with a sufficently advanced technology, to terraform the
planet...

In which case, this page might be useful:
http://www.lunar-reclamation.org/papers/venus_rehabpaper.htm

You _might_ be able to wiggle out of that in a scenario that had
Venus
as far less geologically active than it is. Then carbon dioxide
would not have built up so quickly in the atmosphere, and a runaway
greenhouse _might_ have been avoided.

Or you could give Venus a smaller carbon inventory to start with. Or give it
more calcium to help lock up CO2 in the crust.
Or start it out with very little water, so as to avoid a runaway water vapor
greenhouse effect. You need water to help calcium to bind CO2, though....
Perhaps all three: give Venus less carbon and more calcium to start with,
and scattered shallow seas.

Mars? If you put enough air and water on Mars, could
it warm up enough to be livable? And how long would
such an atmosphere last?


Yes, it would be habitable, if the atmosphere was right. If it had
Earth-composition atmosphere the Greenhouse effect would warm it up
to the point where water would be liquid during the day but freeze
at night. Put in another 20-degrees C worth of greenhouse gases
and you have something warm but with air still breathable.

Cite? And how long would it last?

The above about Mars sounds right to me. Taking a wild guess,
I'd
say such an atmosphere would last a million years or so. Of course,
carrying out such a terraforming project would be no easy matter.

Alien space bats, again.

It could be made to last indefinitely with appropriate technological upkeep
to simulate the silicate/carbonate cycle, greenhouse gas concentrations,
etc.

As for moons of Jupiter and Saturn, I _really_ don't see how
they
could have possibly been human-habitable. Even a major greenhouse
effect seems inadequate to get the temperature up.

The idea in the pulps seems to have been energy from Jupiter, which
I don't think could work, plus heat from the moons' core--volcanic
heat, I guess--which seems rather unlikely, too, but not as certainly
false.

Here we are supposing the moons to be of similar rocky composition
as the inner planets. I realize that's not the case but I don't
think that was known in the pulp-SF era.

Well, Enceladus seems to have liquid water, so who knows?
If you allow technological intervention, though, big mirrors will do the
trick.

-l.
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