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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
    Feb 4 '20 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. $\endgroup$ Feb 5 '20 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. $\endgroup$ Feb 5 '20 at 20:52
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Fast particles, which radiate energy from Moon, are not stopped by any atmosphere. So Moon is general is colder than Earth, just like Venus/Mercury. Due to additional atmospheric isolation, Venus is hotter, though Mercury is closer to Sun.

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  • $\begingroup$ The question was about the thermosphere. $\endgroup$
    – Uwe
    Feb 5 '20 at 11:08
  • $\begingroup$ But also about general intuitive expectation. $\endgroup$ Feb 5 '20 at 17:37

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