THe sixth essay in the Only A Trillion collection is, Planets Have An Air About Them, which talks about planets and their atmospheres, and what Asimov thinks they might be like based on current knowledge.
Ever since it wa recognized that other planets existed besides our own, there has been considerable speculation concerning the possibility of life on these planets and on the kind of life that could be possible on them. Intimately bound up with such speculation are considerations of the kind of atmosphere that might be expected to surround a given planet. What do we actually know, or what can we reasonably speculate concerning planetary atmosphere?
Imagine a planet the size of Uranus in the position of Mars. It has just managed to hang on to enough hydrogen to allow it to be a major component of the atmosphere, along with ammonia, methane and carbon dioxide, and yet the planet is just warm enough to allow the presence of liquid water.
Plant life on such a world might split water to hydrogen and oxygen. ?It would then combine oxygen and methane (which it breathes) to form starch, liberating the hydrogen into the atmosphere. The methane would be replaced by hydrogen, the carbon dioxide would be reduced to methane and then replaced by hydrogen, the ammonia would stay put. The atmosphere of the world would end as only hydrogen and ammonia.
Animals would eat the starch, breathe the hydrogen, recombine the oxygen of the starch with the hydrogen to form water, and breathe out methane gas.
Our situation exactly, but in reverse.
With which thought, I'll step out of the backyard to take a deep, invigorating breath of oxygen and stare fondly at the grass which is so busy making more of it.
The book I have was revised and updated in 1976, and Asimov includes a note:
In the twenty years since this was first written (July 1956) astronomers have discovered more about the details of planetary atmospheres than they had in all the time prebiously - thanks to the coming of the space age and of the launching of satellites and probes. However, the material in this article is essentially correct. ...
Asimov gives a table of atoms abundant in the universe (Hydrogen leading the way, sulfur at the bottom of a 10-item list, with "all others" at the end.
Which of those materials are suitable for atmosphere making, Asimov then questions. He goes on to explain how each one combines with others and what they would produce, with yet more tables...
Surface temperatures of planets of the solar syatem
Boiling points of the common elements
Boiling points of the common compounds
Molecular weights of possible atmospheric components
Atom abundance of oxygen and possible substitutes
Temperature ranges for gas-liquid-solid states of various substances
Nothing particular quote-worthy in this essay.