According to Isaac Asimov's Essays, (a site that lists them and what books they are collected in, and reviews them, but doesn't go in-depth as ths blog does), "Planets Have A Air About Them" first appeared in the March, 1957 issue of Astounding, and anthologized for the first and only time in 1957 in Only A Trillion.
The Quotable Asimov
None in this essay.
Ever since it was 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?
Asimov discusses the planets and the types of elements that could exist in their atmosphere. As with all his essays for Astounding, published in this anthology, they are long, and full of mathematics, and tables, such as "Atom abundances in the universe," "Surface temperature of the planets in the solar sytstem," "Boiling point of the common elements in degrees above absolute zero," "Boiling points of the common compounds in degrees above absolute zero," and so on.
Imagine a planet the size of Uranus in the position of Mars. It has just managed to hang onto 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 sty 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, and with my head humming slightly, I'll step out into the back yard to take a deep, invigorating breath of oxygen and stare fondly at the grass which is so busy making more of it.
Asimov comments that in the 20 years since this article was written, the launching of satellites and probes had enabled scientists to learn more about the atmospheres of other planets than ever before, but that the material in this article remains essentially correct..
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