It was briefly talk about in the manga that Luna and Artemis weren't natives from the moon. They're actually from a planet called Mau where the inhabits are cats and can shape shift into a human form. 

So I have two theories for the crescent moons on their foreheads:

Theory One: It is mentioned both in the anime and manga that the moons on their heads give them powers of speech. Then in the manga most likely allows them to shape shift. But only the problem with this is why a moon? Does every Mau have moons on their foreheads? Or maybe each Mau has their own symbol. The reason this bothers me is because the crescent moon is a symbol of the moon family.

Theory Two: Queen Serenity grant Luna and Artemis the crescent moons on their foreheads. To symbolize their loyalty to the moon family and the moon kingdom. Diana has one because of genetics? Or maybe NQS did the same thing as her mother.


Thoughts???

I think it's more when they became the advisers to the Moon Kingdom, they were given the crescent moon. Mau has their own language and without the crescent moon on their heads, Artemis and Luna aren't able to understand the language of the Moon and Earth Kingdoms. We see when it was covered that Luna couldn't speak to Usagi at all. I wonder if she was able to understand what Usagi was saying.

There has been another topic on this more recently started. I won't merge them since that will throw off the slightly different question the other OP started with, but the discussion is virtually the same. Locking and linking here~.

https://www.thegalaxycauldronforums.com/t14007-theory-luna-s-artemis-s-and-diana-s-crescent-marks