# Moon that changes color with phase

In a world I am building, I would like to have the moon change color when seen from the planet's surface as it goes through its phases (e.g., the crescent after new moon is blood red, a quarter moon is greenish, and the full moon is white). Is there a scientifically reasonable explanation for this?

Assume:

• Any known orbital pattern
• Any known material properties (but not necessarily an existing material)
• The moon appears round when viewed from the planet

The ideal solution will be one where the color changes are due to the properties of the moon rather than the properties of the planet.

• Does the moon have to be naturally formed? Jun 13, 2018 at 18:04
• No. I am more curious about what properties would be required for the behavior than the mechanism of the formation. Jun 13, 2018 at 18:06
• The chosen answer will give you striped full moon, not monotonous one. Look the answers worldbuilding.stackexchange.com/a/115114/38925 and worldbuilding.stackexchange.com/a/115113/38925 - they are the only correct. You need the color depending on phase only. Phase = angle source-surface-observer. And iridescent materials visible colour depends exactly on that angle. Only your moon must be a piece of a sole crystal, they forgot to say. Jun 17, 2018 at 9:06

Our moon is tidally locked so we only ever see one part of it (one rotation per orbit). But if your moon's rotation was instead matched to the phases then you would see the same part of the moon during each phase. Now you can just have the moon's soil create your desired coloring.

You could explain red by way of iron oxide like Mars. Perhaps a (very) large iron asteroid smashed into and covered a small section of the moon which is only visible during the crescent phase.

The green could be Chlorite, Actinolite, or other greenish mineral (or even green plants if your moon supports life.) The white could be Albite or some other mineral (or ice). As the phases progressed each colored area would rotate out of sight.

It wouldn't change colors overnight but instead would transition from one color to the next.

• Iapetus is an example of a moon with very dramatic coloration changes, in this case brown and white. The cause there is thought to be uneven surface heating (between the day and night sides; Iapetus has a very long local day) which led to one side having more ice, the other more dirt. Jun 13, 2018 at 20:11
• @Cadence Wow, iapetus seem weird... Thanks for the link :-) Jun 16, 2018 at 21:48
• And you will never see white moon in the full phase and red moon in the growing phase this way. This is NOT the answer, really Jun 17, 2018 at 8:47

The Moon can appear to be different colors, as seen from Earth:

• A red, orange, or yellow moon can appear when the moon is near the horizon, and light has to travel through more of the atmosphere, and more light is scattered
• A blue moon (not figuratively, literally) can appear if particles of size ~500-800 nm are in the air, scattering red light but not blue light.
• A red moon can appear during totality of a total lunar eclipse, thanks to - you guessed it - scattering.

Now, all of these have to do with the properties of Earth's atmosphere, and the Rayleigh scattering that happens thanks to it. All of the above colors are possible. You can make conditions more or less favorable by changing the atmosphere - for instance, making it more or less diffuse to achieve redness and blueness in varying capacities. Perhaps outgassing of some sort periodically changes its composition; elemental levels could fluctuate, like methane does on Mars. The alignment with the moon's orbit would be coincidental.

Mad Physicist suggested that the moon itself could have an atmosphere. That does seem like a possibility; our Moon's atmosphere is quite tenuous, but other natural satellites in the Solar System, like Titan, have much more dense atmospheres. You'd therefore need some set of gases that change over time. This honestly might be preferable to my original scenario involving the planet's atmosphere.

Now, a color like green could be achievable with a certain type of gas cloud. Doubly-ionized oxygen would give off a green tinge; similarly, $\text{H}\alpha$ is red (as are certain nitrogen lines), and shows up in a number of nebulae, often dominating emission. If this sort of gas is periodically accreted and then dissipated on the right timescales, the moon would appear to change color regularly.

A number of different colors are possible:

• Red: $\text{H}\alpha$
• Blue: $\text{H}\alpha$/$\text{H}\beta$/$\text{H}\gamma$, with appropriate dust absorption
• Green: $[\text{O III}]$ (doubly-ionized hydrogen)
• Pink: $\text{H}\alpha$/$\text{H}\beta$/$\text{H}\gamma$, with $[\text{O III}]$ or $[\text{S II}]$
• Orange: Dust
• This is an excellent answer. However, my ideal solution would involve the moon itself rather than the planet. If you would like to post another answer in that vein, feel free, but I have already upvoted this one. Jun 13, 2018 at 18:27
• @Aliden. The moon could have its own atmosphere. Jun 13, 2018 at 19:56
• ...or volcanism: make it a non-tidal-locked moon (that is, not always showing the same face to your planet), having volcanism on parts only. The moon's temporary ash/gas/particle cloud will be visible from time to time. Apply Rayleigh scattering as described above.
– jvb
Jun 14, 2018 at 6:03
• Red moon can appear during totality - you mean, like, covered with a Soviet or Chinese flag to celebrate the anniversary of the respective revolution? Jun 14, 2018 at 15:27
• @Pavel with flags, I think the flags on the moon already have been bleached white by the sun anyway!
– Wilf
Jun 15, 2018 at 17:14

The main difference during the various phases of the moon is the relative position with respect to the star and the planet.

Coincidentally, there are structures which have angular dependent properties when it comes to interaction with light: dielectric mirrors.

Dielectric mirrors function based on the interference of light reflected from the different layers of dielectric stack. This is the same principle used in multi-layer anti-reflection coatings, which are dielectric stacks which have been designed to minimize rather than maximize reflectivity.

Having a dielectric mirror naturally forming is pretty though, but if the surface of the moon is covered with opal it can show different color as the angle of incidence of the light changes with the phase.

Precious opal shows a variable interplay of internal colors, and though it is a mineraloid, it has an internal structure. [...] The regularity of the sizes and the packing of these spheres determines the quality of precious opal. Where the distance between the regularly packed planes of spheres is around half the wavelength of a component of visible light, the light of that wavelength may be subject to diffraction from the grating created by the stacked planes. The colors that are observed are determined by the spacing between the planes and the orientation of planes with respect to the incident light. The process can be described by Bragg's law of diffraction.

• For this to work, the moon would have to be tidally locked and have different packing in different directions, right? Jun 13, 2018 at 18:30
• @Aliden, no. Look at the picture of the opal I posted. The different line of sight on the different places of the stone are sufficient to give different color
– L.Dutch
Jun 13, 2018 at 18:46
• Right, but if you have a spherical surface, each part of the surface will be at a different angle. So why wouldn't you get a different color from, for example, the center of the moon at full than the edges, if the packing is the same in all directions, as the light at the edges is almost 90 degrees to the surface while the light at the center is almost zero degrees? Jun 13, 2018 at 18:53
• That looks like an 80s hair band guitar pic Jun 13, 2018 at 22:53
• precious opal contains up to 10% water. This would be a huge boon for colonization. Research would be needed to find out if the water in opal could survive on an airless, sun-baked moon: opals on Earth can dry out somewhat. Jun 14, 2018 at 0:30

The moon is covered with Sun-facing vegetation.

Like sunflowers, they track the Sun movement to always face it for maximum efficiency. If this moon, like ours, is tidally locked, maybe the slower Sun apparent movement helped spread this evolution to much of its vegetation.

When the moon is full, the planet is nearly between it and the star. As such, the plants are all facing the planet.

When the moon is waxing and waning, either side of the plants are facing the planet. The East and West side could even have different coloration.

Alternatively, the body of the plants could change color with the passing day. During the first quarter, the plants are seen in the morning. During the last quarter, they are seen in the evening. (Or vice-versa, depending on the moon's direction of rotation.) For example, if they are ephemeral plants living only one day, in the evening they could be dried up, like leaves in autumn.

The moon is too far away for the individual plants to be seen, but if dense enough, the plants could color its surface. Think about a field of sunflowers as seen from afar, east of you. As each sunflower tracks the Sun, you may see the field as a big patch of green in the morning, and a big patch of yellow in the evening as they track the Sun that is now behind you.

Seasons.

What are the phases? They are the seasons of a moon. The phases track one circuit around its planet, just as the Earth's seasons track one circuit around the sun. The color of the Earth can change with seasons.

So too your moon. Your moon is a meadow, or a forest. It makes sense that for a moon with plant life, the plant life would begin to grow with the coming of the sun (the waxing moon), bloom in the summer of full moon, then yellow and die with the autumn of the waning moon.

Spring (green, if you like) would travel across the moon with the leading edge of the sun, with the color of summer in its wake. The full moon would be all the color of summer with edges of spring and fall. As the summer faded the color of autumn would precede winter's shadow.

I can see this in my mind but I lack the photoshop skills to do it justice. Anyone interested - please feel free to addend this idea with your rendition of the moon's seasons.

• There is a problem with this theory, the moon would not have the same season everywhere all the time - seasons alternate on the north and south hemispheres etc. Jun 14, 2018 at 14:26
• A moon with life never entered my mind - nice. The seasons are a result of the tilt of the planet so your moon would either need a tilt or a rotation. You might need some fast growing plants depending on how long each phase was. Perhaps like an algae bloom - that would make the green. Have to figure out why they only grow when we can see them though - the sun is always shining and always hitting some part of the moon so there might be some difficulty there. Also, life (probably) means water which would probably mean clouds. Perhaps the clouds are what causes the 'full moon' to be white. Jun 14, 2018 at 17:45
• @TimB, the moon could be tilted almost 90 degrees.
– Joe
Jun 14, 2018 at 21:47
• The seasons on a tidally locked moon like ours would be more a function of day and night. If you are on the earthfacing side and it is a full moon, you are illuminated all the time, for many rotations of the earth below. If it worked like our moon, a season would last a month. The plants grow when illuminated which is when we can see them too. When shadow falls on them it is winter. Jun 14, 2018 at 23:15
• @TimB On the Earth the seasons are connected to the axis slant. On that moon they will be connected to the stages of lightning: night-morning-day-evening. Jun 17, 2018 at 8:43

In theory, if your moon was surfaced with an iridescent material - one that reflects different colors in different directions - the natural libration in its orbit would present varying angles to the surface over the course of its phases. If you took a hunk of moon rock from this moon in your hand and turned it around, you would see the different colors resulting from it.

"How" is a bit of a stumper, though; iridescent minerals exist, but not generally in such large quantities, you'd have to have a fairly smooth and regular surface, and in general the arrangement would need to be just so. My first assumption would be "ancient alien art exhibit".

• Not according to direction. According to the angle: light source-surface-observer. And because of THAT, it really works. But the same answer is written by @LDutch already. He was 2 min faster. Upvote, anyway. Jun 17, 2018 at 8:58

This is a bit silly, but: if the moon is artificial, it could have ridges along lines of longitude, painted in contrasting colors on their east and west slopes.

• Or due to the way the moon rotates and gets sunlight, different types of whatevers are living on the east facing slopes from the ones on the west facing slopes due to the different amounts of sunlight/prevailing winds in the minor atmosphere/etc Jun 14, 2018 at 0:34
• And the full moon will look striped, not white. Jun 17, 2018 at 8:59
• Maybe not, if the stripes are fine enough. Jun 18, 2018 at 6:08
• @ivanivan, make that an answer and I'll vote for it. Sep 12, 2020 at 16:18

Your material properties assumption is what I would go with. The moon could be made of a substance that reflects light in different ways based on how much of it is reflecting light or absorbing solar radiation. Small amounts of radiation could cause it to be red when slightly exposed, green when quarter, whatever else at other phases, and fully white when reflecting or exposed to all of the sun.

This opens up options for the people in your planet to have religious ceremonies using rare deposits of the same material composing the moon found on their planet. I don't know what sort of story you're writing, but I've already got one in my head about people worshipping a lunar deity who demands certain things during each colored phase and using those rocks as if they were pieces of the god.

Possibility One:

The moon could be made of material whose chemical and/or physical structure gives it a different color when illuminated at different angles. If the moon is approximately spherical and one side faces the planet at all times, then sunlight will hit different regions of the planet at different angles at the same time.

Thus at full moon, when the moon was on the opposite side of the planet from the sun, the entire planet facing side would also be facing the sun, and it would all be illuminated by the sun. The moon would show concentric areas of different colors or shades, going from the center that was directly pointed at the sun to areas that were slanted away from the sun and had different shades or colors out to the outermost limb of the moon which would be tilted almost perpendicular to the sun.

At new moon, when the moon was passing lose by the sun during the daytime, it would be lit totally by planet light reflected from the planet. And the moon would be brightest at the point facing directly toward the planet, and get dimmer in regions farther away toward the limbs. Since the sunlight reflected from the planet would probably be much differ than the direct sunlight, the parts of the moon might lit by the reflected light might be too dim to show colors, and the sunlight reflected from the planet might have the color of the planet.

And when the moon was a little bit away from the new phase there would be a thin bright crescent on the side facing the sun, that would be much brighter than the rest of the moon and all one color.

And you can figure out how the phases in between new and full would look, I guess.

Of course, the moon will only look one single color when it is in a narrow crescent phase. When it is closer to full it should show concentric bands of different colors.

One solution might be that the atmosphere of the planet usually contains particles of light or dust that make it very hazy. If the moon appears smaller than Earth's moon as seen from the planet, and if the atmosphere is very hazy, the colors of the different sections of the moon might blend together in one color. When the moon is in a narrow crescent it might only reflect red light and appear red, when the moon is full it might reflect light of all colors from different sections which might be blended together by the atmosphere to look white (also the moon might look so bright when full that the colors can't be detected by the natives and it appears white), and in between it might look like some different color overall.

And maybe when the atmosphere is exceptionally clear telescopes might clearly show the different colored regions of the moon.

Possibility Two:

Long ago, super powerful aliens (or a previous fallen highly advanced civilization on that planet) colored the moon in broad bands.

They might have deposited colored materials in broad zones across the moon. Perhaps each zone went between meridians of longitude that were 30 degrees apart, making six zones across the near side of the moon: red, orange, yellow, green, blue, purple.

So as the moon goes through different phases the colors of the brightly lit regions of the moon would be:

1) Red.

2) Red & orange.

3) Red & orange & yellow.

4) Red & orange & yellow & green.

5) Red & orange & yellow & green & blue.

6) Red & orange & yellow & green & blue & purple.

7) Orange & yellow & green & blue & purple.

8) Yellow & green & blue & purple.

9) Green & blue & purple.

10) Blue & purple.

11) Purple.

Thus the moon of the planet will appear to change color with various phases. Of course it will only appear a single color when it is a very narrow crescent. When it is fuller it will show bands of color.

If the moon has a much smaller apparent diameter when seen from the planet than the Moon has from Earth, the bands of color might seem to blend together as seen from the planet. And if the atmosphere of the planet is usually very hazy it may usually blend the colors of the different regions of the moon together.

So as the moon goes through various phases it may seem red at first crescent form when only the red region is illuminated; orange when half full and the red, orange, and yellow regions are illuminated; white when it is full and all regions are illuminated and all the colors blend together to make white; blue when half full and and the green, blue, and purple regions are illuminated, and purple when crescent and only the purple region is illuminated.

Of course if the bands are laid out and colored differently the moon might have different colors in its different phases.

And maybe when the atmosphere is exceptionally clear telescopes might clearly show the different colored regions of the moon.

Perhaps the moon is a crystal of sufficient clarity that it acts as a prism. Then the earth would drift through different bands of the visible (and invisible?) spectrum as the moon moves around the earth. This might make for interesting near and actual eclipses too.

When the phases of the moon changes, we still see the same region of the Moon, just with the light coming at different angles.

So you need your moon's surface to reflect the light differently at different angles.

The surface of the moon is exposed to vacuum and solar wind and micrometeorite erosion, and those will naturally bleach most substances to a dull matte gray (which may appear white when illuminated). So a first thing to note is that whatever made the moon look like this, it happened recently on a (astro)geological time scale.

A "laser etching" of the moon, something that the planet will have survived thanks to its atmosphere - perhaps caused by a nearby supernova - might have left one half of the moon looking varying in luminosity, but not in color.

Or the moon might have been bombarded by very tiny crystals formed out of a planetary nebula. The effect would be slight, but visible on such a scale.

Such a bombardment would also probably have caused a nuclear winter on the planet, possibly not so long ago; they might have some wonderful myths about how from the Fimbulwinter emerged a shiny Moon, like a pact from the Gods never to do such a thing again.

• No white full moon this way, only striped one. Jun 17, 2018 at 9:01

The easy solution (from an orbital-mechanics perspective) is a moon with a rotational period equal to the planet's orbital period. With one side of the moon always facing the sun, and the other always facing away, you'll always see the same part of the moon with each phase.

As a side benefit, the extreme temperature differences mean that the color differences could form naturally. Constant solar heating could give a highly-volcanic sunny side, producing a bright yellow surface similar to Io's, while the dark side is a good place for ice to form, giving a highly-reflective surface that appears a dim blue from Earthshine.

• But, then the lit part of the moon would always be the same, not changing colors at different phases, no? Jun 14, 2018 at 2:39

If the moon were a rapidly spinning oblique spheroid with a thick atmosphere and tumbling pole over pole relative to the planet, the light reflected from the moon to the earth would have to travel through more of the moon's atmosphere when it's equator is facing the planet than when the pole is facing the planet. Since the color of the light reaching the planet would be affected by the scattering in the moon's atmosphere you could get a moon that changed color based on the angle between the pole of the moon and the pole of the planet. If the periodicity of the tumbling moon corresponded with the periodicity of the moon's orbit of the planet you could have a moon that changed colors as it changed phase.

• One of the constraints in the question was that the moon appear round from the earth. If it's an oblate spheroid tumbling pole over pole, when the equator is facing the planet, it will appear oval rather than round. Jun 14, 2018 at 16:43
• @Aliden You do have a point about the oblate spheroid looking like an oval if the full moon were to occur when the equator was facing the planet, however, if it occurred when the pole faces the planet the full moon would still appear as a circle. As the moon progressed through its phases it would appear to bulge into a more oval shape as it waned so the other phase of the moon would appear slightly off. Jun 14, 2018 at 17:03

Luminescence could provide an interesting possibility in this case. In the real world the advancing phase of the moon is caused by the moon slowly coming out of the shadow of the Earth. As such the lunar surface soaks up increasing amounts of solar radiation as it's phase progresses. If the source of moonlight is re-emitted rather than reflected radiation it will increase in intensity as exposure time increases.

So a lunar surface made up of several different luminescent minerals with differing luminescence thresholds that luminise in different colours will change colour and intensity as it goes through it's different phases. I'm not sure of what mineralogy you'd need but it would probably have to exploit both Photo- and Radio-luminescence effects to create the exact effect you're looking for. The lunar surface could be any colour including almost black when the sun isn't shining directly on it.