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Say the Earth's moon is covered in human blood 4,444 meters deep, would this achieve the visual effect of deep red moonlight for people on Earth under a full moon?

There are two ways to understand this question.

  1. When someone standing on Earth looks at the moon, they see a red moon. The light reflected from the moon is colored enough to be perceived as red.

  2. When someone walks with their sweetheart in a meadow under a full moon, the meadow is cast in red. The light actually shining on Earth has enough of a red cast that people would find the environment alien.

This question is about #2.

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    $\begingroup$ Hmm, I bet it would significantly decrease how much light the moon reflects, between blood being darker than regolith and the moon rapidly developing a thin atmosphere of evaporated blood. So it would make the night sky darker, but I don't know if the reflected light would be red. $\endgroup$ Commented Dec 29, 2024 at 11:25
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    $\begingroup$ Nope. The oxygen would get released in the low pressure turning the blood dark blue/purple. $\endgroup$ Commented Dec 29, 2024 at 19:40
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    $\begingroup$ @Escapeddentalpatient. deoxygenated blood is still (dark) red, it's just the skin making it look blue/purple $\endgroup$ Commented Dec 29, 2024 at 22:36
  • $\begingroup$ @JBH Thank you very much. This is definitely much clearer. Hopefully, it didn't invalidate previous answers... $\endgroup$ Commented Dec 30, 2024 at 6:32
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    $\begingroup$ "Weird and worrying questions from my inbox." You didn't by any chance calculate that the blood of 8 billion people would result in a 4km layer on the moon, and your couple on their romantic stroll are two vampires looking up to their retirement package?? $\endgroup$ Commented Jan 1 at 0:31

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If we cover the surface of the moon with a four kilometre deep sea of blood - at the traditional temperature of 37°C - it will appear to be red... for a little while:

Red ball

Image from http://blog.toddbodnar.com/2010/05/ray-tracing-for-beginners.html

The problem is that blood is not an inert solid, it's a suspension of red blood cells in water. Now, let's consider that the surface of the moon ranges in temperature between -133°C at night and 121°C during the day. It's also in the vacuum of space, and the moon's gravity is not strong enough to hold on to all that water.

Pretty much immediately, the water is going to start to boil and sublime away into space, while the blood cells are heavy, and will start to settle to the bottom of the blood sea. Since the moon has some gravity, the moon will be surrounded by a thin atmosphere of water vapour that will act to slightly increase the pressure on all that liquid water while it freezes.

On the dark side of the moon, where it's cold, the surface of the lunar blood sea will begin to freeze at the same time that it boils, forming a white rime of frost and ice... maybe with some blood cells in it, but not many. The ice may appear to be a pale pink. However, since the dark side of the moon is... well... dark, we won't really see that... we'll just see it reflecting the light from Earth, so it'll appear a slightly brighter dark grey than usual, but still dark.

Meanwhile, on the light side of the moon, the surface of the blood sea will be sublimating more rapidly into the vacuum of space... but still beginning to freeze. While the surface of the moon is above the boiling point of water at 1 ATM, the amount of energy reaching a given area of the surface of the moon is about the same as that reaching Earth, so the entire depth of the blood sea isn't going to boil like a pot on a stove. However, with the blood cells settling out of suspension, the surface will boil until its temperature drops low enough for it to freeze.

At that point, we'll have a moon covered in a layer of ice. With the hottest water molecules continually sublimating off into space, the temperature is going to continue to drop, and the blood is going to freeze from the top down.

When the moon is completely covered in ice, it'll no longer look red, but white... or pinkish white. Blood boiling and freeze-drying on the moon will not behave like blood freezing at low temperatures on Earth, where it can remain red.

The blood ice covering the moon will take a long time to sublimate away, but given that the moon is in the liquid water zone of the sun, it will happen eventually. However, the ice will have increased the albedo of the moon, so less of the sun's incident energy will be retained at first. This would be like an ice-age on Earth, where ice prevents its own melting.

However, over the course of years or centuries, the ice will sublimate away until the settled red blood cells at the bottom of the frozen blood sea are revealed. At this point, the colour of the moon will begin to change again... to that of freeze-dried blood:

Freeze dried blood

Image from https://www.danubeadornments.com/product-page/freeze-dried-blood

Instead of grey lunar regolith, we'll have reddish-brown freeze-dried blood. At first, it'll be mottled reddish-brown and white, since the ice won't sublimate away evenly, but eventually it'll become entirely reddish brown. That'll probably take well over a human lifetime to occur.

So, to sum up, yes, the moonlight will be red for a little while, until the blood freezes over, which should happen within a day. Then it will be back to white (or pinkish white) for many years or even centuries, before becoming reddish brown, or at least redder than it was before being covered with blood, but not as red as it was in the first moment after the blood was deposited.

Edit in response to the question edit:

As to whether there would be visibly red light from the moon on Earth... that's likely to be subjective.

A blood-drenched moon will have a higher reflectivity than the pre-blood moon, and it is possible that the light levels will be bright enough that many people would see red light from the moon at night before all that blood freezes.

Then, after the blood freezes, forming a pink-white crust on the moon, you'll just have particularly bright moonlight... it might be a little pinker than normal, but it may not be perceptible to many people.

Once the ice sublimates, leaving a surface of freeze-dried blood, the moon will be less reflective than it was when it was icy, and it may not be bright enough for any reddish tint to be visible to humans on Earth.

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    $\begingroup$ Just a quick note: The dark side of the moon is not actually dark. It simply never is oriented toward Earth because the moon is tidally locked to the Earth. The same side of the moon is always facing Earth, regardless of what phase the moon is in. Even if we just see a thumbnail sliver of the moon, the "dark side of the moon" is still the opposite side of the moon (which would be in partial sunlight). $\endgroup$ Commented Dec 30, 2024 at 1:59
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    $\begingroup$ @ComradeJoecool You are wrong. In correct English and correct astronomy "the dark side of the Moon" is the half which is darkness at the moment, and the side which is eternally turned away from the Earth is the far side, whether it is illuminated or in shadow at the moment. A lot of people call the far side "the dark side", but they are inaccurate. $\endgroup$ Commented Dec 30, 2024 at 2:11
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    $\begingroup$ @M.A Golding That is totally fair! I appreciate correct English and correct astronomy. :D I just wanted to point it out in case anyone read the answer using the historical definition of the "dark side" which actually means the "far side". scienceblogs.com/catdynamics/2014/06/09/… Perhaps a better phrase for better English and astronomical clarity may be something more like "the portion enshrouded in darkness" or "the portion in shadow" $\endgroup$ Commented Dec 30, 2024 at 6:17
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    $\begingroup$ So, can the moon make red moonlight on earth? $\endgroup$ Commented Dec 30, 2024 at 6:43
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    $\begingroup$ Are you sure about the "thin atmosphere of water vapour" part? 4km deep is a LOT of water, that should suffice for a quite chunky atmosphere and at that point all the freezing and sublimating stuff goes out the window. $\endgroup$ Commented Dec 30, 2024 at 9:51
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A major issue with this is the way the eye adjusts colour and brightness according to ambient lighting.

Many people will be familiar of the viral photo at https://en.wikipedia.org/wiki/The_dress . This is an overexposed picture of a blue dress with black lace, which many people perceive as white with gold lace based on the colours recorded by the camera.

This is a question of context. People tend to adjust their perception of colour according to the ambient light. Therefore anyone present in the room with that dress would adjust their vision accordingly to tune out the excessive strength of the ambient light (perceiving it more or less as "normal" light) and therefore perceive the dress correctly as blue and black.

As the moon will be the main source of light in the sky, vision will be adjusted to perceive it as whitish (perhaps pink or magenta) and reasonably bright, regardless of the actual shade of the moon's surface.

The normal way to perceive a red sun or moon is when it is just on the horizon. In this case, vision adjusts to the bluish ambient light from the sky and perceives the sun or moon to be redder. This is due to atmospheric absorption/scattering of blue and green light away from the direct path of sun/moonlight because of the increased thickness of atmosphere through which the light must pass through due to the low angle.

In other words, the way to make the moon look red is not to make the moon itself red; it is to introduce blue backlighting across the rest of the sky.

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  • $\begingroup$ There's a limit to how much adaptation your vision system will do. It works best with a blackbody spectrum, and a deep red is far beyond what it will adjust for. $\endgroup$ Commented Dec 31, 2024 at 1:02
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    $\begingroup$ I would also add that moonlight is generally too dim for humans to perceive much color when cast on the landscape (the original question) - even if the moon looks red, the landscape illuminated by it probably won't. We keep grayscale vision to much lower light levels than color vision. $\endgroup$ Commented Dec 31, 2024 at 17:02
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Only for a moment before the blood turns the moon black

At the exact moment you do this, the ablito of the moon will decrease from 12% to about 8% but it's cross sectional area will increase 10.9 fold do to the blood ocean making the moon almost as big as Earth. This means that for a moment, the moon will shine a red that is about 7.3 times its current brightness, and it will bath the landscape in a noticably red light, but this red light will quickly fade.

While the accepted answer sounds plausible, it makes the assumption that the blood cells will settle fast enough to allow us to treat the blood like an Ocean, but blood seperates pretty slowly, even under Earth's gravity, but the top layer will begin to sublimate instantly not giving the blood time to seperate. This means that the top layer of blood cells will be freeze dried into a deep brown colored solid shell before a geologically significant amount of moisture can evaporate from your oceans. The rapid pressure drop will also trigger the platelets in the blood to try to form a scab. Sublimation loses effectiveness through a solid barrier very quickly meaning that once the top 1/2 inch of the blood scabs, the rate of sublimation will significantly slow down, and this scab will continue to grow thicker until it fully contains the sublimation. This whole process will not take more than a few minutes.

That said, the day time temperature on the moon is 121°C. In these parts, the scab will be charred. After a few more minutes of direct exposure to the sun and vacuum of space, the surface layer will fully dry out and start to burn down into a charcoal. This will drop the albedo of the moon down to about 3% giving your moon only 2.7 times its total current luminosity while being only barely more red than it is right now.

At first the moon will look like a dark brown ring around a black center, but the black will quickly spread to fill the everything but the edge of the crescent/gibbous as the moon goes through its phases, as there will be a temperate transition zone where the frozen scab has not had a chance to be cook yet, but by the start of the second month, the whole moon will have a char covered surface.

Over a longer time period, liquid blood will occasionally erupt up though the crust, but this blood will become less and less red as the cells under the crust eventually settle to the ocean floor. These eruptions will never form a noticeable atmosphere though because the surface gravity will only be 1.75m/s^2. Even though the blood ocean will make the moon much bigger, it will have such a low density that its surface gravity will not be much higher than it is now.

Transitional colors of blood scabbing cooking and burning and what it would look like for a blood covered moon

If you want to make the moon blood red for more than a few minutes, it is better to cover it in Cinnabar instead of actual blood

Cinnabar (mercury sulfide) is a naturally blood red mineral that will be unaffected in color by the surface temperatures on the moon . On Earth, this mineral naturally oxidizes and turns black over time, but on the moon where there is no oxygen, it will remain blood red indefinitely. Furthermore, it is 27% reflective meaning that you will more than double the brightness of moon without making it noticably bigger. So, your full moon will not only be blood red, but it will glow much more intensely than our moon covering the landscape on Earth in a noticeable red glow while also preventing any unintended consequences of adding so much mass to the moon.

enter image description here

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  • $\begingroup$ I think you confused metres and kilometres there - it's almost 9 km of added diameter which as opposed to nearly 9 thousand km (which would make the Moon indeed very large) $\endgroup$ Commented Jan 11 at 3:00
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Short Answer

No (maybe for a very short while)

Longer Answer

It would look first red (very shortly), than brown (also shortly), than white, than maybe brownish with white clouds sprinkled over, than who knows how (in the long run).

Long Answer

So you poured an ocean of blood on the moon. To now its red, like blood.

But the moon has no atmosphere so the water at the surface will immediately start sublimating, freeze drying the top most layer and making it a dark brown (see other answer for a picture).

Now, 4km deep of blood is a LOT of blood, so it generates a lot of water vapor. How much? Well, earths oceans are about comparable in depth, and they weight about 275 times as much as earths atmosphere (compare https://en.wikipedia.org/wiki/Atmosphere_of_Earth -> 5.1480×10^18 kg with https://en.wikipedia.org/wiki/Ocean -> 1.4×10^21 kg).

That means, we have enough water to generate an earth sized atmosphere many times over. So what will happen is, as more and more water sublimates the pressure rises until a equilibrium is reached. At about 10 mbar we can have liquid water if the temperature is right, and it likely is when all that blood had 36°C to begin with.

I'm going to assume assume it will not freeze through from the inside because the inner parts of the moon are probably not that cold, but could not find any good info about the temperature a few km deep in.

The moon will eventually lose that atmosphere but that will take a quite some time (compare https://space.stackexchange.com/questions/12576/could-the-moon-keep-an-atmosphere) and there is enough water to replenish it many times over.

Now that we have a sizeable water vapor atmosphere, thick clouds will begin to form as the vapor cools down by radiating heat into space. The moon will be obscured and turn white.

The blood ocean will have quite a bit of convection going on, breaking up the surface and mixing it through, turning it red again, but we won't see that below all the clouds.

As the clouds get thicker and thicker it will eventually begin to rain, starting a water cycle. Weather systems will emerge, but probably quite different than on earth - the coriolis force on the moon is much weaker so we should expect less swirly patterns.

On the surface microorganisms in the blood will begin chewing on it, without an organism keeping everything in check it will all over time decompose in various ways into a brown sludge.

At some point when the weather systems reach a equilibrium of evaporation and rain the clouds may break up and partly reveal the surface.

In the long run it depends on what kind of microorganisms can survive, stuff like Co2 and other molecules will be released, altering the composition of the atmosphere.

Eventually all the water will have escaped to space and all that remains on the surface is whatever all that blood (or whatever temporary ecosystem that was kickstarted by it) has been decomposed to. But that could take millions of years.

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  • $\begingroup$ Where are these microorganisms coming from? Blood is alive, but doesn't typically contain anything but blood cells... which don't generally 'chew' on their own... unless they're involved in an autoimmune disease. $\endgroup$ Commented Dec 30, 2024 at 16:55
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    $\begingroup$ @Monti You will be very hard pressed finding any organic material that does not contain some microorganisms. Especially not when in that large of a volume. $\endgroup$ Commented Dec 30, 2024 at 16:57
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Our actual moon has the same color as the tar cover you can see on every road in the world. Yet it doesn't exactly shine dark-ish black.

the moon is essentially covered in various shades of grey: from the very light highlands to the dark basaltic flood plains.

I suspect that the kilometer deep blood sea would achieve a similar result: blood puddles look already dark-black, so not really different from asphalt at the naked eye.

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  • $\begingroup$ Everything I can find suggests it would disappear relatively quickly. Blood in vacuum vaporizes. $\endgroup$ Commented Dec 29, 2024 at 18:19
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    $\begingroup$ You're confusing brightness with hue. If the moon stopped reflecting blue-green wavelengths, that would certainly change the color of moonlight. $\endgroup$ Commented Dec 29, 2024 at 19:36

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