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Worldbuilding

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    $\begingroup$ Unfortunately, what counts is not the time needed for all the rock to cool down, but rather the time needed for the topmost layer of the rock to cool down. For example, the interior of the Earth is still molten, four and a half billion years after it was formed; but the thin crust is cold, and as a result Earth does not glow at all. $\endgroup$
    – AlexP
    Commented Apr 26 at 20:55
  • $\begingroup$ Very true, like I said this is a pretty fast and loose calculation. Part of my simplification was to disregard any radiative cooling (since it will have a negligible effect compared to the "heatsink" of the moon), which means in my simple model the hottest portion will actually be at the surface of the moon. A better model would account for that, but then you're talking about heat gradients and black bodies etc. Since the simplified model spit out numbers MUCH larger than needed, I wouldn't worry too much about it myself, but you are welcome to do those calculations! $\endgroup$
    – Kyle G
    Commented Apr 26 at 21:11
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    $\begingroup$ On the contrary, I would expect that the topmost layer at the surface will freeze quite fast, exactly because radiative cooling. Yes, below it the rock may remain molten for a long time, but the question is about the surface layer. $\endgroup$
    – AlexP
    Commented Apr 26 at 21:16
  • $\begingroup$ Even adding a term for the radiant emissivity (assuming the mare/rock interface is about the same area as the mare/moon interface) doesn't really move the needle on the orders of magnitude involved here. Is this calculation exact? No. Is it enough to completely suspend any disbelief in a rather cool plotpoint? I think so but YMMV. $\endgroup$
    – Kyle G
    Commented Apr 26 at 21:48
  • $\begingroup$ @KyleG thank you for being the first person to try and give an actual time estimate! :) I appreciate it! I think the radiation cooling would play a large role irl and my gut tells me it probably wouldn't be visible for long, since the heat gradient even in the "vacuum" of space is pretty extreme toward heat rapidly leaving warm bodies, but you're exactly right the purpose of this isn't to make this happen irl, it's to make it sound plausible. I WANT the Moon to have a glowing scar. If people read it and go "is that possible? Actually it might be, hmmmmm..." That's good enough for me :) $\endgroup$
    – Chromatic Realms
    Commented Apr 27 at 16:08