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In the far future the moon is terraformed and maintains earth-like conditions for people to walk around freely. Gravity is a 6th of earth's. We assume the atmosphere is "well-functioning for human beings" and kept so by permanent maintenance.

What would the temperatures be like?

Things to consider

  • the moon has 29 earth-day long days (14,5 days sunlight, 14,5 days night time, a sunset taking about a day)
  • depressions and craters have filled up with water to a depth of 10 meters
  • I assume wind is more turbolent, caused by the extreme length of nights and days, resulting in higher temperature differences
  • there are forests and cities, but also vast deserted places either left untouched or mined. About 40/60. These areas are mainly bright, reflecting sunlight.

What I'm looking for is a plausible estimate. Would one freeze to death during night time for example, or would a temperature drop to "a slight chill" be believable.

You can make the atmosphere what it needs to be in your assumption.

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    $\begingroup$ Sorry, but I don't get your question. If the moon is terraformed then its temperature should range where life is possible, else it won't be terra-formed. $\endgroup$ – L.Dutch - Reinstate Monica♦ Sep 17 '19 at 12:22
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    $\begingroup$ "You can make the atmosphere what it needs to be in your assumption" -- this basically means the temperature can be anything. As @L.Dutch said, if it's supporting "forests and cities", then the temperature would be in the survivable range of ... forests and cities. I'm not sure I really understand what you're looking for. Can you clarify your question? $\endgroup$ – cegfault Sep 17 '19 at 12:25
  • $\begingroup$ "We assume the atmosphere is "well-functioning for human beings"" : I know it's presumed in your question so isn't pertinent but I can't help but wonder what (with its gravity) the densest atmosphere the moon could plausibly hold might be, how are you terraforming it again, is this a dome situation? $\endgroup$ – Pelinore Sep 17 '19 at 14:26
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    $\begingroup$ I've seen estimation that a breathable atmosphere on the moon would last about a thousand years, but I don't have a reference for that. There's a thirteenth of the surface area to cover compared to Earth, but the atmosphere needs to be roughly six times taller if only gravity is keeping in the pressure. $\endgroup$ – notovny Sep 17 '19 at 14:49
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    $\begingroup$ "vast deserted places either left untouched or mined ... areas are mainly bright, reflecting sunlight" The non-terraformed Moon has an albedo of 12% (which is relatively low). Desert sand, however, has albedo of 40% (but we don't have it currently on Moon). Clouds can have albedo as high as 80% (but they can contribute to climate both ways). $\endgroup$ – Alexander Sep 17 '19 at 16:23
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What I'm looking for is a plausible estimate. Would one freeze to death during night time for example, or would a temperature drop to "a slight chill" be believable.

Obvious answer: do you want people to freeze to death? Then fine: they freeze to death overnight.

Personally, I'd rather not. The long nights are obviously problematic in this regard, especially given a thin atmosphere at a high-albedo surface. I'm not sure what sort of vegetation could survive this kind of regular deep-freeze punishment, but I bet it woudl resemble lichen a lot more than it would resemble anything that might be described as a forest on earth.

Problem then: how do you increase insolation on the moon to ensure that everything doesn't freeze to death? Well, one way of fixing that is by arranging for a day length somewhat shorter than is currently available.

Gregory Benford's take on lunar terraforming involves hammering it with ice-rich asteroids to provide water and other volatiles whilst at the same time speeding up its rotation and maybe giving it a bit of a twist at the same time to arrange for some seasonal variation. The end result would be a 60 hour day and a deep, damp atmosphere that would be warm and cloudy. Benford compares it to a murky Florida, but doesn't specify which bits of Florida he was thinking of, and there's quite some variation. Most of florida is covered by a humid subtropical climate zone, which gets you winter temperatures around freezing and summer temperatures around 20-30 degrees C.

You might also consider orbital mirrors and sunshades to produce artificial days on the dark side and nights on the light side. These can be made bigger or smaller as required to ensure the optimum temperature, which is of course something that you would decide for yourself. This is perhaps slightly less destructive than hammering the moon with ice and rock for years at a time, and you might be able to make the mirrors and shades with the material you're processing to extract water and oxygen as part of the terraforming effort. Benford's Moon has a thicker, wetter atmosphere than yours, so solar heating in this way is probably essential. Heat would be lost quickly through the thin atmosphere, so your moon would be more like a cold desert... perhaps more like the Köppen BWk (cold desert) or BSk (cold semi-arid) classifications, with cooler summers (10-20 degrees C) and cold winters (-15-0 degrees C).

(FWIW, I think trying to terraform the moon is crazy, and paraterraforming with massive domes or even just building some nice big space stations in cislunar space would be much more sensible, so there you go).

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    $\begingroup$ I like the Florida moon! Thanks for link. It would make sense to have as much atmosphere as you could load on, to provide a buffer in case maintenance fails for a while. Also atmospheric pressure comparable to Earth will keep things from dessicating easily. $\endgroup$ – Willk Sep 17 '19 at 13:44
  • $\begingroup$ @Willk eh, if your technology isn't up to making reliable domes or space stations, what business do you have trying to terraform something as unsuitable as the moon? A thick atmosphere is clearly possible on a small world (see also: Titan) but getting that much gas onto the moon in the first place seems like it would be a major challenge all by itself. A thin oxygen atmosphere is the least hard option there, I think. $\endgroup$ – Starfish Prime Sep 17 '19 at 13:53
  • $\begingroup$ the moon is close. Location, location, location. Love the one you're with. Love the one you're with. Do-doo doo doo doo doo do-doo! $\endgroup$ – Willk Sep 17 '19 at 14:22
  • $\begingroup$ @Willk LEO is closer, more hospitable, and handy for the local amenities... $\endgroup$ – Starfish Prime Sep 17 '19 at 14:51
  • $\begingroup$ @StarfishPrime Appreciated answer! On Gregory Benfords idea I would comment that until we have the technology to smash ice to the moon a lot of moon bases and mines will already be put up, and governments as we know them will probably not come to terms to have these threatened and rather agree to a "kinder" approach. I see not why atmosphere would need to be thin though (if we don't smash ice on), expect that it takes a lot of effort to have smaller deliveries? Any other reason? $\endgroup$ – Backup Plan Sep 18 '19 at 9:01
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I think it would be very hard to make the atmosphere as dense as it is on Earth, because the Moon has low gravity. And you should take into account that even on Earth you can freeze to death but only on some places or in specific season. On the Moon there will be better and worse regions to live, too.

The temperature on the Earth scales from around -90 to +60 Celsius degrees, with average about +14 degree (about one third from the upper limit).
Based on facts about Mars it has some atmosphere too so we can see how temperatures are different (-140 to +30). The average temp there is around -60 degree (it is around the middle of this range - see the difference compared to Earth).
Currently the Moon temperature ranges from -150 to +120 degrees Celsius.

Simplified - the gravity is one of the main factors of how dense the atmosphere on the planet/moon can be.
The gravity on the Mars is about 38% of the Earth's gravity.
The gravity on the Moon is about 16% of the Earth's gravity - even less than on Mars.

So, if the terraforming process (or other technology) enhanced the atmosphere density, you could narrow the range by some amount (e.g. -130 to +100). Assuming that the presence of humans caused some degree of climate change (like on Earth) scale them both up (e.g. -120 to +110). That is my fictional result, the average can be around +20 degree. Excpecting the days in summer really hot and nights in winter really cold.

Not a scientific answer, just my imagination.

Update

An enhance - in far future it is possible that some tech creates strong enough magnetic field that can lead to more dense atmosphere. And occasionaly there can be some failure in the mechanism that can cause to some interesting story twists.

As correctly pointed out by @AlexP the magnetic field has no big effect on atmosphere in short mankind lifetime scale. But it still can be used to protect people and communication technology from space/sun radiation. Another nice article on atmospheres and more at astronomynotes.com

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    $\begingroup$ There is no relationship between the intensity of the magnetic field and the density of a gas. $\endgroup$ – AlexP Sep 17 '19 at 14:06
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    $\begingroup$ @AlexP well I am not an expert, but I read that the magnetic field is protecting the planet's atmosphere from solar wind for exampe here $\endgroup$ – Zavael Sep 17 '19 at 14:15
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    $\begingroup$ The magnetic field is protecting the atmosphere from the solar wind . . . over geological timescales. The efect of the solar wind is negligible at human timescales -- a decade, a century, a millennium. No human project ever was or will be interested in stuff which happens over geological timescales. We don't build bridges to last millions of years, we don't build dams to last millions of years, we don't dig canals to last millions of years. We don't care that sometime in the next two million years we will need to replenish the lunar atmosphere. $\endgroup$ – AlexP Sep 17 '19 at 14:25
  • $\begingroup$ @AlexP I see your point, ok that is reasonable :) $\endgroup$ – Zavael Sep 18 '19 at 7:38

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