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The moon in question is composed entirely of a radioactive substance, and showers the earth with strong radiation. It is responsible for a particular kind of cancer that causes the cells to multiply out of control fairly quickly. Over time, the victim will mutate into a shambling monstrosity that is a danger to everything around it and will eventually die. The moon is responsible for a number of other anomalies, such as stillbirths and miscarriages. Even more rarely, pieces of moon rock break off and fall to earth, emitting radiation in the surrounding area and putting people at risk.

Despite this, most of the population are unaffected by the moon, and the majority will not contract this disease. Most areas on the planet are also unaffected by the radiation from the moon. The ozone layer doesn't seem to be a viable reason, since it doesn't explain why a relatively few contract and others do not. Why would this be the case? What prevents the moon from mutating and killing the entire planet as well as the population?

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    $\begingroup$ Welcome to Worldbuilding, Incognito! If you have a moment please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox (both of which require 5 rep to post on) useful. Have fun! $\endgroup$ – FoxElemental Jun 24 '18 at 19:23
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    $\begingroup$ Wait radioactive substance? What is the primary form of radiation, alpha particles, beta particles or gamma rays? $\endgroup$ – Ash Jun 24 '18 at 19:27
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    $\begingroup$ Why would "pieces of moon rock break off and fall to earth?" What else is going on to cause that? $\endgroup$ – user535733 Jun 24 '18 at 20:01
  • $\begingroup$ @Ash Pretty much has to be gamma radiation; alpha or beta would be absorbed by the atmosphere far above the surface. $\endgroup$ – Mike Scott Jun 24 '18 at 20:14
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    $\begingroup$ Is Earth's moon far enough away that Earth would be protected by its magnetic field to any meaningful degree? If so, you might find that the only radiation source is from lunar debris. $\endgroup$ – Cadence Jun 24 '18 at 20:37
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The simpler explanation would be that the population has evolved a very efficient antimutagenic mechanism, possibly based on something like a specialized human helicase enzyme.

But the mechanism isn't perfect: perhaps the enzyme requires some element that is present in most areas of the planet, but not all, to work (e.g. like molybdenum on Earth for nitrogenases); perhaps stresses, or some viruses, or lack of a different micronutrient make the mechanism go awry.

The end result is the same: most people, most areas of the planet are unaffected by the gamma radiation, but some others are not - and the results are quite messy.

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  • $\begingroup$ good solution, another addition or similar option one could be that some virus or bacteria on this world could denature the enzyeme to stop it working. could even be weaponised if the OP wanted it to be $\endgroup$ – Blade Wraith Jun 25 '18 at 9:51
  • $\begingroup$ I like this solution too. I imagine it either being something like an allergy for us, or an HIV-like virus which attacks that aspect of their immune system $\endgroup$ – Brae Jun 25 '18 at 14:48
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A more interesting turn of this question might be "what are the health benefits associated with a radioactive moon?"

In Taiwan, there are a number of buildings from the 80s that were accidentally built out of scrap metal containing radioactive substances. At least one long term study of this phenomenon has yielded a curious result - people are living longer. If true, and attributable to the radiation, it requires a kind of 'goldilocks zone' that amounts to extremely low level radiation therapy - mutated and damaged cells, like precancerous tissue, are being cleared out before they become a problem, resulting in statistically significant changes to mortality rates.

But it's not like these people never get cancer. They just get it less, and later in life. And this suggests a framework for an answer.

On the whole, statistically, and for most people, the radiation isn't necessarily doing harm. Damaged, mutated and precancerous cells are, statistically, being cleared out faster than the immune system can do the job by itself.

The problem arises when that one cell doesn't finish dying, it's broken in a very specific way, and it's in a place an immune system can't really go. This is basically how cancer works in people anyway - the only thing left for you to handle in this situation is "why monsters?"

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Your moon is not made completely of the radioactive substance. It has a radioactive meteor embedded at one location in the surface. When that meteor points at the world below, the world is hit with radiation.

The meteor comes around once each spin of the moon. The moon spins very slowly. It might complete several rotations around the earth before it brings its hot side to bear.

You could make it even less frequent by having the moon in an elliptical orbit. The radioactive meteor might be pointing inwards but the moon is at the far point of its ellipse, and so does not radiate its planet.

So: the radiating of the planet would be an infrequent and much dreaded occurrence. I propose that entities planetside could see the meteor when it faced them, because it was a different color or perhaps glowed.

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The average dose people get from the Moon and environment lies well within the safe thresholds for radioactivity exposure.

According to XKCD, the normal early background dose radiation a person recieves lies around four millisierverts: (click to expand)

Gotta love XKCD Quoting Munroe himself: I waive all copyright to this chart and place it in the public domain, so you are free to reuse it anywhere with no permission necessary.

I am placing this chart here so that you can check what kinds of things are radioactive and how much radiation they give out, so that you can tune the radiation levels of your Moon during special events if you wish to give your audience an idea of the radiation levels involved.

So everyone might be getting just a few microsieverts from your Moon during a year, but on special occasions it will emit more radiation than usual. Some ideas:

  • During supermoons, blue moons, or blood moons. All of these happen a few times per year. If you wish to make it more rare, consider a combination of those events - for example, we've had one super blue blood moon in 1866, and then another in 2015.

Supermoons will give more radiation because the Moon is full and at its closest to Earth. Blue Moons are the second full moon of a month - these are only a thing because of our calendars, but in your world, you could make it so that full moons don't happen at regular intervals. Therefore, two full moons close to each other would give out more radiation than normal. Finally, Blood Moons are lunar eclipses in which the Moon appears red. Maybe solar wind actually sweeps radioactive particles from the Moon away, so its darker side is more radioactive. New moons will be more radioactive than normal, but Earth will be protected by some solar wind still. But during a lunar eclipse, there is much less solar wind between the Moon and the Earth than during a new moon.

  • During a solar eclipse, alignment of the sun and moon might cause the solar wind to combine with the Moon's own radiation to become more dangerous. Perhaps the particles in the solar wind would trigger some transformation on the Moon (which could be the cause for its radioactivity in the first place), and the fact that the solar wind will now carry Moon particles as well helps them pierce the Earth's atmosphere in larger quantity.

  • The Moon in your universe is not tidally locked to Earth. It does not always expose the same face and it has an axial tilt of its own. One region near one of its poles is more radioactive than the rest of its surfaces - so when the Moon season is right, it will shine more radiation onto part of Earth's surface for a few nights.

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