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In 2005, some astrobiologists proposed that methanogenic life on the surface of Titan would consume both hydrogen and acetylene from the atmosphere, causing a downward gradient in the concentrations of the gases

My naive thinking would be that, the most natural reason for a downward gradient for molecular hydrogen on any atmosphere would be that, being the lightest gas that is out there, it will tend to accumulate in the upper layers of the atmosphere.

Why isn't natural buoyancy of molecular hydrogen enough to explain the observed downward gradient?

Nasa - What is consuming hydrogen and acetylene in Titan?

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My first thought was convection, but I'm finding lots of reports that hint that the bulk of the atmosphere is stable against convection. –  Chris White Dec 23 '12 at 2:22
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up vote 2 down vote accepted

In the Titan atmosphere hydrogen is in dynamic equilibrium. It is being continuously created by uv splitting of hydrocarbons and it is being continuously consumed by reaction with other gases in the atmosphere.

In NASA's models of the atmosphere the equilibrium concentration of hydrogen turns out to be roughly independant of height. I'm not familiar with their models, but this is presumably because the ratio of the production and reaction rates is roughly independant of height so there is no net flux of hydrogen in the atmosphere. I'm sure their model includes upwards diffusion of hydrogen :-)

Now assuming that all the physical parameters in NASA's model are correct, if a concentration profile with height is observed it must mean there is either a source of hydrogen or some process consuming hydrogen that isn't included in the model. The suggestion is that there is some process near the surface that consumes hydrogen, and that one possible process is metabolism by living organisms.

We'd all love there to be life on Titan, so it's very tempting to use the apparent deficit of hydrogen near the surface to jump to this conclusion. But there must be loads of other possible mechanisms, and there must be a good chance it's just down to a purely physical process not included in the atmosphere model.

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if hydrogen is formed by UV splitting hydrocarbons, i would expect such UV (from the sun right? what other sources of UV are in titan?) i would expect most UV to interact with hydrocarbons at the higher layers of the atmosphere, and the bouyancy of hydrogen would mean that it will stay on those upper regions. What i don't get is what is the mechanism in those models that would have molecular, unreacted hydrogen go downward in the atmosphere? –  lurscher Dec 23 '12 at 14:28
    
i would trust them to have incorporated each gas specific weight in their model, but still i want to know what mechanism could possibly transport the hydrogen from the upper layers where it is formed, to the lower layers, in order to be uniform (in their model) –  lurscher Dec 23 '12 at 14:30
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I found the details at astrobiology.jhu.edu/wp-content/uploads/2010/06/…. I haven't had time to read the paper so I can't comment on it. –  John Rennie Dec 24 '12 at 9:10
    
looking now.. thanks –  lurscher Dec 24 '12 at 13:15
    
feel free to add any more comments if you wish, but i'm accepting this answer now –  lurscher Dec 24 '12 at 13:16
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