The last entry discussed possible biochemistries for planets hotter than Earth, ranging from "hot" to "incomprehensibly hot." Such life forms may actually exist in the Universe, but for fictional purposes they're a little inconvenient. When either the aliens or the humans have to make brief visits to the other civilization wearing heavy protective gear, it limits your opportunities for dramatic interaction.
So what kind of life might exist on worlds with temperatures comparable to Earth, other than Earth-type life itself?
Solvents which can exist at liquid-water temperatures include sulfuric acid (which I covered last time) and liquid hydrogen cyanide. Sulfuric acid is so inimical to Earth life that temperature would be relatively minor concern for a visiting human astronaut.
Cyanide is poisonous — but it's not corrosive. A pressure suit could keep it out and a human could walk around as long as his or her air tanks last. Cyanide could support DNA-based life, though obviously it would be very different from Earthly organisms.
Well, what about alien water worlds? Can we have weird life on an otherwise Earthlike planet? Yes!
One obvious difference would be to have an ecosystem based entirely on anaerobic respiration, with no free oxygen. Maybe plants never evolved on that world — my first novel A Darkling Sea takes place in a lightless ocean on an ice-covered moon, and the entire ecosystem is powered by chemosynthesis. Plant-like organisms feed on chemicals from seabottom thermal vents, and then other organisms eat them, and so on.
Anaerobic respiration is a lot less energetic than aerobic. Oxygen really is rocket fuel, after all. In my novel, that meant the Ilmataran characters could only remain active for brief bursts using stored energy, then would go torpid until they recover.
There are several different kinds of anaerobic respiration. There's fermentation, which breaks down complex organic molecules, releasing energy and waste products like lactic acid or alcohol. Fermentation requires a source of complex organics in the first place, which means it can't be the primary level of an ecosystem — you need some way to make those organics, and you can't do it by breaking down other organics for obvious reasons.
There's sulfur chemosynthesis. I alluded to this briefly in the last entry. Some bacteria on Earth use inorganic sulfur compounds as reducing agents, getting energy from reactions that release hydrogen sulfide. This is the deadly "sewer gas" which can turn an enclosed manhole or mine into a deathtrap. Like other anaerobic reactions, it's not as energetic as oxygen respiration.
Finally there's chlorine. It's very reactive element, and could take the place of oxygen in an energetic ecosystem. The only problem is how you produce it. Certainly a salt-water ocean contains plenty of chlorine, but I haven't heard of any reactions which liberate free chlorine in any quantity. Maybe some kind of biochemistry reliant on sodium compounds would produce chlorine as a by-product, just as photosynthesis gives off oxygen. That would give you a planet with a nitrogen-chlorine atmosphere, rainstorms of hydrochloric acid, and blue-green skies.
And of course, even if you have life doing photosynthesis, oxygen respiration, and storing information with DNA, it can still be startlingly weird. Just rearranging what kinds of organism use certain molecules gives us very alien life forms: animals with wooden bones, or skins of natural latex; plants made of chitin or calcium; cell nuclei with just a single giant chomosome, or branching DNA of fractal complexity; and so on.
Next time I'll look at life on cold worlds.
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