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Old May 30th, 2003, 10:53 AM
Geonee Geonee is offline
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I wanted to flesh out an alien evolved on an exotic Nitrogen atmosphere world which colonized a corrosive atmosphere world. The idea is, for them it is just a "tainted atmosphere". I need some help with this, my chemistry is all but forgotten.

1) A Nitrogen Atmosphere is classified as Exotic. However, a Nitrogen-Oxygen atmosphere can range from breathable (earth) to corrosive.

From the CT supplement "Exotic Atmospheres":

If nitrogen and oxygen are present in an atmosphere in certain combinations, the atmosphere becomes somewhat more hostile. Nitric acid (HNO3) can form under certain conditions, as can other interesting compounds which can be irritants in low concentrations, or can make the atmosphere corrosive in higher doses.
Hence I was thinking of having the homeworld have a nitrogen atmosphere, and the colony an oxygen "taint" turning it corrosive. Does anybody know what the certain combinations of oxygen and nitrogen and the certain conditions have to be to yield such an atmosphere? Nitric acid is liquid at normal temperatures, so we get oceans of the stuff. But what are the higher doses of the other interesting compounds which make the atmosphere corrosive? I am guessing those compounds are nitrogen oxides, which are deadly at extremely low concentrations. But, does "corrosive" refer to that, or to the atmosphere "eating" your skin?

2) I was reading Rob. O'Connor's Xenobiology stuff at Freelance Traveller. Aliens in a nitrogen atmosphere could make use of a "breathing" reaction inhaling Nitrogen and exhaling either ammonia or urea, with vegetal life using the inverse reaction (as photosynthesis). However, that should mean at least a small but significant amount of free ammonia in the atmosphere, I guess (is this true? We exhale CO2 and plants use it, but there is not much in our atmosphere). However, again from exotic atmospheres:

Atmospheres containing ammonia will have to be extremely mild to classify as exotic rather than corrosive, for in any kind of concentration ammonia will cause serious damage to exposed skin
Actually, ammonia is life-threatening in concentrations in excess of 500 parts per million, which is just 0.05%... close to nothing. Does this mean that a nitrogen-breathing alien (who would exhale ammonia) would quickly turn an exotic atmosphere into corrosive?

The alternative might be having it exhale urea (NH2-CO-NH2), which seems to be solid at standard temperatures... colorful, isn't it?

3) In the colony world, I was thinking of having local life use another "breathing" reaction, which inhales ammonia and turns it into (liquid) nitric acid, with vegetals using the opposite reaction. Of course, that means the colony world does have ammonia (generated by plants) in the atmosphere apart from the oxygen "taint". Is there something wrong with the picture?
Carlos - BTE Ref.
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