Orbital elevators and long, long days

[Anders]

Originally Posted by Fred Brackin

You've got to have photosynthesis or there will be no atmospheric oxygen. That's what I was getting at.

Not necessarily. Most of the oxygen will be bound up with hydrogen - i.e., water. However, hydrogen is a very light gas and will escape, leaving an excess of oxygen. There was a recent article in Scientific American about how photosynthesis may not have had such an enormous influence on atmospheric chemistry.

[Agemegos]

Originally Posted by Asta Kask

Not necessarily. Most of the oxygen will be bound up with hydrogen - i.e., water. However, hydrogen is a very light gas and will escape, leaving an excess of oxygen. There was a recent article in Scientific American about how photosynthesis may not have had such an enormous influence on atmospheric chemistry.

In theory you can have water dissociated into oxygen and hydrogen by UV light, and that hydrogen can then escape to space by the Jeans Escape process.

But last I heard:

• water tends to freeze out in the atmospheric "cold trap", so the upper atmosphere is rather dry;
• free hydrogen and oxygen tend to recombine more quickly than hydrogen escapes;
• as the process establishes some concentration of oxygen in the upper atmosphere, that tends to form a UV-opaque layer which shuts off photodissociation (unless the ozone layer is below the cold trap).

And therefore the conventional wisdom was that the process was an order of magnitude or two too slow to have been the origin of most of Earth's atmospheric oxygen.

What's changed? I stopped subscribing to Scientific American when it started being written by journalists and that smirking ignoramus Steve Mirsky.