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The Moon has a large number of dark spots, as can be seen on the photo below (Wikipedia), that differ significantly from the lighter areas. The dark areas often seem to be roundish, but not exclusively. What is the source of the darker areas of the moon?

enter image description here

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2 Answers 2

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The dark and large areas are solidified lava beds from earlier periods of Lunar evolution, when it was still volcanically active. Notice that they might be somewhat less covered by still visible impact craters than on average the lighter areas are, as then still liquid lava would completely cover any traces of impacts from the early Lunar history. As the lava eventually solidified with the Moon cooling down, later impacts would of course still remain visible. These large lava beds are called Maria (Latin, pl. for Seas, sing. Mare), for example, Apollo 11 Lunar Module "Eagle" landed in the area called The Sea of Tranquility (Lat. Mare Tranquillitatis), calling the landing site The Tranquility Base.

Smaller dark regions, but still large enough to be clearly observable on the attached to the question photograph and indeed also to the naked eye when observing Luna on the night's sky, are a product of larger meteor impacts that were energetic enough to literally melt the impact area's Lunar surface and penetrate deep enough to cause mixing of the molten rock from the Lunar surface with materials from the Lunar inner, denser layers.

Denser areas of smaller impact craters are however just casting shadows on the Lunar surface, also making them appear darker than the average darkness of the Lunar surface, covered in regolith and with better light reflection properties.

This answer is not based on any external reference. Please, let me know in the comments, if they are required.


Edit to add: Another interesting thing about the darkness of the Lunar surface is that the far side of the Moon has relatively few lunar maria, compared to its near side that faces the Earth at all times due to this Earth's natural satellite being in a tidally locked orbit with our planet. Knowing that, we could argue that the most commonly used term for Lunar far side, the dark side of the Moon, is in fact a misnomer:

    Side-by-side comparison of the near (left) and the far (right) sides of the Moon, taken by the NASA's Lunar Reconnaissance Orbiter.

Additionally, and as luck would have it, NASA has just released a 360 degree view of the Moon, a video of stitched together still images taken by its Lunar Reconnaissance Orbiter, and published for example on this CBS News page.

Thanks go to Paul A. Clayton for pointing this out in the comments below!

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    $\begingroup$ Well it's a misnomer anyway as it gets plenty of light; everyone just knows Pink Floyd way better than tidal locking. $\endgroup$
    – Amory
    Sep 24, 2013 at 17:43
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    $\begingroup$ @Amory It should be noted that "dark" can mean things other than unlighted or dark-colored--e.g., referring to lack of knowledge ("I am in the dark about X") or even evil ("dark lord")--the dark side of the Moon is where the forces of evil have located their secret lair :-). $\endgroup$
    – user56
    Sep 24, 2013 at 19:26
  • $\begingroup$ Probably tidal force caused those prehistoric lava flows by near side. $\endgroup$
    – Peter.k
    Feb 23, 2023 at 0:41
  • $\begingroup$ @PaulA.Clayton I would also use "dark side of the Moon" with respect to radio communication (with Earth), in which case it would be the far side as well. Missions directed there come with a lamp/relay to make the link. $\endgroup$ Feb 23, 2023 at 15:00
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Wikipedia differentiates the compositions between the brighter highlands and darker maria on the Moon (citing Ref. 1):

enter image description here

The relatively low silica content suggests that all regions are composed primarily of various mafic minerals, but the highlands have more calcium-rich material while the maria have more magnesium and iron. As is the case on Earth, it is generally the iron content of mafic matter that renders it both dark in color (compare the color of iron(II) oxide with that of the other common oxide components in both regions) and also higher in density. This distribution is actually much like the rock on Earth, where continental crust is poorer in magnesium and iron than the basalt on the ocean floor. In effect the calcium-rich, iron-poor components on both bodies have made it to the top.

Cited Reference

  1. Taylor, Stuart R. (1975). Lunar Science: a Post-Apollo View. Oxford: Pergamon Press. p. 64. ISBN 978-0080182742.
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  • $\begingroup$ I found that map of FeO and TiO abundance in lunar regolith and, as well, map of regolith layer thickness made by radar plays very well with visual albedo map. So it's all about chemical compounds and some physical light distribution within more loose material, and also conductivity/radiation of the lunar soil. $\endgroup$
    – Peter.k
    Feb 23, 2023 at 0:38

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