2
$\begingroup$

Note for clarity: By reddish skies I mean being reddish in the same way Earth's skies are blue. This should apply to the skies in general (not a localized effect), and it's preferable that it's an effect of the sky itself, not the oncoming sunlight (so that the planet can be unique even in that entire star system).

The context is imagining how other planetary bodies can be different from the ones we are familiar with, especially human-habitable planets that don’t look exactly like Earth. This is for science fiction, but I want to be as rigorous as possible in grounding it to real science.

After some research, the best solution I could think of is having sulfate aerosols in the atmosphere (similar to reports after the Krakatoa 1983 eruption), but with extremely uniform particle size around 0.6 microns in radius. Unless I’m interpreting stuff wrong, Mie scattering should then cause red light to be scattered, and I assume having enough of the aerosol in the stratosphere would lead to the red scattering overpowering the blue of Rayleigh scattering. I think the Krakatoa eruption does suggest this being possible, since there are reports of the solar and lunar disks turning greenish or blueish (indicating the scattering of red light). However, I do need it on a more exaggerated degree, as to cause a reddish sky throughout the day, not only nearing sunsets.

Because I imagine violent eruptions would contribute something that is messier than desired, I might need some pretty unusual geophysical processes that can constantly pump the upper atmosphere with this aerosol. Are there plausible ways to have a planet do that? What would the geophysics of the planet look like when compared to Earth, or other bodies in the Solar System?

My main concern is how the sulfates cycle back to the “geysers”, which could imply a very alien configuration of the “crust”, unless I’m missing something that would let Earth geology handle that. My immediate hunch is on a planet that is something like Earth, but also more like the moons of gas giants we know, having more of a liquid interior, perhaps. The only constraints I really have are the surface gravity being below 1.25g (as low as it needs to go), and the fact that a population of human-like inhabitants should be able to exist in the surface indefinitely (assuming thousands of years of acclimating), though a very minor need of breathing apparatuses outdoors is acceptable. Maybe also that the sun doesn’t appear redder than the sky, so the red dwarf type solutions probably won’t cut it.

$\endgroup$
10
  • $\begingroup$ Please set your expectations. We know of one and only one human breathable atmosphere in all the universe: Earth's. Thus, when you ask for "plausible" what you're really asking for is "something less than magic" because we don't have any other data points to work with. We can change the star's color, but that affects human-breathable. We could add dust or organics to the atmosphere... but anything we do compromises human habitability. So... fair warning. $\endgroup$
    – JBH
    Apr 16 at 20:58
  • $\begingroup$ @JBH It's hard to know, depends on the solution. Theoretically, if it's stuff that never leaves the stratosphere for some reason, the surface conditions could stay pretty much normal. But if it's sulfate doing acid rain every day I can see the issues.This is not about adding something toxic like chlorine or NO2 to the whole atmosphere, so whether or not it is a problem depends on the specifics. $\endgroup$ Apr 16 at 21:15
  • $\begingroup$ @PedrohSpaceWolfy why don't try using a red star? $\endgroup$ Apr 17 at 6:44
  • $\begingroup$ @Fallenspacerock Even a very cold M dwarf is still more orange than red, and that's a bit of a hack solution, because if a true "white" light source ended up illuminating the planet, it would reveal the "lie". Same goes for tidal locking and "eternal sunset", it's not the actual sky being red in the same way that Earth's sky is blue. $\endgroup$ Apr 17 at 15:38
  • $\begingroup$ @PedrohSpaceWolfy would it though? May depend on what your atmosphere is made out of and the distance to the Star. Though i don't think it is physical possible to get closer to true red sky if you want it to be blood red. $\endgroup$ Apr 17 at 18:32

2 Answers 2

2
$\begingroup$

The immediate answer that came to mind, which would require the least amount of theorizing about atmospheric chemical compositions their interactions with light, is an eyeball planet. This is a type of tidally locked, terrestrial planet, which means that much like our Moon only shows us a single face, one side of the planet continually faces the star it orbits. This can theoretically (they haven't directly been observed but they are far from unreasonable assumptions for natural formations, and we have a few candidates) result in each side of the planet being uninhabitable, with the star-facing side scorching hot while the deep-space-facing side remains a frozen wasteland.

How does this result in red skies? Well, that's easy! On this hypothetical eyeball planet, where one side is blistering hot and the other numbingly cold, the only habitable zone is along the transition zone between the two sides. This means that any inhabitants of this planet would necessarily be continually exposed to a beautiful, Earth-like sunset, with all the flashy red colours that usually come with it.

Things to consider:

  • This planet can have surface gravity as close or as far from Earth's as you want, as long as you follow through on calculating the proportional change in size.
  • You may want your inhabitants to be forced to stay just behind the edge of the terminator in order to maintain the red skies effect.
  • While this idea proposes/assumes the planet has an atmosphere practically identical to Earth's, this uneven heating of the atmosphere will definitely have effects on weather patterns - fluids will flow from areas of high pressure to areas of low pressure.
  • If this is a culture that has developed on this planet along this narrow twilight zone (i.e. was not colonized by an intelligent species), there are many cool avenues you can take on the effects of having both sides of the planet be uninhabitable death zones where the sky isn't red. To them, is blue the colour of all their warning signs?
$\endgroup$
2
  • 1
    $\begingroup$ This means the skies aren't really red, the sunset is just frozen. Going beyond the terminator would break the illusion. Making it inhospitable to force people to stay in the terminator brings its own issues and isn't a satisfying explanation. This strays too far from what I want since the inhabitants are colonists with advanced space travel technology who named the planet after the sky color. I didn't include this in the question or specify that I want the WHOLE sky to be red, but that's why I focused on asking about geophysical processes. I'll add a note on that. $\endgroup$ Apr 18 at 0:53
  • $\begingroup$ That's totally fair, the additional notes about the technological status of your colonists gives that missing context, probably wouldn't have written this otherwise. I'll leave the answer up in case someone comes across this question in the future and it is what they're looking for. $\endgroup$ Apr 18 at 1:47
1
$\begingroup$

Habitable planets for Man, Stephen H. Dole, 1965

https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf

discusses the composition of an atmosphere habitable for humans. That should give some idea of changes which you can make to atmospheric composition.

Perhaps you can can make the atmosphere require breathing gear when humans venture outside to filter out a lot of red dust in the atmosphere. That red dust might make the sky seem red.

One thing you might do is make the world a giant moon orbiting a brown dwarf instead of a star, and the brown dwarf orbit a class M star emitting mostly orange light. The brown dwarf would emit mostly infrared radiation, and its visible radiation should be mostly reddish and orangish.

And possibly the heat on the surface of the world could come partially from the star's radiation, partially from the Brown Dwarf's radiation, and partially interal heat of hte moon resulting from tidal heating.

If much of the surface heat of the world comes from tidal heating, it may have many more active volcanoes than Earth. Perhaps the surface is only warm enough for life at the proper distance from an erupting volcano. The erupting volcanos will spread clouds of ash and dust for many miles around them, especially on the side the winds blow toward. And the red hot lava coming from the volcanos emits light reflected back down from the ash clouds to the ground and back up to the clouds, bouncing up and down and getting fainter with distance but still quite noticeable at the distances of all the human settlments.

And perhaps in the ares beyond the volcanic clouds where the sun shines brightly during the daytime, the ground is all red for some reason. Maybe it contains reddish iron oxides - rust. Maybe some process has covered the surface with red corundum (ruby) which would be highly reflective and reflect light upwards. And maybe there is a layer of dust or something in the atmosphere which reflects the red light reflected from the ground back down to the ground making the sky seem red.

The Tunguska event in 1904 produced a unusal effect.

Over the next few days, night skies in Asia and Europe were aglow.[13] There are contemporaneous reports of brightly lit photographs being successfully taken at midnight (without the aid of flashbulbs) in Sweden and Scotland.[11] It has been theorized that this sustained glowing effect was due to light passing through high-altitude ice particles that had formed at extremely low temperatures as a result of the explosion – a phenomenon that decades later was reproduced by Space Shuttles.[14][15] In the United States, a Smithsonian Astrophysical Observatory program at the Mount Wilson Observatory in California observed a months-long decrease in atmospheric transparency consistent with an increase in suspended dust particles.[16]

https://en.wikipedia.org/wiki/Tunguska_event

If it is too hot for humans to come out in the day, they might find the skies glowing at night from permanet ice crystals in the sky. And perhaps some process could make that glowing sky red.

If humans can only come out at night on the world in your story, they might see something like the zodiacal light seen from Earth, but many times brighter.

The zodiacal light (also called false dawn24 when seen before sunrise) is a faint glow of diffuse sunlight scattered by interplanetary dust. Brighter around the Sun, it appears in a particularly dark night sky to extend from the Sun's direction in a roughly triangular shape along the zodiac, and appears with less intensity and visibility along the whole ecliptic as the zodiacal band.[6]

The interplanetary dust in the Solar System forms a thick, pancake-shaped cloud called the zodiacal cloud which straddles the ecliptic plane. The particle sizes range from 10 to 300 micrometres, implying masses from one nanogram to tens of micrograms.[8][9]

https://en.wikipedia.org/wiki/Zodiacal_light

So the dust would have to be many times thicker to make a many times brighter zodiacal light. And some process would have to make the reflected light red.

The Star Trek song "Beyond Antares" says: "The sky is green and glowing, where my heart is."

Glowing sky reminds me of the aurora.

An aurora[a] (pl. aurorae or auroras),[b] also commonly known as the northern lights (aurora borealis) or southern lights (aurora australis),[c] is a natural light display in Earth's sky, predominantly seen in high-latitude regions (around the Arctic and Antarctic). Auroras display dynamic patterns of brilliant lights that appear as curtains, rays, spirals, or dynamic flickers covering the entire sky.2

Auroras are the result of disturbances in the magnetosphere caused by the solar wind. Major disturbances result from enhancements in the speed of the solar wind from coronal holes and coronal mass ejections. These disturbances alter the trajectories of charged particles in the magnetospheric plasma. These particles, mainly electrons and protons, precipitate into the upper atmosphere (thermosphere/exosphere). The resulting ionization and excitation of atmospheric constituents emit light of varying colour and complexity. The form of the aurora, occurring within bands around both polar regions, is also dependent on the amount of acceleration imparted to the precipitating particles.

Red: At its highest altitudes, excited atomic oxygen emits at 630 nm (red); low concentration of atoms and lower sensitivity of eyes at this wavelength make this color visible only under more intense solar activity.

Green: At lower altitudes, the more frequent collisions suppress the 630 nm (red) mode: rather the 557.7 nm emission (green) dominates. A fairly high concentration of atomic oxygen and higher eye sensitivity in green make green auroras the most common.

The Carrington Event was the most intense geomagnetic storm in recorded history, peaking from 1–2 September 1859 during solar cycle 10. It created strong auroral displays that were reported globally1 and caused sparking and even fires in multiple telegraph stations. The geomagnetic storm was most likely the result of a coronal mass ejection (CME) from the Sun colliding with Earth's magnetosphere.2

Auroras were seen around the world in both northern and southern hemispheres. The aurora over the Rocky Mountains in the United States was so bright that the glow woke gold miners, who began preparing breakfast because they thought it was morning.[8] People in the northeastern United States could read a newspaper by the aurora's light.[14] The aurora was visible from the poles to low latitude areas such as south-central Mexico,[15][16] Cuba, Hawaii, Queensland,[17] southern Japan and China,[18] and even at lower latitudes very close to the equator, such as in Colombia.[19]

https://en.wikipedia.org/wiki/Carrington_Event

So you would need the star to produce a constant stellar wind much stronger than the Carrington Event to provide constant world wide auroras.

And you might need a layer of dust in the atmosphere below the layers which produce green auroras, so it would block the green light from the ground and let only the red light through.

And I can think of a few other ways to make the sky red.

To be continued.

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .