I've got a lunar colony that was set up to be self-sustaining before a collapse of life on Earth. The colony has survived on its own for several thousand years through a period of technological regression. But now they've recovered to roughly their original technology levels, and realize that they've been slowly losing something which will soon threaten their ongoing survival. What could a colony on the lunar surface slowly lose that would ultimately prove fatal?
Some notes on their technology:
- The colony is a shielded crater about 12km across.
- They have advanced, but not magical, 3D printing technology.
- They have large solar collectors for power, but no fusion.
Some thinking so far:
- The obvious answer is oxygen, but lunar regolith is 40% oxygen so that's easy to replace.
- My understanding is that lighter elements are more likely to escape even tightly-sealed systems via atomic diffusion or simpler processes like opening and closing airlocks. So a slow loss of hydrogen (rare in lunar regolith) seems likely, especially if there's a mechanism via which it would become unbonded from water.
- Humans need a lot of trace elements to survive (zinc, magnesium, iodine, etc). Is there some process via which a key one of these would be slowly lost, no matter how carefully things are recycled? (i.e. soil is tilled to recover, air is scrubbed, etc.)
- I know certain substances exposed to space for long periods of time will degrade or chemically alter (space weathering), possibly in ways that wouldn't be easy to reverse. For instance, the flags planted on the moon are supposedly bleached of color because of ~50 years exposure to UV rays. What substances are most susceptible to this kind of degradation? I have a notion that some of these processes might cause, say, atoms to bind together into molecules that can't easily be separated back into constituent elements, but haven't found good sources with more detail. But if so, the parts of the colony exposed to vacuum/space (solar wind/flares, cosmic rays, micrometeor bombardment etc.) might be a weak point.