Atmospheres D, E, and F

[daryen]

Malenfant (or any other actual scientist out there),

Just how "realistic" are these weirdo atmospheres?

On the surface, it would seem F could happen, and E is completely nonsensical. Is this right?

And could D happen? Can a planet hold onto that much atmosphere? Could it support enough live to keep the oxygen content breathable at the higher levels?

Just wondering.

 

[Malenfant]

I just posted something about this on the TNE playtest board

.

It's all down to "Scale Heights". atmospheric pressure drops by a factor of 0.37 (1/e) with each scale height. On Earth, the scale height is about 7.3 km. On an atm D or F world, the scale height must be smaller - the only way it can get smaller is if the world's gravity is higher, the atmosphere is made of heavier gases, or the temperature is colder.

Atm D worlds have very high "sea level" pressures but have small enough atmospheric scale heights that the pressure drops to more habitable levels at higher altitudes.

Atm F worlds have extreme topographic lows and small scale heights - this means that the atmosphere is thick in the lowlands but thin at the mountaintops. While that doesn't sound much different to an atm D world, with atm F the lowlands must be rare (eg big canyons) and the highlands must be common (so most of the surface is "high" and therefore above most of the atmosphere).

As for realism, D is possible, but it's probably unlikely to be an N2/O2 mix. F is quite possible, depending on the topography (if Mars was bigger and had more atmosphere, the Vallis Marineris and the Hellas Basin would probably be deep enough to have a breathable atmosphere while the rest of the surface couldn't).

Ellipsoid worlds as described in the text are indeed completely nonsensical. Personally, I'd replace them with "Thin, Low" and make atm F "Standard, Low" or something.

 

[aramis]

Ellipsoids are, to my mind, going to be possible, but not exactly as described in the text...

they are merely worlds where the poles are SIGNIFICANTLY closer to center of mass than the equators, but the atmosphereic effcts would be minimal above size 2 or so... (The gravitational gradient is too low to generate such a situation) The earth's 25 mile deviation from true has no directly measurable effects; the atmosphere conforms to it.

The Dense low-lying is quite possible. Likewise, a thin, but breatheable at low levels atmosphere would also be possible... but rare. Since D, E, & F are "Custom Fit Only", unless you're palying in the OTU, you need never encounter them!

 

[Malenfant]

I think Ellipsoids were based on Mesklin in Hal Clement's "Mission of Gravity" book, which was a huge solid world that had a truly obscene rotation period that supposedly flattened it and made a big bulge at the equator.

Those extremes just aren't realistic - they'd fly apart for starters, and there's no way you could get a solid world that big rotating so rapidly.

D & F atmospheres are in the right place, since you'd expect those around larger worlds. But E atmospheres - even if they could world - don't have any particular bias towards large worlds.

 

[daryen]

Thanks for the input.

So, just to make sure I am understanding this right, a world with a D atmosphere, even if it had the right atmospheric pressure at some altitude, would still be unbreathable due to the absense of oxygen (and nitrogen). Right?

Obviously, if we can have worlds smaller than our moon with a breathable atmosphere, a breathable D atmosphere shouldn't be a problem, though.

If I remember right, E worlds are more idiotic that you are giving them credit for. I think they are described as "egg" shaped, meaning that it is stretched pole-to-pole rather than at the equator.

 

[Malenfant]

Not quite. Technically, D atms are supposed to be N2/O2 mixes, but they're unbreathable at 'sea level' because the pressure is too high (IIRC nitrogen narcosis sets in around 3-5 atms with earthlike oxygen percentages?). So since pressure drops with altitude, the idea is that they become breathable at some higher altitude (eg on a mountainside). What I wonder is how likely you are to get a world that has that much N2 and O2 anyway. One could imagine having a world with 3+ atms pressure and lots of N2 and little O2, but that isn't helped by going up in altitude since the O2 decreases with height too so it'd still be unbreathable.

So I imagine that atm D is supposed to be a very dense (3+ atms pressure) atmosphere that is mostly N2 with some O2, but is unbreathable at the mean surface level without protection. As you go up in height, the pressure drops and you will reach some level where the nitrogen pressure isn't high enough to be toxic.

The alternative is that you have a high pressure world with an exotic atmosphere - but then of course that negates the whole point of having a special atmosphere type. There's a point in Venus' atmosphere where the pressure is around 1 atm, but since it's unbreathable anyway then you may as well still call it a type B, right?

The other thing is that realistically there is no way that you can have worlds that are smaller than our moon that have a breathable atmosphere (or indeed any atmosphere at all) at habitable temperatures. They're not massive enough to hold onto the gases at those temperatures - unless they have superdense cores which are ludicrously unrealistic. I managed to explain one away (Enos in the Sword Worlds), but you can't use the same ridiculously unlikely explanation for the many size 1 worlds that are generated with atmospheres.

 

[Straybow]

I vaguely recall 5 atm (roughly 160 feet) is the limit for scuba diving with atmospheric mix.

 

[daryen]

Malenfant,

That was my point. It is physically impossible to have a breathable atmosphere on a planet the size of the moon. (Without massive hacks like Enos.) So, if we can accept that, since there are so many of them, then we should be able to accept an occational type D, or maybe even a type E, atmosphere, too.

(From what I understand, and from what GT:FI had, a world of size 4 or less is pretty much unable to retain a breathable atmosphere.)

 

[daryen]

A new point on these atmospheres.

Right now any world with an atmosphere of A+ and a hydrosphere of 1+ is defined to have fluid oceans (trade code Fl).

This should be amended to atmosphere of A-C and a hydrosphere of 1+. Worlds with atmospheres of D-F and a hydrosphere of 1+ should still have water oceans, not fluid oceans.

(I guess a case could be made with D atmospheres, but I doubt it.)

This has griped me for a while, but it hasn't really shown up until I have been going through the Domain of Deneb worlds. I really want to change the definition of "Fl".

 

[Malenfant]

Makes sense. Though technically an F atmosphere could be exotic too. After all, there's no particular reason why an exotic atmosphere couldn't settle into the low places just like an N2/O2 mix.

 

[daryen]

I don't think F can be exotic. By definition, D-F atmospheres are breathable at some location (D->high, E->two latitudes, F-> low). Therefore, they must all be N2/O2 mix.

The only possible exception I see is D, which might have some odd effects with the higher pressure down on the surface, even with an N2/O2 mix.

 

[Malenfant]

Not by definition, perhaps. But is one supposed to classify an exotic F atmosphere as just a plain old type A though, even if it's settled into the lowlands?

 

[daryen]

I would say that, yes, if an atmosphere is exotic, it is type A, regardless of its other characteristics.

The reason I said "by definition" is that F is defined as "Thin, low. Breathable below certain altitudes." I interpreted "breathable" as N2/O2 mix.

 

[Psion]

Would an E atmosphere be possible for a close tidally influenced moon?

(Of course by the canon tables, you can't generate a moon that big, so it may be besides the point. So you'd just as well ignore it.)

 

[Malenfant]

I don't think so. If it's that stretched by tides, then the world's going to be torn to shreds.

That said, the E atm description in bOok 6 is just plain silly anyway. It says "the world's surface is ellipsoidal, not spherical in shape. Because the atmosphere remains spherical, surface atmospheric pressure ranges from very high at the middle to very low at the ends. Breathable bands may exist at some point within the range of pressure".

See the problem? It claims that the solid, rocky, non-responsive surface is ellipsoidal, but the fluid, responsive atmosphere is spherical! But tides will, if anything vastly affect the atmosphere and ocean much more than the solid body, so it should be the other way round - the atmosphere is ellipsoidal and the surface is spherical. Besides, I can't think of any way that the surface could be so distorted without the atmosphere also being distorted.

Now, it might be possible for the atmosphere to be distorted by tides and the surface to be effectively spherical. Look at somewhere like Titan - the tidal bulge raised there by Saturn is on the order of 100 metres - it's probably a bit more in the atmosphere because that can respond more easily. If the orbit was circular, then this bulge would stay in the same place relative to Saturn in both the solid body and the atmosphere. But the orbit is eccentric, which means that the bulge moves relative to Saturn so the thicker part of the atmosphere "wobbles" over the equatorial regions. But of course the pressure difference between the poles and equator because of the bulge isn't so large as to change it's atmosphere type.

There's an example in vol 2 of the the awesome 2001 Nights manga comic trilogy of a world orbiting Algol that suffers from horrendous tides. I don't think it's particularly realistic, but the stellar tides basically pull the atmosphere so much that pretty much all the air rushes from the poles towards the equator during an orbital cycle. The three expeditions that go visit end up getting crushed by huge boulders blasted around by the winds at the equator, drowned in tidal waves of sand moved by the wind in a desert at midlatitudes, and nearly killed at the poles when they're frozen to the ground as the air pressure drops at they end up in a perfect vacuum there. Perhaps that kind of thing might be more what a Type E should be like... (it's probably not much more realistic than the default, but it's cooler IMO

)

 

[Psion]

Alrighty then. Since GtD doesn't have any type E atmosphere's, I'll just ignore it.

 

[Malenfant]

So what would you do when a type E atmosphere result is rolled up? Or do you not make your own worlds ever?

 

[Psion]

I am at the stage in my life where I am not using the CT generation system. My only reason for using Traveller's system generation is to use the backdrop. Lose the backdrop, I have no reason to use the system. (Well, that's not totally true. I still dig the UWP format, just not the system for generating it.)

If I were to generate something from scratch, it would be something that generates 3D sectors, possibly using GURPS Space, First In, and/or the Ross Smith system as references.

My post about spectral type densities is actually in an effort to help the programmer brush up the StarGen program so it's realistic enough for my happiness.