Dorothy J Heydt wrote:
No. Venus's atmosphere is way the heck too thick, and a reducing
atmosphere besides.

It's thick now. It's far from certain that it was thick to begin with.
Carbon dioxide may well have outgassed from rocks as the heat began to
build. In other words, it may have been a case of positive feedback.

And the Earth *had* a reducing atmosphere to begin with. It was partly
the introduction of life that didn't depend on molecular oxygen and
carbon dioxide that helped to convert the reducing atmosphere into an
oxidizing one.

The reason Earth has such a thin
atmosphere compared to that of Venus is that quite early on,
while the solar system was still in the final stages of assembly,
proto-Earth was hit by a Mars-sized planetoid. There was molten
rock flying all over the place; part of stony exterior of the
planetoid went into orbit, regained a spherical shape, and became
the Moon, while its heavy iron core sank into the Earth-mass and
amalgamated with the Earth's core. In the process, a large part
of Earth's primordial atmosphere was blasted away, leaving us
with a much slighter greenhouse effect than Venus has.

That's all plausible, but it's far from certain. I mean it's a pretty
good bet that the Moon was created in such a cataclysmic collision, but
it's not clear what that effect that would have had on the atmosphere.

See above. Maybe, but not as likely. Chlorine, for instance,
has sometimes been suggested, or silicon; but it's all
speculation.

Silicon, sure, but chlorine? How does one form chains with the single
valence chlorine atoms? I can see some kind of organic molecule (and
by organic I mean life-sustaining) involving chlorine, but I don't see
how it can be based on chlorine. Do you have any details?

--
Brian Tung
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