[doc30]

You are quite right about the carbon dioxide in Venus's atmosphere as compared to our own. Earth has just as much carbon dioxide, but it is tied up in limestone as calcium carbonate. All that limestone can from living things. The ammonia is a likely form for nitrogen in an early reducing atmosphere. A surplus of H2 and NH3 would be completely compatible with each other.

As for the outer planets, the mean surface temperature refers to the extreme upper atmospheric layers since these planets have no defined surface boundary so it is difficult to make absolute comparisons of physical properties between the inner planets and the gas giants. The point from the post I replied to was the presence of hydrogen on the early Earth. The gas giants still retain their hydrogen because of their intense gravity. The inner planets would have lost their atmospheric H2 because their combination of lower gravity and higher temperature pushes a significant proportion of the hydrogen molcules past escape velocity based on their Boltzmann distribution of speeds.

[AndrewC]

The inner planets would have lost their atmospheric H2 because their combination of lower gravity and higher temperature pushes a significant proportion of the hydrogen molcules past escape velocity based on their Boltzmann distribution of speeds.

That's right. And there is no gas lower in weight than hydrogen.