Atmospheres and world sizes

[Gruffty]

This is a tricky one, but I'm looking for a simple answer.

We know that a size 8 world can "hang on" to a type 6 atmosphere (i.e. Terra). We know that a size 0 or 1 world can't hold on to an atmosphere. But what about the other world sizes?

I know that a lot of factors affect a planet's ability to hold on to an atmosphere, but as this is Traveller, I don't really want to go down too complex a route. I'm just looking for a simple table, perhaps something like this:

</font><blockquote>code:</font><hr /><pre style="font-size:x-small; font-family: monospace;">Size Atmos
0 0
1 0
2 0 or 1
3 0, 1 or 2
4 0 to 3
5 0 to 4
6 0 to 5
7 Any
8 Any
9 Any
A Any</pre>[/QUOTE]Any thoughts, suggections, ideas on this?

 

[mike wightman]

This is the atmosphere range for each planet size that you will get most of the time, with the median result favoured:
</font><blockquote>code:</font><hr /><pre style="font-size:x-small; font-family: monospace;">world usual
size atmosphere
0 0
1 0-3
2 0-4
3 1-5
4 2-6
5 3-7
6 4-8
7 5-9
8 6-A
9 7-B
A 8-B</pre>[/QUOTE]If you want to use this table - or an even quicker way to do it - take the world size and roll a die and apply the resulting number to the world size:
1 = -2, 2 = -1 3or4 = 0, 5 = +1, 6 = +2

 

[Flynn]

Are you looking for a scientific response or a game-statistic response, Grufty?

-Flynn

 

[Anthony]

Originally posted by Gruffty:

We know that a size 8 world can "hang on" to a type 6 atmosphere (i.e. Terra). We know that a size 0 or 1 world can't hold on to an atmosphere. But what about the other world sizes?

Talking realistically, for most likely planet materaisl, sizes 5-10 could have just about any atmosphere type, 4 probably can't hold onto a breathable atmosphere but can hold CO2, 3 can have a trace atmosphere, 0-2 won't have any atmosphere.

 

[Gruffty]

Originally posted by Flynn:
Are you looking for a scientific response or a game-statistic response, Gruffty?

-Flynn

I'm looking for a realistic but simple system. The only other factor I want to have any effect on the atmosphere type is the planet's distance from the star. Even bringing that factor into the equation will complicate matters further than I'd like, but it can't be avoided

Thanks for the probabilities analysis Sigg, BTW. That will come in useful, but isn't quite what I'm after at present.

Given the limitiations of the Traveller worldgen systems, I'm starting to move towards building worlds by hand (i.e. no dice involved) using construction principles as opposed to random die rolls, then converting the results into Traveller UWPs.

Hence this post...

 

[Gruffty]

Going by Anthony's parameters:

</font><blockquote>code:</font><hr /><pre style="font-size:x-small; font-family: monospace;">Size Atmos
0 0
1 0
2 0
3 1
4 ? which code would best represent CO2?
5+ Any atmosphere type</pre>[/QUOTE]I suspect I'll have to look at creating a table like the above for each star type....

 

[Pickles]

Do it ... you know you want to ...

Really, I think look-up tables are worth the trouble if you're going to do any amount of world generation. There's not that many star types ...

 

[Gruffty]

I was thinking of something like this (Refs/GMs would need LBB6, MT:RM, WBH, GT:FI or some such to hand):

</font><blockquote>code:</font><hr /><pre style="font-size:x-small; font-family: monospace;">Star: G2 V

Size IZ Atmos HZ Atmos OZ Atmos
0 0 0 0
1 0 0 0
2 0 0 0
3 0 1 ?
4 ? ? ?
5 9(?) Any 9+
6
7
8
9
A</pre>[/QUOTE]IZ = Inner Zone
HZ = Hab Zone
OZ = Outer Zone.

Teh. Just the gaps to fill in then..........

 

[Merxiless]

2300 has a good system, based on the density of the planet, and its gravity, because it really depends on planetary gravity, not size.

Rocky core worlds are not typically going to have the denser atmospheres, unless they have been terraformed, typically.

2300 uses a different system to determine how gas giants and planetary atmospheres are formed, i.e. a core that is so big, He, and H won't escape into space in large amounts, thus = Gas Giant.

By the same token, larger non-gas giant worlds will have (or can hold) heavier gases.

The general problem as always is that Traveller's system coded atmospheres based on pressure, and taint, not on specific content. It was made generic to be simple, and easy to use.

I'm not knocking it, just that if you want very specific flavor, you need to go Grand Survey, or plug in 2300 atmospheric tables, or there are some good articles written on the situation, in the Traveller literature.

I'll look around in my Challenge mags and JTAS collection. Someone has done work like you are looking for, about 15 years ago, as I recall.

It really is a case of, if you want it really detailed, you have to go beyond the UWP.

But getting there first, is not so easy, if all you have is the UWP to reference.

 

[Ptah]

Originally posted by Gruffty:

I'm looking for a realistic but simple system. The only other factor I want to have any effect on the atmosphere type is the planet's distance from the star. Even bringing that factor into the equation will complicate matters further than I'd like, but it can't be avoided

Hence this post...

A quick scientific rule of thumb, whether an planet has an atmosphere depends primarily on the age of the planet, the escape velocity (gravity), and the RMS velocity of the gas. There are of course other factors and outgassing from the planet can maintan an atmosphere where none should exist. The oft cited work on this is Martyn Fogg's articles on terraforming.

Temperature increseas RMS velocity as the square root so the closer in to the star the lower the atmosphere rating and/or the heavier the gas (more CO2 instead of N2 and O2). RMS velocity goes down as the inverse square root of the mass. The temperature is usually taken as that at the top of the atmosphere since this is where gas escapes. Technically, the escape velocity there is the one to use as well.

For "earth-like" worlds the range of temperatures is not enough to make a big differnce IMO from a game mechanics point of view, maybe +1/-1 max from outer to inner zone.

What might be more interesting is IZ planets have more of a CO2 taint, while OZ planets might have more N2 relative to O2 but still a hig partial pressure of O2.

Gravity is a big thing as well. Mars is a good example. It's about half the size of Earth but it's density is about 3.9 g/cc versus Earth's 5.5 g/cc. So instead of having an escape velocity sqrt(2) of Earth's it's about half.

A quick GOOGLE search turned up lots of info on this topic, the equations at wikipedia look OK, and Martyn Fogg's general ratios for escape velocity to RMS velcotiy for atmosphere lifetime are being bandied about.

 

[Gruffty]

Without wishing to offend, be rude or appear ignorant:

Originally posted by Ptah:
A quick scientific rule of thumb, whether an planet has an atmosphere depends primarily on the age of the planet, the escape velocity (gravity), and the RMS velocity of the gas. There are of course other factors and outgassing from the planet can maintan an atmosphere where none should exist. The oft cited work on this is Martyn Fogg's articles on terraforming.

Temperature increseas RMS velocity as the square root so the closer in to the star the lower the atmosphere rating and/or the heavier the gas (more CO2 instead of N2 and O2). RMS velocity goes down as the inverse square root of the mass. The temperature is usually taken as that at the top of the atmosphere since this is where gas escapes. Technically, the escape velocity there is the one to use as well.

"This means nothing to me, oooooohhhhh, Viennaaaaaaa.........."

Sorry - that all flew straight over my head

This is why I'm looking for simplistic approaches, cos the science baffles me most of the time...........

 

[Gruffty]

I suppose what I'm really trying to say here is that I am working from the assumption that Refs/GMs either don't want/need that level of detail or if they do, they can/are able to generate that level of detail for themselves.

Call me stoopid if you like....

<All: "You're stoopid!">

 

[Ptah]

Sorry Gruffty, I admit it's a bit much and when I went to do some calcs myself realized it would take a bit to put it all together in a simple format.

I guess this is the takehome bit:

For "earth-like" worlds the range of temperatures is not enough to make a big differnce IMO from a game mechanics point of view, maybe +1/-1 max from outer to inner zone.

What might be more interesting is IZ planets have more of a CO2, [edit] i.e., +1 mod. towards taint or insidious, while OZ planets might have more N2 relative to O2 but still a hig partial pressure of O2, [edit] i.e., +1 mod towards insidious.

I'll try a bit with the equations at lunch today.

 

[Plankowner]

I don't think you need to create a table for each star type, just make a table generically for the zones.

I doubt it will make much difference if a planet is in the Inner Zone of a G2 star or the IZ of an M5 giant star. If it is in the IZ, it will have xxxx types of atmospheres. The same could be said for the outer zone, although you might want to break up the outer zone into a couple of different temperature ranges since at very low temperatures gasses like Oxygen and Nitrogen start to solidify. Of course, out that far the ATMOS codes allowed should only be 0, 1, B, C. With LOTS of the 0, 1 and less B and very few C, but possible.

I use the standard 2d6-7+SIZE but have different tables for the IZ, HZ, and OZ. I break the OZ into 2 areas OZ1 and OZ2. with my break at about the 4th OZ orbit if you are using LBB6.

 

[Chaos]

Originally posted by Anthony:
</font><blockquote>quote:</font><hr />Originally posted by Gruffty:

We know that a size 8 world can "hang on" to a type 6 atmosphere (i.e. Terra). We know that a size 0 or 1 world can't hold on to an atmosphere. But what about the other world sizes?

Talking realistically, for most likely planet materaisl, sizes 5-10 could have just about any atmosphere type, 4 probably can't hold onto a breathable atmosphere but can hold CO2, 3 can have a trace atmosphere, 0-2 won't have any atmosphere. </font>[/QUOTE]I disagree. Saturn´s moon Titan is a size 3 world, and it has more than a trace atmosphere; the recent Cassini probe has found that atmosphere to be hazy enough to block the view to the ground from orbit.
I guess Titan´s atmosphere would qualify as Thin, as far as pressure is concerned, and of course Exotic as far as composition is concerned.

Of course, in Traveller UWP terms, there is only 0, 1, A, B and C as far as unbreathable atmospheres are concerned. I´d go with A (Exotic), but a size 3 world can´t get an A atmosphere via random generation.

 

[Anthony]

I was assuming a world in the life zone. At 9.5 AU, Titan can hold on to atmosphere as well as a size 5 planet in the life zone.

 

[BillDowns]

Related to this subject, I happen to find an old copy of JTAS #17, 1983, which had a Special supplement on Exotic Atmospheres. Rather good rules, I thought.

 

[Plankowner]

Titan has an atmospheric pressure of 1.4 Terran, so I don't think you can call it thin, I think it might even qualify as dense. Probably needs to be a "B" atmosphere though since full enviro suit is required, not just oxygen mask like you need for an "A".

This brings up an interesting point. Per LBB6, on a throw of 10+, ANY Outer Zone world can have an atmoshere "A". Perhaps that should be changed to "B" to reflect the low temperatures. Only worlds rolled in the HZ (12+ reqd) would get the "A" atmosphere.

A big thing to remember is that Traveller was designed with LOTS of VARIETY in it's worlds. Any table you want to make should include that overall theme.

Also, the more we learn about the rest of our solar system and other solar systems, the more we realize that we don't know.

Ten years ago, no one thought a Gas Giant could exist in the Inner Zone, but we know better now. Don't let our small view of the universe prevent you from having lots of variety in your game. (Climbs down from soap box).

I HATE it when real world science interferes with this game.

 

[Ptah]

Originally posted by Plankowner:
I don't think you need to create a table for each star type, just make a table generically for the zones.

I doubt it will make much difference if a planet is in the Inner Zone of a G2 star or the IZ of an M5 giant star. If it is in the IZ, it will have xxxx types of atmospheres. The same could be said for the outer zone, although you might want to break up the outer zone into a couple of different temperature ranges since at very low temperatures gasses like Oxygen and Nitrogen start to solidify. Of course, out that far the ATMOS codes allowed should only be 0, 1, B, C. With LOTS of the 0, 1 and less B and very few C, but possible.

I use the standard 2d6-7+SIZE but have different tables for the IZ, HZ, and OZ. I break the OZ into 2 areas OZ1 and OZ2. with my break at about the 4th OZ orbit if you are using LBB6.

I think this is dead on as the thing that tends to define the zones is temperature. And besides gravity, the thing that tends to define atm pressure is temperature.

 

[TheEngineer]

Hi !

I have to agree with Plankowner.
It might make less sense to press a dimly understood complex matter into a simple chart.
The result might be as wrong as any other simplyfication (statistical error condensation).

As long as its not perfectly provable wrong (and thats a rare situation), I would leave good old Traveller assumptions as they are.

What I strongly would suggest, is to recognize, that any atmosphere is a dynamic construction, which always changes over geological times. This could provide a greater flexibility, when dealing with somehow strange compositions.

Regards,

Mert