Atmospheres - gases detail

[Frankymole]

I recall that Traveller:2300 aka 2300AD had a useful table on which gases could be retained by certain levels of gravity and temperature, and which would escape from a planet's atmosphere due to the molecules being too energetic.

I haven't seen much guidance on what colour a planet's sky would appear to be with different gas-mix compositions.

Is there a system somewhere that can be adapted for Traveller to give such guidance? I don't want an impossible mix of gases for a planet's gravity/temperature range; and I'd like to know what it looks like!

Many thanks in advance for your thoughts!

 

[Timerover51]

Steven Dole, in his book, Habitable Planets for Man, gives some formulas and data for determining that. I will have to see if I can dig out my copy.

 

[sabredog]

Here's something to get you started.

http://www.orionsarm.com/xcms.php?r=oa-page&page=gen_skyonalienworlds


Don't forget, too, that the color of the star is going to be a factor in what colors the sky ranges in. The color of the star will shift the color of the atmosphere similar to a filter on a lens.

 

[shaunhilburn]

I recall that Traveller:2300 aka 2300AD had a useful table on which gases could be retained by certain levels of gravity and temperature, and which would escape from a planet's atmosphere due to the molecules being too energetic.[

You're referring to minimum molecular weight retained (MMWR). My boilerplate reply:

"... the MMWR is generally proportional to the radius of a planet and inversely proportional to its mass, e.g., a planet with twice the mass-radius ratio as Earth retains molecules two times lighter than in Earth's atmosphere. A planet with half the mass-radius ratio retains gases two times heavier than Earth (for the same exobase altitude and temp).

Earth's exobase temp @ 600 km is ~ 1275 K and MMWR (for Jeans escape only) = 9.9 g/mol"

Minimum molecular weight retained = MR x 10. Say the planet in question has mass 0.25 earth and radius 0.68 Earth. The then MR ratio = 0.68 / 0.25 = 2.72, and MMWR = 2.72 x 10 = 27.2.

This solution is approximate, but very close. It's better than a table lookup. More precise methods require a spreadsheet.

I haven't seen much guidance on what colour a planet's sky would appear to be with different gas-mix compositions.

Rayleigh scattering normally results in a blue sky regardless of atmospheric constituents. Suspended particles like dust, aerosols, mists will turns skies a different color. The spectrum of the host star is also a factor.

Based on test renderings I've done in Terragen2, Rayleigh scattering from suns below spectral type ~K7 yields a de-saturated blue sky that's no quite the turquoise color of Earth's skies. As the sunlight becomes redder in spectral class M, the effect becomes more pronounced, and the blue color less vivid. Skies become ugly. Skies lit by an M5 dwarf have a sort of dark steel-blue color like an approaching thunderstorm. Dialing the star down M7V yields a charcoal gray sky that's hard to describe; "blue" only near the horizons.

Dialing the sun the other direction, there's not much difference in the blue sky between spectral class K7 and K2. Imagine a desaturated Earthly sky approaching sunset. All the stars further up the main sequence after G8 appear to produce much the same color of blue sky.

I made a contact sheet from one of these tests, but I have no idea how to post the jpg.

 

[Carlobrand]

...

I made a contact sheet from one of these tests, but I have no idea how to post the jpg.

Maybe we can persuade someone to coach you through it. That's something I'd like to see.

When discussing what gaseous elements are going to be retained or lost, there are additional factors that need to be kept in mind besides gravity and molecular weight. A geologically active planet with a large molten core and a lot of volcanic activity will be generating gases that replace some of the gases lost. A planet which lacks a magnetic field* lacks a shield to protect its atmosphere from solar winds; that seems to make it very hard for the planet to hold water, as water molecules in the upper atmosphere get broken by the solar wind and the hydrogen escapes, and the result depending on the planet's size seems to be either a Venus or a Mars. The magnetic field is created by a molten core in a spinning planet, so planets which have little or no molten core (Mars) or which spin too slowly (Venus) will lack a field. Temperature is also a factor, though it shows up more in extremes: a planet very close to its sun may be too hot to hold an atmosphere for very long (in geological terms), while planets far from the sun may hold atmosphere more effectively than they might if they were closer in - assuming of course they're not so far out that those molecules precipitate out as ices. (Not that this would do us much good.)

Age is a factor too; if the system is young, it may not have been around long enough for a significant percentage of the borderline molecules to escape to space. Whether that can come into play in a Trav setting is an issue I don't know. I don't know if stars in a given region tend to form within a few million years of each other or if they all form independent of each other.

*that is to say, an intrinsic magnetic field generated by the planet's core. Apparently the interaction of the solar wind with Venus' upper atmosphere does create a bit of a magnetic field, but it's too late to save the atmosphere from the solar wind effects at that point.

 

[epicenter00]

The color of the star will shift the color of the atmosphere similar to a filter on a lens.

It's my understanding that the "colors" of stars in reference to temperature (OBAFGAKM) is greatly exaggerated and may be entirely fictional; if you're near a star, certainly in the habitable zone, one star's light is going to look much like another star's light to the human eye. The whole "we're near a red dwarf so everything is going to red" isn't true.

 

[Enoki]

It would also be interesting to know how the spectral color of the star affects things like the color of clouds (which would also be affected by their chemical composition of course) and other atmospheric phenomena.

Another interesting affect might be different colored lightning based on the charge and chemical composition of the atmosphere.

It would also be useful if there were a quick way to determine the arc segment that a star took up in the sky of a planet. While this can be done from a mathematical reference book it is a bit more drawn out to convert Traveller data like in the Scouts LBB to an answer.

 

[aramis]

It's my understanding that the "colors" of stars in reference to temperature (OBAFGAKM) is greatly exaggerated and may be entirely fictional; if you're near a star, certainly in the habitable zone, one star's light is going to look much like another star's light to the human eye. The whole "we're near a red dwarf so everything is going to red" isn't true.

There are a dozen articles by astronomy professors which imply strongly that your POV on the issue is in fact wrong.

The Color of the star won't always be readily visible to the naked eye, but it certainly will be affecting what the color of the plants is...

In the case of the star itself, the classification is from the part of the spectrum where that star is brightest... but given the dark areas of the spectrum are still blindingly bright, it's going to be white with a tinge of X to the clear-helmeted eye.

The plants, however, will probably adapt their chlorophyll analogues to be absorbing the peak intensity wavelengths.

Plus, ever been on stage? Having played with lights, there's a very qualitative and obvious difference to a bank of white floods with one red vs the same bank plus one blue, one yellow, or one green. Everything looks just slightly ___ish... but colors are still discernable, and yet, the extra spot's color is also readily apparent.

http://www.astrobio.net/topic/deep-space/alien-life/colors-of-alien-plants/
http://www.solstation.com/life/a-plants.htm

 

[Kilgs]

Steven Dole, in his book, Habitable Planets for Man, gives some formulas and data for determining that. I will have to see if I can dig out my copy.

Thanks for the reference. There are also some books cited in First In that I'm planning on ordering soon. I'm doing the same thing as the OP (with no science background). It's pretty fun. Well, at least when you're doing it for fun...

 

[Timerover51]

I guess that I go with more of the attitude, if it feels right for atmosphere, go with it, and not worry about the formulas.

 

[Frankymole]

Here's something to get you started.

http://www.orionsarm.com/xcms.php?r=oa-page&page=gen_skyonalienworlds

You're referring to minimum molecular weight retained (MMWR). My boilerplate reply:

Thank you both, and all the thread contributors - very helpful and interesting.

As to the arc subtended by a sun or other body in the sky - doesn't it simply depend on diameter and distance? An object twice the width of our sun at the same distance would appear twice as wide in the sky, and if it were twice as far away it would appear as our sun does. The moon appears sun-sized to us, but is actually 200-ish times smaller and closer.

The tricky one I always found to be determining times of sunrise and sunset on worlds with large axial tilts, and/or at extreme latitudes - luckily "Practical Astronomy With Your Calculator or Spreadsheet" helped, though twilight and length of time for amounts of light before sunrise or after sunset are still problematical and based on atmosphere.

 

[shaunhilburn]

Thank you both, and all the thread contributors - very helpful and interesting.

As to the arc subtended by a sun or other body in the sky - doesn't it simply depend on diameter and distance?

Yes.

An object twice the width of our sun at the same distance would appear twice as wide in the sky, and if it were twice as far away it would appear as our sun does. The moon appears sun-sized to us, but is actually 200-ish times smaller and closer.

Simple ratios (half size, twice as close) may work for short distances, but diminishing angular size is not linear with distance.
The correct solution is 2 x arctan(radius/distance). Excel uses radians; you have to convert using DEGREES(2*ATAN(R/D))

 

[Frankymole]

Ah, cool. Thanks! I've also found this site which has some stuff on angular diameters and also on brightness: http://evildrganymede.net/rpgs/worldbuilding/


A slightly-related question: is the speed of sound likely to differ in different densities (or compositions) of atmosphere? Is that calculable?

(I presume it would, if the speed of sound on Earth varies at different pressures, under water, etc)...

I was thinking about it because of a recent off-forum discussion of sonic booms.

 

[shaunhilburn]

A slightly-related question: is the speed of sound likely to differ in different densities (or compositions) of atmosphere? Is that calculable?

(I presume it would, if the speed of sound on Earth varies at different pressures, under water, etc)...

I was thinking about it because of a recent off-forum discussion of sonic booms.

This hyperphysics links has the formula: http://hyperphysics.phy-astr.gsu.edu/hbase/sound/souspe3.html#c1
Also http://en.wikipedia.org/wiki/Speed_of_sound


For ordinary sea-level air, it's: 20.05 x SQRT(Kelvin_Temp), 4.3 times faster in water.
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