PDA

View Full Version : Chromophores and sky color and Raleigh scattering



Sanbeko
2009-Jun-15, 12:47 PM
I have a question.

Theoretically, what trace elements could be in the atmosphere in enough concentration to change the color of our sky, yet maintain breath ability?

As I understand it, higher frequencies (blues) are absorbed more often than the lower frequencies (reds). The longer wavelengths pass through the atmosphere. The blues are absorbed, then radiated around, so we see blue. Aka Rayleigh scattering.

Would, say, even something as simple as a thicker layer of atmosphere cause a change in light absorption, like what occurs at sunset when the light rays need to travel further to get to our eyes?

Or am I simplifying it too much?

I know I am stretching science! I'm just having some fun playing with ideas. I recently read a fiction book that stated colonists on Mars would see a yellow sky. I just wondered if conditions were slightly different on Earth what possible sky color we would see...though still be breathing the air to see it.

Thank you!

Sanbeko

cfgauss
2009-Jun-15, 12:52 PM
Yes, a thicker atmosphere would produce redder light, when the blue light is scattered too much! Also, in more heavily polluted areas, you sometimes see more colorful sunsets as a result of whatever's in the local atmosphere. You also get that "gooey LA sky" look in general...

But a more fun option is to grow eyes that can see into the near-UV, because IIRC the sky would look totally crazy. I vaguely remember once reading that the crazy patterns you can see in the near UV help birds navigate, since they're oriented along a north-south axis, but I cannot back that up with a source.

grant hutchison
2009-Jun-15, 02:02 PM
But a more fun option is to grow eyes that can see into the near-UV, because IIRC the sky would look totally crazy. I vaguely remember once reading that the crazy patterns you can see in the near UV help birds navigate, since they're oriented along a north-south axis, but I cannot back that up with a source.I think you're probably recollecting Haidinger's Brush (http://www.opticsforkids.org/teachersparents/articles/pdfs/haidinger's brush.pdf) (240 KB pdf), which is a sky phenomenon produced by polarization of the Rayleigh-scattered light. Since some insects are sensitive to polarization, it's hypothesized that they might use it for navigation when the sun is obscured. It's slightly detectable by humans, too.

Seeing in the near-UV would produce an increased haziness in the air: since the short wavelengths are preferentially scattered, there's a an increase in the aerial perspective effect.

Grant Hutchison

Spaceman Spiff
2009-Jun-15, 11:23 PM
Atmospheric physicist Craig Bohren had a lot to say about this. Here (http://homepages.wmich.edu/%7Ekorista/colors_of_the_sky-Bohren_Fraser.pdf) is a non-technical discussion of the general topic, and this one (http://homepages.wmich.edu/%7Ekorista/atmospheric_optics.pdf) is slightly more technical but covers many of the phenomena of light in the sky. See especially the discussion around Figure 5. Here are a couple of teasers:

1) The fact our sky is blue was not an inevitable outcome of its molecular atmosphere.

2) Before the sky became a dim, ruddy red (within a much thicker molecular atmosphere), the sky would become white -- as it already does along the horizon. Of course, the details would depend on the content of condensed particulates (aerosols) and the spectrum of incident starlight (if planet orbits some other star).

Enjoy.

PraedSt
2009-Jun-15, 11:55 PM
That first one is excellent.

Sanbeko
2009-Jun-16, 02:03 PM
"Of course, the details would depend on the content of condensed particulates (aerosols) and the spectrum of incident starlight (if planet orbits some other star)."

Thanks for the links! Great articles.

Now I am wondering what would happen to our sky color if we were orbiting an F star...in its HZ, of course, because burning up would be a nasty option!
Any thoughts?

Thanks,

Sanbeko

grant hutchison
2009-Jun-16, 02:30 PM
Now I am wondering what would happen to our sky color if we were orbiting an F star...in its HZ, of course, because burning up would be a nasty option!
Any thoughts?Probably not much noticeable. Although there would be more blue photons and fewer red photons to scatter, we'd also reset our white point to match the sunlight (as you do when you move from incandescent light to fluorescent light to sunlight). So the bluer sky would be offset by a bluer white point: it would perhaps look brighter (if you could remember Earth's sky for comparison), but not bluer.
Likewise for cooler stars, all the way down to brown dwarf territory. A 3000K red dwarf is at the same temperature as an incandescent light bulb filament, which looks glaringly white if it's the dominant light-source. The sky of an Earth-like atmosphere under such cool stars would be darker, but would probably appear no redder.

We'd get more aerial perspective around an F star, I guess: distant scenery would have more scattered blue light along the sightline, so would fade towards sky colour faster with distance.

Grant Hutchison

Sanbeko
2009-Jun-17, 11:41 AM
Thanks for the info!

Sanbeko