I am writing a science-fiction/fantasy novel and I've come to a sci-fiesque part in my story where I am entertaining several options for a scene, one of which could involve some pretty serious science. The short version is, my planet has two moons, there is a ritual on the planet that involves one of the moons turning red, and there are two narratives involving this scene: one is from the viewpoint of several characters on the planet, and the other is from a character on the moon itself (in a lunar colony). The easy out could be to use Rayleigh scattering and work in a lunar (or bi-lunar, since my planet has two moons) eclipse, but that's not as fun as my other idea, which is to have my moon characters somehow cause the red tinge of the moon, unbeknownst to the planet-dwellers (who think the reddish hues are caused by magic :)).

My thoughts on pulling off option 2 so far are this: Mars is red due to Iron Oxide dust in its atmosphere, right? I also read that Iron (III) oxide has many industrial uses, so is it a far stretch to play with the idea that, somehow, my characters could accidentally release a pressurized vat of iron oxide into the moon's atmosphere, suctioning out into the vacuum of space, and causing the red expanse? Aside from obvious writing challenges galore, (I'll have to be hand-wavy, to be sure), I've realized that the expansion rate of a gas (or dust, in this case) may be sooooooo slow into space, that it would take forever to cover something like the surface area of a fraction of a moon. This just wouldn't work from the story-telling point of view, as the expansion has to be near real-time to be effective for the scene.

My question is this: do the equations governing pressurized gas/dust expansion into a vacuum (space) preclude me from using this idea? If so, does anyone have any other cool ideas that could fit the bill?

Appreciate any advice that you could provide. I'd really like to make the science as believable as possible for this scene.

Thank you all!

I appreciate all of the ideas here, thank you everyone!

For clarification, my moon has no atmosphere (like Luna) and colonists live in a kind of biodome culture, as is depicted on Mars in many sci-fi series. Thus, atmospheric pressures combined with absolute pressures (of some compressed material) could come into play with the release of some substance X out of the biodome and into space.

As for the ritual and defining “real-time” effects, the ritual is actually not a seasonal thing but a one-time deal put on by a cult who are attempting to resurrect a three-thousand year old wizard whose soul has been trapped in a sword made of a dragon’s tooth (which captures the essence of creatures which would have otherwise eaten or, in this case, which the blade came in contact with). In other words, this is not an annual occurrence. That also means that it would be ideal if the colour change of the moon could occur over a timespan that coincided with the length of the ritual, ie something along the lines of an hour or less. Thus, any ideas involving a seasonal phenomenon wouldn’t work, unfortunately.

Appreciate any further input and advice!

  • $\begingroup$ Just having seasonal moon weather seems simplest. $\endgroup$
    – DKNguyen
    Commented Apr 7, 2021 at 21:35
  • 1
    $\begingroup$ What kind of atmosphere does this moon have? $\endgroup$
    – Spencer
    Commented Apr 7, 2021 at 23:15
  • $\begingroup$ Is there an atmosphere that allows plant growth on the moon, even if it's microorganisms? $\endgroup$
    – Trioxidane
    Commented Apr 8, 2021 at 7:10
  • $\begingroup$ Related: How to make the Earth red again? $\endgroup$ Commented Apr 8, 2021 at 16:48
  • 1
    $\begingroup$ Red when seen from space, or red when seen through the atmosphere of the parent planet? And, as mentioned, does the moon itself have an atmosphere? $\endgroup$ Commented Apr 9, 2021 at 11:03

8 Answers 8


If the "red ritual" is an annual ritual associated with a festivity it could a festivity associated with the reproductive cycle of a red colored animal that requires the mass migration of the animals to one side of the moon. Something similar to the migration of red crabs on Christmas Island, in the Indian Ocean.

Alternatively the moon could be covered with red flowering trees.

Maybe combine both events, red flowering trees with migrating red crabs.

Edit - several days later

Since there is no atmosphere on the surface, adapt what the city of Perth, Western Australia, did in 1962 when John Glenn flew over Perth while orbiting the Earth.

Cover the moon with red lights and turn them on when required.

An adaptation of this could be placing a screen in front of the moon, at the correct distance so that it appears to completely cover the moon from the planet and illuminate the screen, from the edge of the screen, with red lights to give the moon a red glow.

  • $\begingroup$ Very cool idea! Unfortunately, the phenomenon can't be natural but caused by my characters. Thank you for the input though. :) $\endgroup$
    – Mitch
    Commented Apr 12, 2021 at 1:32

I can't think of any (non-magical) way for your characters to cause the change, but there is a reasonable way that such a color change could happen naturally.

One hemisphere - say the north - of the moon is heavily forested with deciduous trees similar to maples & red oaks, except the old foliage is all red (no orange or yellow) and doesn't drop off until midwinter. (The other hemisphere is mostly ocean.) Every fall, the trees turn color, a la New England in leaf-peeping season. Here's a sample of it happening on Earth: https://mashable.com/2014/10/03/fall-colors-space/ You could even extend this a bit, with grasses &c having the same seasonal color change.

Now if the moon has a pronounced axial tilt, the planet dwellers will sometimes see mostly the northern hemisphere, and so will see a red moon.

  • $\begingroup$ Thank you for the input! As above, characters would need to cause the red shift. If I can't make it feasible, I'll probably just go with Rayleigh Scattering as it involves the least amount of work on my part :P $\endgroup$
    – Mitch
    Commented Apr 12, 2021 at 1:33

If not for OP's reference to expanding into vacuum, I would have suggested life of some sort. An Algal Bloom, for example, would be perfect.

But: criteria=vacuum. There's a handy writeup about the behavior of a gas expanding into vacuum here:

In short: Determine your speed of sound in the gas. The expansion proceeds outwards at a speed of 3 * speed of sound.
For normal air, this means your expansion is about 1km/s
For our rather outsize moon, from a single source in the middle of the visible disk, the whole moon will appear to be covered in 2730 seconds. About 46 minutes.

  • $\begingroup$ Wow, thanks for this! And what a crazy old article; from the 50s! Didn't even know they were doing serious science back then. Thought it was all drive-ins, girls on roller skates, drinking in the office like on Mad Men. (J/k; I'm fully aware of Oppenheimer and all those chaps). I'll admit, the math is way above my head but I think I could glean something from this. I wonder if dust follows the same rule as ideal gas...? $\endgroup$
    – Mitch
    Commented Apr 12, 2021 at 1:36
  • $\begingroup$ @Mitch 50's was also the era of First Space, SpeedOfSound breaking, Thermonuclear bomb development. You know that B52 bomber that is still the main heavy bomber of the US? 1952. As for the dust: No, it will not follow ideal gas law. But it will approximate it, if in gas form or carried by a gas. Totally dry dust will just sit there and look silly, of course. That's what 99% of the Moon's surface is, after all, just ultrafine dust from millennia of micrometeorite abrasion. $\endgroup$
    – PcMan
    Commented Apr 12, 2021 at 6:36
  • $\begingroup$ Thanks for the input about the gas/dust dichotomy. This essentially puts my idea to rest about using compressed Iron Oxide. What you are saying makes sense; if the vacuum energy were strong enough to fight the moon's gravity, we should see all the dust evaporating into space, which of course we don't! However, there is the part about the dust coming out of a contained atmosphere within the biodome, where there is a large pressure differential. Perhaps, if there is a breach in the biosphere, this would change the situation as dust is expulsed along with the pressurized gas into space...? $\endgroup$
    – Mitch
    Commented Apr 12, 2021 at 19:58

Your moon is much like ours, but it has evolved a simple life form. Or perhaps it is only a self-catalytic phenomenon ... it is simple. There is a silicate mineral, similar to quartz, that has formed very small lenses across the surface of the moon. The lenses accumulate energy from photons that strike them and/or from their deformation by the heat changes of the day/night cycle. Occasionally this bursts forth in a piezoelectric breakdown that vaporizes a few atoms of the raw regolith beneath. These recrystallize onto the lens above.

As such, it is a very slow growing "desert pavement" for the moon that needs no atmosphere and will regrow to cover meteor craters and other disruptions. But the growth of each tiny crystal stops short of overrunning the next, because the charge that builds up on each one prevents them from coming into close contact. The resulting structural color gives the moon an eerie iridescence, silvery, but different from our Moon.

Your characters have aimed a powerful particle accelerator, more typically used to disintegrate incoming nuclear missiles, at the distant Moon. Or they have placed it on the Moon and aim it into space. In either case, in the short term at least, the Moon's overall potential is becoming different from the potential of the extremely thin atmosphere that exists due to the breakdown of bits of regolith by the organism I mentioned. As that electrical potential difference builds up, the crystals are pushed a little further apart, causing an overall increase in the wavelengths reflected. This is viewed as waves of eerie redness crossing the surface of the moon.

  • $\begingroup$ This is super fun! Unfortunately, the timescale wouldn't work as the effect needs to be "real-time" (the change must be perceptible to the viewers on the planet). Also, the effect needs to be caused by my characters, for dramatic effect; red-shifting crystals would be a natural (if not perturbed) phenomenon, and there aren't any super-charged particle accelerators in this case, unfortunately! Would love to see you use this idea in your own material, though. Let me know if you drum something up. :) $\endgroup$
    – Mitch
    Commented Apr 12, 2021 at 1:25

If you don't need it to be cleaned up again, just small rockets carrying dust.

Allow me to quote a bit from Heinlein's "The Man Who Sold the Moon":

Harriman was shown into the office of the president of the Moka-Coka Company ("Only a Moke is truly a coke"— "Drink the Cola drink with the Lift"). He paused at the door, some twenty feet from the president's desk and quickly pinned a two-inch wide button to his lapel.

Patterson Griggs looked up. "Well, this is really an honor, D.D. Do come in and—" The soft-drink executive stopped suddenly, his expression changed. "What are you doing wearing that?" he snapped. "Trying to annoy me?"

"That" was the two-inch disc; Harriman unpinned it and put it in his pocket. It was a celluloid advertising pin, in plain yellow; printed on it in black, almost covering it, was a simple 6+, the trademark of Moka-Coka's only serious rival.

"No," answered Harriman, "though I don't blame you for being irritated. I see half the school kids in the country wearing these silly buttons. But I came to give you a friendly tip, not to annoy you."

"What do you mean?"

"When I paused at your door that pin on my lapel was just the size—to you, standing at your desk—as the full Moon looks when you are standing in your garden, looking up at it. You didn't have any trouble reading what was on the pin, did you? I know you didn't; you yelled at me before either one of us stirred."

"What about it?"

"How would you feel—and what would the effect be on your sales—if there was 'six-plus' written across the face of the Moon instead of just on a school kid's sweater?" Griggs thought about it, then said, "D.D., don't make poor jokes. I've had a bad day."

"I'm not joking. As you have probably heard around the Street, I'm behind this Moon trip venture. Between ourselves, Pat, it's quite an expensive undertaking, even for me. A few days ago a man came to me—you'll pardon me if I don't mention names? You can figure it out. Anyhow, this man represented a client who wanted to buy the advertising concession for the Moon. He knew we weren't sure of success; but he said his client would take the risk.

"At first I couldn't figure out what he was talking about; he set me straight. Then I thought he was kidding. Then I was shocked. Look at this—" Harriman took out a large sheet of paper and spread it on Griggs' desk. "You see the equipment is set up anywhere near the center of the Moon, as we see it. Eighteen pyrotechnics rockets shoot out in eighteen directions, like the spokes of a wheel, but to carefully calculated distances. They hit and the bombs they carry go off, spreading finely divided carbon black for calculated distances. There's no air on the Moon, you know, Pat—a fine powder will throw just as easily as a javelin. Here's your result." He turned the paper over; on the back there was a picture of the Moon, printed lightly. Overlaying it, in black, heavy print was:

"So it is that outfit—those poisoners!"

"No, no, I didn’t say so! But it illustrates the point; six-plus is only two symbols; it can be spread large enough to be read on the face of the Moon." Griggs stared at the horrid advertisement. "I don't believe it will work!"

"A reliable pyrotechnics firm has guaranteed that it will—provided I can deliver their equipment to the spot. After all, Pat, it doesn’t take much of a pyrotechnics rocket to go a long distance on the Moon. Why, you could throw a baseball a couple of miles yourself—low gravity, you know."

  • $\begingroup$ This is legit. Aside from stealing from the Great Heinlein, I think the logistics of one of my moon characters somehow commandeering enough rockets to pull something of this scale off is just not feasible with the situation in my book. I was going for something along the lines of a "grand accident" or mishap; this is a coordinated strike! Very cool though, thanks for sharing! :) $\endgroup$
    – Mitch
    Commented Apr 12, 2021 at 1:41

(a) Ritual bloodshed

The moon is densely populated - too densely populated. Every year, there is a lemming-like ritual where everyone over a certain age is killed and their blood used as fertiliser on the fields or spilled in the lakes. This causes a brief redness until the blood dries and turns back to its normal reddish brown.

(b) St. Patrick's Day

The inhabitants are red-green colour-blind. Every year, every available stretch of water is dyed what they think is green.

Chicago River dyed green for St. Patrick's Day enter image description here https://www.nbcnews.com/news/us-news/chicago-river-dyed-green-st-patrick-s-day-surprise-move-n1261046

  • $\begingroup$ Lol. Thanks for this. When you mentioned using blood as fertilizer, all I could think of was Idiocracy: "Brawndo has what plants crave!" youtu.be/kIZ9YuPm_Ls $\endgroup$
    – Mitch
    Commented Apr 12, 2021 at 1:38

Put a red filter on the dome.

You don’t need to actually turn the moon red. Your people are in a transparent dome! Put a red filter on top of the dome. It would just be in the place where the moon will be when they want to trigger the effect. Ideally your characters could turn it on and off with a remote. Alternatively they just scramble up there and flop it down then later pull it off with a long string.

Persons who were collecting rags and bones out at the edge of the dome will have the wrong angle; they would not be looking at the moon thru the filter and would miss the effect. Those folks aren’t participating in the ceremony either. All your ceremonious people are in the dome Times Square so your filter would be placed to color the moon when seen from that angle.

Later they can use that filter to color lights at their ad hiv disco. It turns more colors than just red!

  • $\begingroup$ Er... sure? Only problem there is the bio dome would barely be visible from planetside—who are exactly the population who need to see the moon become red. Even if I decided to go 1970s and make my colony a bunch of disco jockeys, there’s still the little problem of the bio dome being...little. From space, that is. No way a pyschidelidome would cover the entire disc of the moon! Ex: cover every inch of the Great Wall of China (only manmade structure visible from space) with tar and feathers and any ET flying by wouldn’t even notice the difference. (Maybe birds would?). See my point?! $\endgroup$
    – Mitch
    Commented Apr 13, 2021 at 3:22
  • $\begingroup$ @Mitch - I thought the viewers were inside the dome looking out at the lifeless Luna like moon in the sky. $\endgroup$
    – Willk
    Commented Apr 13, 2021 at 13:59

Photo-chromism-Reverisbal light-based chemical reactions *********

Photo-chromism is the reversible transformation of a chemical species (photoswitch) between two forms by the absorption of electromagnetic radiation (photoisomerization), where the two forms have different absorption spectra.[1][2] In plain language, this can be described as a reversible change of colour upon exposure to light.


As you would be in a vacuum, I expect chemical behavior would be different(unless you want your moon to have an atmosphere) So in short it's absolutely possible if the limit occurs when the light level reaches a certain amount, which gives you the opportunity to just coat the moon and keep coating it while the entire moon will end up turning red within a certain time frame. If it stayed, then the moon would KEEP turning red on a circular basis based on the time of the moon's orbit and how much energy it takes to turn the moon red on a regular basis once it hit the light level required to turn red.

I have no clue what chemical compound could be used to cause such a thing. That's up to somebody else.

Photo-chromism page

  • $\begingroup$ Thanks for this! Definitely, going to look into photo-chromism as my next lead. $\endgroup$
    – Mitch
    Commented Apr 12, 2021 at 1:42
  • $\begingroup$ Been doing some research on thermochromic pigments. It looks like, in the solid form, a given pigment would show discolouration. This could totally work for my idea since thermo-pigments have so many applications, I could easily write a compressed vat of this stuff into my story and have it be punctured and evacuated into space, where the extreme temperature drop causes a redshift in the pigment as it spreads over the moon for a time, before it settles (which perfectly coincides with red expanse followed by subsequent clearing). I think we have a winner! $\endgroup$
    – Mitch
    Commented Apr 12, 2021 at 21:37

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