Re: Hottest and coldest habitable worlds...

I'm not sure I'd worry a lot about the magnetic field. As has been
pointed out, we don't seem to have a good firm grasp on how a
planetary-scale self-exciting dynamo seems to work. This is perhaps
complicated by the fact that we have only very rough long-term data
for one planet (Earth), and at best very limited snapshots in time and
space of a handful of others. We don't even know if Venus's cloud-
covered, high-albedo state is a constant, or just a current transient
state - it's hard to say much with certainty about it's magnetic field
(maybe it's going through a very low-field magnetic reversal... OK,
not likely :-).


I've always thought the dynamo theory a bit suspect myself, but it' s
the only one out there. Maybe some kind of piezolectric effect is the
alternative... who knows.

I don't think even a good strong magnetic field would help retain
hydrogen, however, as at the Venusian (Cynthian?) exobase, H would
have a Vrms of 2.7 kps, and Venus has a slightly lower Vesc than Earth
- reference previous calculation on the expected lifetime of H in the
atmosphere.

I was thinking that ionised hydrogen molecules night be retained by
the magnetic field and sent back to the poles. Something like protons
being trapped in radiation belts. That's just a naive assumption on my
part with no real research behind.

The lack of plate-style tectonics is likely due to a lack of water,
which again got cooked away - get a planet warm enough that you lose
an effective tropospheric water trap, and the water leaks up to where
photodisassociation can strip it of H, which then promptly (in a few
million years or so) escapes. End of the water, end of the oceans, and
plate subduction and melting stop because the melting point isn't
lowered by having lots of handy hydrated silicates at the subduction
zones.

--
Brian Davis

In other words, rain doesn't make it to the ground? Our problems with
the early climate of Venus are things like the planetary rotation
rate, concentration of water and so on. The water trap could still
rain out on the night side... depending on overall global temperatures
which depend on atmospheric pressure, which depends on albedo and
temperature, blah blah etc. Sounds like an n-body problem to me. It'll
be interesting to see what new discoveries are made about Venus in the
coming years.

I ran some crude models of a post-terraforming Venus once, and got
temperatures going up to and beyond boiling point for peak daytime
temps, with Earth's albedo. Atmospheric density mediated the whole
process, evening out temperature differences between equator and pole.
With a higher albedo, indicating a cloud-wrapped Venus, temperatures
got down to reasonable levels... but more water vapour aloft and
higher atmospheric pressure, coupled with

Hydrated silicates are pretty much necessary, but at what point did
plate tectonics stop - before or after? Could plate tectonics also
depend on rotation and mass? Magma's also a fluid, and as such would
be deflected by coriolis forces in a rotating body. The presence of
the Earth's moon has also been cited. All the moons with vulcanism do
so because of tidal forces, it seems.

.

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