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I have been designing a planet and it has one moon that is slightly smaller than ours. Just like how mars appears red and its dust is red, I realized that maybe if I altered the composition of my moon, the color that the moon looks like in the sky from the viewers on my planet would change. Is this accurate?

If not, are there any other ways to achieve this?

FYI, my star is a K2V[e] star, the exomoon is 0.8 times as massive as our moon, and the semimahor axis of the moon is about 291292km.

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The apparent colour of a body in space, such as a moon, depends upon a number of factors:

  1. The spectrum of light emitted by the light source (the system's star or stars).

  2. The light absorption and reflectance properties of the body's surface material.

  3. The light absorption properties of the atmosphere (if any) of the body upon which an observer is located.

  4. The spectral sensitivity of the optical apparatus observing the body.

Assuming that we want to keep 1, 3 and 4 constant, that means changing 2: the light absorption and reflectance properties of the body's surface.

With respect to Earth's moon, the lunar regolith is grey, absorbing some incident light and reflecting some, at a level effectively constant across the visible light spectrum. Mars' surface is covered in iron-rich substances that reflect more red light than other parts of the visible spectrum, hence its generally red appearance.

By surfacing a moon with a material such as olivine, it would be possible to give the body a green appearance.

There is an additional way that the apparent colour of a body could be changed. This involves red/blue shift, but an orbiting body would not have any significant doppler shift, that would likely be reserved for rogue objects approaching or retreating from the observer.

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  • $\begingroup$ Let's say that I made the moon covered in olivine. The moon would look green to some viewers, but is it possible to get that much olivine in one place? Also, could the amount of olivine be a thin surface veneer, or could it have to be thick? $\endgroup$
    – Neil Iyer
    Commented Sep 5, 2023 at 2:40
  • $\begingroup$ A moon is pretty big, but you'd only need a thin layer, maybe only a millimetre if crushed small. It could still add up to quite a large mass though. $\endgroup$
    – Monty Wild
    Commented Sep 5, 2023 at 2:45
  • $\begingroup$ Ok then. That could work. You said that olivine works for green, but what about other colors (red, orange, yellow, blue, and purple)? $\endgroup$
    – Neil Iyer
    Commented Sep 5, 2023 at 3:40
  • $\begingroup$ @NeilIyer I think for yellow you could use pure sulfur, however check if it's stable enough under direct sunlight. Anything else, check for ore reflection spectrum (aka color), probably you might find a suitable one. $\endgroup$
    – Vesper
    Commented Sep 5, 2023 at 6:38
  • $\begingroup$ One last question, would the olivine or sulfur have to be a thin layer of dust on top the lunar regolith or would they have to be solid? $\endgroup$
    – Neil Iyer
    Commented Sep 5, 2023 at 13:39

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