[AndrewC]

Some people believe that it was in the form of ammonia, NH3. Also, the atmosphere of Venus contains almost the exact same quantity of nitrogen as Earth (on a mass basis). Only Venus has a heck of a lot of CO2 so the proportion of nitrogen is a lot lower. Also, only the upper atmospheres of the gas giants are cool. Jupiter is quite hot. It emits more infrared (heat energy) into space than it receives from the sun.

I believe the premise here is hydrogen not ammonia. The reason for more carbon dioxide in the venusian atmosphere might be that life does not exist on the planet and carbon is not tied up in living or previously living things. As for Jupiter, it is cold. Mean surface temperature: -150°C

[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.