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In my previous question I asked if two moons, one with a red surface, the other with a blue surface, were each to reflect different light wavelengths of an equal luminosity at the same time if they would combine into a purple light being shined upon the local planet at night. Apparently they would generate a purple tint to the landscape, even if one comment highlighted that their orbits would eventually make the phenomenon impossible due to each moon's orbit stablizing on opposite sides of the planet. Now only one question remains in my mind regarding these moons...

What would the two moons most likely be made out of, at least on their surface, to so strongly reflect their respective colours?

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    $\begingroup$ Don't be fooled by the comment that said that the 2 moons would stabilize on opposite sides of the planet. There are many examples of multiple moon sysems in our solar system. If there are two moons, the one with the inner orbit will orbit faster and pull ahead of the outer moon until it is 180 degrees ahead, on the opposite side of the planet, and then catch up with the outer moon until it eventually passes it, over and over again. Sometimes the two moons will appear close together, sometimes only one will be seen at a time. Continued. $\endgroup$ Dec 4 '21 at 17:07
  • $\begingroup$ Continued. With 2 moons in Trojan orbit, one will alwys be c. 60 degrees from, the other, but it will be too small to reflect much light. You could have a ring of 7 to 42 moons of equal mass equally spaced in the same orbit, but if they were massive enough to reflect enough light, the planet would probably have to be too massive to be habitable for humans. planetplanet.net/2017/05/03/… Maybe the best way to have moons visble together is to have two equal size moons orbiting around their barycenter as they orbit the planet. $\endgroup$ Dec 4 '21 at 17:14
  • $\begingroup$ Related: worldbuilding.stackexchange.com/questions/40195/… $\endgroup$ Dec 4 '21 at 18:45
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There are many options for two different materials

I found a search link for you. nice lookup table.. pick red colored minerals, blue colored minerals..

http://webmin.mindat.org/determin/color.php?color_search=Red&keyword=#.YaqKfboo-Uk

There are numerous minerals that can make one moon shine red and the other moon blue

However..

You can pick two minerals and be happy with your purple moonlight. But one problem explaning this is having two different moons near one planet: why would a planet have two big moons of completely different color ? Could such moons originate from the same planet ?

.. suppose when the moons were chipped off the planet, your atmosphere had plenty of oxigen and the planet crust provides lots of Beryllium and Silicon, some Aluminium, and some Manganese .. you'd get

Emerald moons and how they could have formed

Both moons have huge deserts consisting of Beryl sand, or pebbles, that is Emerald

https://www.google.com/search?client=firefox-b-d&q=red+beryl

https://www.google.com/search?client=firefox-b-d&q=maxixe+beryl

Both are the same mineral. Blue Beryl can have any color between green and blue. The darker blue is called Maxixe Beryl. Red Beryl has manganese ions (Ma3) mixed in.. like dissolved in the crystal. That type of Beryl looks red.

The red moon originated from an early collision that took most Manganese out of the crust. The blue moon was formed a billion years later, in another region, devoid of Manganese.

Heat and pressure during the impacts formed the Beryl, as colored sand, and billions of emerald pebbles.

enter image description hereenter image description here


NOTE: these moons are heavy, similar to earth's moons. A situation of mixed (purple) moonlight would only occur when the moons are both on one side of the planet, which would increase the chance of colission and instability. In above answer, I tried to solve instability issues by putting a billion years between the two moon-formation events. At the time of the second impact, the system has stabilized. The second impact could cause the new moon to have a different inclination angle (say 60 degrees). Assuming this as a starting point, both moons can be seen in the sky, without orbit disturbances or collisions.

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    $\begingroup$ Did you mean manganese? $\endgroup$ Dec 4 '21 at 6:15
  • $\begingroup$ That'd mean the local planet would need to have loads of beryllium as well... equal to if not more than its silicon content... May lead to some pretty deserts and beaches I'd imagine. $\endgroup$
    – Rubrikon
    Dec 4 '21 at 7:23
  • $\begingroup$ The moon only has to look like an emerald, it does not need to be one solid emerald. When the moon was chipped off the planet, the heat and pressure created huge amounts of Beryl emeralds, which fell back on the surface of the new moon as colored pebbles, or sand. $\endgroup$
    – Goodies
    Dec 4 '21 at 9:02
  • $\begingroup$ @chrylis-cautiouslyoptimistic- thanks for your remark, I've put an edit to correct "Mangane" to "Manganese" $\endgroup$
    – Goodies
    Dec 4 '21 at 9:03
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    $\begingroup$ I'm not well-versed in orbital, and cannot help but wonder how the planet would keep its first moon when the second moon is created by collision. Wouldn't the orbit of the first moon be so disturbed by the event that the moon would likely be ejected, or crash into either of 2 new objects? $\endgroup$ Dec 4 '21 at 11:56
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For red it can be easily iron oxide or vulgarly called rust, like it happens with Mars, aka the red planet.

enter image description here

For the blue one, you can also go for iron oxide, this time the blue one, also called magnetite

Blue iron oxide, also known as iron oxide compound blue, is mainly composed of magnetite, which makes it has strong magnetism. Its crystal belongs to the oxide mineral of the equiaxed crystal system. It is mostly dark blue or light blue powder in appearance with the performance of bright color and high colouring power.

enter image description here

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  • $\begingroup$ That's Fe3O4. Hematite is Fe2O3. Both are (can be) blue and both are found on Mars $\endgroup$
    – Mazura
    Dec 4 '21 at 4:48
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There are objects in our solar system that do have red and blue color. However, the process to make these can be very different.

D-type asteroids and other objects in the outer solar system can develop a red hue over time. It was assumed Omuamua was a red color, possibly due to chemical reactions of volatiles and cosmic rays. the presence of excess of iron oxides on the surface. Although, in the vacuum of space, that amount of iron(III) oxide probably wouldn't form naturally. It could be a remnant of a once habitable planet that ended in a catastrophic event.

Blue objects tend to be icy worlds with lots of frozen water and hydrocarbons. Enceladus has some blue stripes on its surface. Again, this may not form as a natural satellite at the location of a habitable world. Again, maybe a captured large comet or icy rogue moon... maybe? Or it can be copper oxides on the surface, but that would be in quantities well in excess of what would typically be found on a body in a solar system, and would again have to form in an oxygen rich environment. Most likely option would be a water world. It would reflect blue light, but would not necessarily make everything look blue.

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This is a frame challenge to Rubrikon's satement that:

Apparently they would generate a purple tint to the landscape, even if one comment highlighted that their orbits would eventually make the phenomenon impossible due to each moon's orbit stablizing on opposite sides of the planet.

Don't be fooled by the comment to your previous question that said that the 2 moons would stabilize to orbit on opposite sides of the planet.

There are many examples of multiple moon systems in our solar system. There are no examples of 2 moons orbiting at the same distance from the planet and always opposite to each other.

Part One: Moons in different orbits.

If there are two moons, They will (almost always) orbit the planet at different distances. And so the moon with the inner orbit will orbit faster and pull ahead of the outer moon until it is 180 degrees ahead, on the opposite side of the planet, and then catch up with the outer moon until it eventually passes it, over and over again. Sometimes the two moons will appear close together, sometimes only one will be seen at a time.

Sometimes one moon will be seen in the day, and the other moon will be seen in the night. Sometimes both moons will be seen in the day. Sometimes both moons will be seen in the night, making the night purple as requested.

With different red and blue moons, there will be constant changes in the color of the night.

Part Two: Moons in Exchange Orbits.

It is commonly said that the moons Epimetheus and Janus of Saturn share an orbit, but that is not exactly true. They have very similar orbits, but one orbits slightly closer to Saturn than the other. Because of the very similar orbits and orbital speeds, it takes many orbits round the planet for the inner one to pull ahead of and then catch up with the outer one, and they orbit so close that every time the inner one catches up their gravitational interactions make them switch obits.

https://en.wikipedia.org/wiki/Janus_(moon)

Epimetheus's orbit is co-orbital with that of Janus. Janus's mean orbital radius from Saturn is, as of 2006 (as shown by green color in the adjacent picture), only 50 km less than that of Epimetheus, a distance smaller than either moon's mean radius. In accordance with Kepler's laws of planetary motion, the closer orbit is completed more quickly. Because of the small difference it is completed in only about 30 seconds less. Each day, the inner moon is an additional 0.25° farther around Saturn than the outer moon. As the inner moon catches up to the outer moon, their mutual gravitational attraction increases the inner moon's momentum and decreases that of the outer moon. This added momentum means that the inner moon's distance from Saturn and orbital period are increased, and the outer moon's are decreased. The timing and magnitude of the momentum exchange is such that the moons effectively swap orbits, never approaching closer than about 10,000 km. At each encounter Janus's orbital radius changes by ~20 km and Epimetheus's by ~80 km: Janus's orbit is less affected because it is four times more massive than Epimetheus. The exchange takes place close to every four years; the last close approaches occurred in January 2006,[14] 2010, 2014 and 2018. This is the only such orbital configuration of moons known in the Solar System[15] (although, 3753 Cruithne is an asteroid which is co-orbital with Earth).

https://en.wikipedia.org/wiki/Epimetheus_(moon)#Orbit

If your planet had two moons in very similar orbits like Epimetheus and Janus, they would be close together in the sky for a long time, and be farther away in the sky for a long time, and would be far enough apart that only one was visible at a time for a long time. Years could pass when only one moon was visible at a time, when the red and blue moon alternated in visibility and when only one lights the night sky at a time. And then after that there could be years when the two moons were close enough that they usually were seen together and light the night sky together.

If you want such a sequence of lighting effects you should go with a Janus and Epimetheus type orbit. And naturally some characters would worry that maybe something will go wrong the next time the moons exchange orbits, and fear that one of the moons might crash onto the planet.

Part Three: Moons in a Trojan Orbit.

With 2 moons in a trojan type orbit, they will both be the same distance from the planet, and one will alwys be about 60 degrees from the other. Being separated that far will mean that sometimes they will be seen together and sometimes one will be below the horizon while the other is visible (and sometimes both will be below the horizon). More importantly, one of the moons would have to be tiny compared to the other for long term orbital stability, and so it will reflect only a tiny amount of the light that the other one does.

Part Four: A Ring of Moons.

You could have a ring of 7 to 42 moons of equal mass equally spaced in the same orbit and alternating red and blue in color of surface material.

https://planetplanet.net/2017/05/03/the-ultimate-engineered-solar-system/

https://ui.adsabs.harvard.edu/abs/2010CeMDA.107..487S/abstract

But if the moons were massive enough to reflect enough light, orbital stability would probably require that the planet be too massive to be habitable for humans.

Maybe the habitable world would be a giant moon orbiting around a giant planet, and the ring of alternately red and blue moons would orbit the planet outside the orbit of the habitable moon.

Part Five: Double Moon - Two Moons Orbiting Each Other as They Orbit the Planet.

In a comment to th eother question Ilmari Karonen suggested a double moon, with the two moons orbiting each other as they orbited the planet.

That might be the best way to have moons visble together for the mos time. They would be two equal size moons orbiting around their center of gravity or barycenter as they orbit the planet. The orbital distance between the two moons would be a rather small fraction of the total circumference of their orbit around the planet. Both the moons would be visible almost half the time. Both the moons would be out of sight below the horizon almost half the time. The red moon would be seen alone low above the horizon a small percentage of the time, and The blue moon would be seen alone low above the horizon a small percentage of the time.

They don't have to be exactly the same size. If one had half the diameter of the other one it would have a quarter of the surface area to reflect light, and one eighth of the mass of the larger one. It might appear to be only one quarter as bright as the other moon which may be a problem mixing colors to make purple light.

But if the two moons have different surface materials which have different colors, those surface materials could have different albedos, reflecting different percentages of the light that hits them. The larger moon's surface could refect a relatively small percentage of the light that hit it and the smaller moon could reflect a higher percentage of the light that hit it.

The variation in the albedo of various objects in the solar system is great enough that a moon with only one quarter the surface area of another one could reflect much more light than the larger moon.

And of course the star of the system might not emit equal amounts of red and blue wavelengths of light for the moons to reflect back at the planet. In fact it would probably be almost impossible for a star with a habitable planet to do so.

Of course designing a stable planet-moon system with a double moon orbiting the planet might be tricky, but it should be possible.

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