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I would have expected the surface of the Moon to be hotter. Both become hot owing to solar radiation.

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Because while the lunar surface is being heated, some of that heat is conducted away from it into a large heat sink, the moon. Isolated atoms in the thermosphere lack such a heat sink.

(Both the lunar surface and the thermosphere are significantly colder "at night" when shaded from the sun, by the moon and the earth respectively.)

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    $\begingroup$ Heating a very thin gas requires much less energy than dense solid moon dust. $\endgroup$
    – Uwe
    Commented Feb 4, 2020 at 22:14
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    $\begingroup$ There was a Heat Flow experiment in Apollo J-type missions. According to this source, "The HFE found that the surface layer temperature during the night was 76ºK (-197ºC) rising to a maximum of 358ºK (+85ºC) during the day. The temperature at 1.5 meters under the surface was a constant 253ºK (-20ºC), indicating the regolith is an excellent thermal insulator." If I'm not misinterpreting this information, the idea of the Moon being a heat sink is not really correct. $\endgroup$ Commented Feb 5, 2020 at 10:42
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    $\begingroup$ The previous paragaph says "temperatures in the upper part of the regolith vary as the amount of incident sunlight changes throughout the lunar day and night." So the upper few inches(?) are still a better heat sink than the complete absence of one. $\endgroup$ Commented Feb 5, 2020 at 20:52
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There is thin layer of gas particles over the surface of Moon or similar planets, which is practically same thin thermosphere.
Particles in gases=atmospheres tend to stabilise in wide range of velocities which look like this

Right side of these distributions corresponds to small amount of very fast particles, speeds of which in Moon day case get close to escape velocity (2400 m/s).
Escape velocity for the Earth thermosphere is much higher, therefore higher expected temperature.

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  • $\begingroup$ The question was about the thermosphere. $\endgroup$
    – Uwe
    Commented Feb 5, 2020 at 11:08
  • $\begingroup$ But also about general intuitive expectation. $\endgroup$
    – halt9k
    Commented Feb 5, 2020 at 17:37

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