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.
This post was sourced from https://worldbuilding.stackexchange.com/q/115109. It is licensed under CC BY-SA 4.0.
1 answer
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
0 comment threads
0 comment threads