The Colors of Alien Plants

[kmsoice]

Want to add some color to your campain? What does Dunkelheim look like from orbit?

If you have not seen it already there is a fascinating article in Scientific America (April 2008) on the likely colors of alien plants. See
http://www.sciam.com/article.cfm?id=the-color-of-plants-on-other-worlds

To summarize the article: It all depends on spectrum of the alien sun. When the sun type is
F type stars ( Ka’ra!’ah -Kafer homeworld) would have planets with bluer colored plants

G and K type stars (Earth) would have planets with green colored plants

M type stars (Dunkelheim) would have planets with darker, even black, colored plants. On planets with older suns (Ellis?) the plants could be any color and may not produce oxygen.

So what is the color of Dunkelheim from orbit? I am going with thin splotches of purple that fades to tan in the deep desert areas.

 

[Peter Schutze]

another summary (of a summary?) of the original NASA sponsored paper ?

does it mention anywhere in the article that blue is out or why in an oxygen atmosphere ?

 

[epicenter00]

There's an article in one of the Challenge magazines that talks about stuff like that. If you really want to get into hard sci-fi, when sunlight is tinted something other than "yellow" (what we're familiar with), the entire world looks different to those of us whose eyesight has evolved to take advantage of yellow star light. Colors like blue or purple would look black to us, things like caution striping wouldn't be very visible at all, and so on. "White" clouds wouldn't exist anymore and so on. I don't remember the exact extent of the article but it was something like "Putting the Science back into the Fiction" or something. It's a good read, I suggest you check it out.

 

[Anders]

The original NASA article is likely this pair:

Spectral Signatures of Photosynthesis. II. Coevolution with Other Stars and the Atmosphere on Extrasolar Worlds
http://pubs.giss.nasa.gov/docs/2007/2007_Kiang_etal_2.pdf

Spectral signatures of photosynthesis I: Review of Earth organisms.
http://pubs.giss.nasa.gov/docs/2007/2007_Kiang_etal_1.pdf

Looks meaty, I have not had the time to read it. A light version of the conclusions can be found here:

http://astrobio.net/news/modules.ph...=article&sid=2477&mode=thread&order=0&thold=0

The key issue is the number of photons that can get absorbed, not the maximum intensity. Planets around flare stars can have thriving underwater ecologies protected from UV yet able to photosynthetize.

I think real plants are often not light-limited but water limited (think of C3/C4 plants on Earth) - many environments may induce other odd adaptations due to temperature, radiation or changed length of day. Today I came up with an ecosystem for Gamma Virginis, and given the extreme grand seasons that planet goes through it would not be inconceivable that there are a range of photopigments that are expressed depending on light levels and climate. The flora literally shifts color as the hot and bright season occurs.

 

[Valarian]

They're blue, or purple. Trust me!

 

[rfmcdpei]

The key issue is the number of photons that can get absorbed, not the maximum intensity. Planets around flare stars can have thriving underwater ecologies protected from UV yet able to photosynthetize.

From what I remember of the Colonial Atlas, Niebelungen is the only garden world orbiting a star that flares. The flares can't be that severe, if the biosphere is as diverse as described and 92 or so million people live on that world, but ...

One question, regarding the astrobio.net article you linked to:

"She then used that to look at planets around other stars. For example, if you’re orbiting an F star, the peak of the F radiation is towards the blue, and the ozone just makes it more towards the blue. So if you were a plant on a planet going around an F star, which is a star hotter than our sun, then your pigments are more likely to be in the blue, and so you as a plant are more likely to reflect orange or red radiation.

Another thing she did was look at safe ocean depths. M stars tend to flare quite a bit, so in the very early stages of planetary development -- when you haven’t got enough photosynthesis to build up oxygen so you don’t have your ozone shield yet -- you are susceptible to large bursts of UV radiation up to 30,000 times what we’re used to in any given day. So if you’re a life form you don’t want to be out on the surface. What can you do? Well, you can go under the surface, or you can go down in the water. "

If I'm correct in my understanding of the article and stars generally, the peak output of K stars would be in the orange range and the output of M stars in the red range. Would that mean that plant pigments on garden worlds orbiting those stars would be in the orange and red ranges respectively and reflect infrared radiation?

 

[Anders]

From what I remember of the Colonial Atlas, Niebelungen is the only garden world orbiting a star that flares. The flares can't be that severe, if the biosphere is as diverse as described and 92 or so million people live on that world, but ...

Since people are living along the terminator the light will also be filtered through ~40 atmospheric heights since they see the sun along the horizon, likely reducing the risk a lot. The flares of Doris are probably a bigger problem (and Colonial Atlas mentions the shelter system).

Hmm, what about Dunkelheim, Austin's World and Ellis? Especially Ellis would probably be very vulnerable to flares. I don't quite buy that it wouldn't be tidally locked (and hence lack magnetic field).

I came across this blog post (pointing to some papers) that suggest that terrestrial planets around M-class stars *need* flare activity in order to become garden worlds (otherwise they will not form the complex pregarden chemistry needed for life): http://www.centauri-dreams.org/?p=993

Overall, having a M-class sun likely means more luminosity variations due to sunspots and flares (and the narrow life zone means that any eccentricity of the orbit is amplified temperature-wise), so the climate is going to be messier.

If I'm correct in my understanding of the article and stars generally, the peak output of K stars would be in the orange range and the output of M stars in the red range. Would that mean that plant pigments on garden worlds orbiting those stars would be in the orange and red ranges respectively and reflect infrared radiation?

They claim plants at K2 stars would look rather like terrestrial ones, while M stars become complicated because their spectrum is messier due to strong absorption lines. They conclude that "the dominant photosynthetic organisms would most likely harvest light over 0.4–1.1 microns, with potential but unlikely extensions to 1.4 and 2.5 microns." This range covers all of the visible spectrum and a bit more, so they would be black even in near infrared. They could look completely black, or maybe reflect some blue (which, in the local light, would look like they were black anyway).

 

[rfmcdpei]

Hmm, what about Dunkelheim, Austin's World and Ellis? Especially Ellis would probably be very vulnerable to flares. I don't quite buy that it wouldn't be tidally locked (and hence lack magnetic field).

Aren't flare stars usually young M-class stars? The primary of Ellis is described, I think, as a very old star.

They claim plants at K2 stars would look rather like terrestrial ones, while M stars become complicated because their spectrum is messier due to strong absorption lines. They conclude that "the dominant photosynthetic organisms would most likely harvest light over 0.4–1.1 microns, with potential but unlikely extensions to 1.4 and 2.5 microns." This range covers all of the visible spectrum and a bit more, so they would be black even in near infrared. They could look completely black, or maybe reflect some blue (which, in the local light, would look like they were black anyway).

With late K-class stars somewhere in between?

 

[Anders]

This report suggests that even old dwarves have some spice in them:
http://www.astronomy.com/asy/default.aspx?c=a&id=3658

As for K stars, I think the issue is messier than the paper describes - I have read it, and it is a very good paper, but it mostly looks at the star's spectrum and a terrestrial atmospheric absorption. My current bedtime reading is this little paper: http://homepages.wmich.edu/~korista/atmospheric_optics.pdf
which is full of gems like the spectral effects of thicker atmospheres. And that seems to shift things around a lot: on a planet with dense atmosphere the sky becomes overall brighter and whiter at first, and then starts shifting the light towards red. Lots of other complex scattering effects. I found some interesting discussions here:
http://www.bautforum.com/archive/index.php/t-60031.html
http://www.shatters.net/forum/viewtopic.php?t=11138
including a piece of software for simulating alien skies:
http://homepages.tcp.co.uk/~goodcompanions/skies.html
(see this page for the names of the skies: http://homepages.tcp.co.uk/~goodcompanions/graphics/Skies/ )

So for a plant, air pressure might matter quite a bit because it gives rise to photons that could drive photosynthesis coming from everywhere in the sky or a high degree of directionality on low-pressure worlds.

Another cool page for alien skies is this, http://www.atoptics.co.uk/halo/oworld.htm which looks at parhelia like halos on different planets.

 

[shaunhilburn]

There's an article in one of the Challenge magazines that talks about stuff like that. If you really want to get into hard sci-fi, when sunlight is tinted something other than "yellow" (what we're familiar with), the entire world looks different to those of us whose eyesight has evolved to take advantage of yellow star light. Colors like blue or purple would look black to us, things like caution striping wouldn't be very visible at all, and so on. "White" clouds wouldn't exist anymore and so on. I don't remember the exact extent of the article but it was something like "Putting the Science back into the Fiction" or something. It's a good read, I suggest you check it out.

None of that is true.

The 'color' of other suns is greatly exaggerated, in part because astronomers insist on referring to them with color names and the use of colored inks on HR diagrams.

For purposes of human color perception and scene illumination, the type of star makes little difference. It *can* affect outdoor photography if the proper filter or white balance isn't used, and it does affect sky color. But the direct rays of the local sun always contain all the colors of the spectrum, so reds and blues are always recognizable.

The perceived hue starts to become a factor in scene illumination around spectral class M6V and below. An ordinary 100 watt incandescent bulb is a near-perfect red dwarf (M5V) simulator.

 

[Anders]

The 'color' of other suns is greatly exaggerated, in part because astronomers insist on referring to them with color names and the use of colored inks on HR diagrams.

Good point. Most of the variable colours on the sky are after all due to various scattering processes rather than differences in the colour of sunlight (the atmospheric optics paper I mentioned earlier was very good, in particular pointing out many misconceptions).

So the biggest difference looks like it is going to be due to different light intensities. But even this is not true. Cooler stars are less luminous but will have terrestrials orbiting closer. A planet in the lifezone will be sqrt(L) AU away and the luminosity in watts/m^2 will be roughly constant (L/4pi(sqrt(L))^2), about 1300 W/m^2 outside the atmosphere.

The size of the sun will be different though. Assuming the star radius to scale as R ~ T^(-2)L^0.5 the angular diameter is going to scale like ~Rstar/distplanet = T^(-2)L^0.5/L^0.5 = T^(-2) - hot stars will be pinpricks of searing light, cool stars will cover more of the sky.

This affects horizon light and colour perspective a lot. It turns out that the brightness of the horizon is set by how much of sky sphere is covered by the sun. So on planets with hot suns the horizon will be much darker and the colour perspective will be weak - distant mountains will not look as bluish as they do here. On a M star planet there will be more of a mistiness in the air, and the colour perspective will be more extreme (and likely a bit tinted; "look at yonder turquoise mountains!").

If the peak of the spectrum is at longer wavelengths than for the sun, there will be less Rayleigh scattering of the sunlight and the sky will look darker at the same light intensity from the sun. However, particle scattering may matter more.

Ozone scattering is apparently responsible for keeping the sky blue at sunset and especially twilight, otherwise it would be a greyish-green blue at sunset and yellowish at twilight (this is from E.O. Hurlbut's paper in 1952). Planets with UV-rich stars might hence have much bluer twilights than planets with stabler stars (or less oxygen).

Overall, I think the moral is that many of these interactions are quite subtle. A Dunkelheim and a Paulo landscape will look alien, but not glaringly alien (beside the Kamelinsekten mounds and Pedro hovering in the sky of course - reflected light from Pedro will definitely be significant). The human eye is adapted to terrestrial sunlight, and might see things differently in different environments.

 

[Anders]

Another messy factor: the scale height of the atmosphere. Pressure decreases exponentially with height, the scale height (a 37% decrease, ~8 km on Earth) is H=kT/Mg, where T is the temperature, M is the mean molecular mass and g is gravity.

On a light planet the scale height will be larger, the atmosphere will be "higher", clouds will grow higher (but more slowly) - and light will have to pass through more air. But since low gravity worlds the total pressure is lower, so the amount of air scales just with temperature and M, not g! This means an equal amout of Rayleigh scattering, but more chances from diffuse scattering from dust and water, reddening the sun a bit and making the sky a more milky blue.

On a heavy world like King M is going to be bigger in addition to a heavy g, so the scale height is going to be short. Clouds will be low, convection strong (fires on King can turn into nasty firestorms due to the chimney effect). The difference between zenith and horizon will larger, and there will be less scattering - a sharper, more dark blue sky with a low but intense horizon light.

 

[shaunhilburn]

Another messy factor: the scale height of the atmosphere. Pressure decreases exponentially with height, the scale height (a 37% decrease, ~8 km on Earth) is H=kT/Mg, where T is the temperature, M is the mean molecular mass and g is gravity.

My 'rule of thumb' is that any breathable atmosphere scales as ~8/g.

On a light planet the scale height will be larger, the atmosphere will be "higher", clouds will grow higher (but more slowly) - and light will have to pass through more air. But since low gravity worlds the total pressure is lower, so the amount of air scales just with temperature and M, not g! This means an equal amout of Rayleigh scattering, but more chances from diffuse scattering from dust and water, reddening the sun a bit and making the sky a more milky blue.

On a heavy world like King M is going to be bigger in addition to a heavy g, so the scale height is going to be short. Clouds will be low, convection strong (fires on King can turn into nasty firestorms due to the chimney effect). The difference between zenith and horizon will larger, and there will be less scattering - a sharper, more dark blue sky with a low but intense horizon light.

The sky on any habitable world can take on almost any appearance depending on sun angle and weather conditions. Each location has its "norm", but can devitate from it dramatically.

 

[shaunhilburn]

They claim plants at K2 stars would look rather like terrestrial ones, while M stars become complicated because their spectrum is messier due to strong absorption lines. They conclude that "the dominant photosynthetic organisms would most likely harvest light over 0.4–1.1 microns, with potential but unlikely extensions to 1.4 and 2.5 microns." This range covers all of the visible spectrum and a bit more, so they would be black even in near infrared. They could look completely black, or maybe reflect some blue (which, in the local light, would look like they were black anyway).

Based on my own experience with aquariums & diving:

o Green algae of all types thrive (sometimes explosively) under normal incandescent lamps - equivalent to M sunlight. Its always pest algae that does best

o Coralline algae (pink to deep purple) grows in shallow marine waters where the the lighting is somewhat bluish, equivalent to spectral class F - A. The deeper one goes, the more saturated the reds and purples become. Corallines and corals require special spectrum lighting equivalent to 8,000 K to 20,000 K.

o A red specimen (microalgae, seaweed, corals, or corallimorphs) is a dead giveaway that it came from depths between 50 - 70 ft.