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Posted by Peter Munn on December 13, 2007, 9:21 am
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Leafing through alt.sci.planetary, I read David Williams's message of
Mon, 3 Dec 2007:
[Sky&T quote:]
>-> But what's fascinating is that the D:H ratio is even higher (2½ greater)
>-> in the planet's upper atmosphere, 50 to 60 miles up. Why so? One
>-> possibility is that Venus is sweeping up water from passing (and
>-> colliding) comets, and cometary water has a huge excess of deuterium.
>-> The problem is that the atmosphere has far less water vapor on top than
>-> it does lower down — a puzzling result for which investigator Jean-Loup
>-> Bertaux and his experiment team have no explanation."
[robert casey:]
>-> Maybe I'm missing something, but isn't the upper atmosphere the place
>-> where the planet loses the water/hydrogen and oxygen? And wouldn't the
>-> heavy hydrogen (deuterium) have a better chance of staying around then
>-> the light (regular) hydrogen)? So the D:H ratio would be higher in the
>-> upper atmosphere. As it is, both hydrogen and deuterium are escaping,
>-> but the regular hydrogen escapes faster. SO that would be why water
>-> vapor is scarce up there. Yes?
I don't follow the article's reasoning as you quoted it, Robert, for
similar reasons. My first guess would be that something scientifically
crucial, that underpins the original scientific report's analysis, is
missing. (If Sky & Telescope are responsible, I'd presume they did it
unwittingly with the intent of avoiding complexity on behalf of their
readers). Anyway, I follow your rationale for being puzzled, Robert,
but can't resolve for you.
[dow:]
>It depends on how rapidly the hydrogen gets mixed in the atmosphere. If
>the atmosphere is essentially stationary, with no vertical currents,
>then the loss of "normal" hydrogen into space would cause a higher
>relative concentration of deuterium than at ground level. But if the
>atmosphere is vigorously stirred, which would seem to be the case since
>there are strong winds, then there shouldn't be much variation in the
>isotopic concentrations.
But if the vigorous stirring does not mix the atmosphere thoroughly by
altitude, which would seem to be the case since we are reading of
measured substantial differences, then how does the rate at which
hydrogen gets mixed in the atmosphere provide the answer to Robert's
questions?
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