An Astrographic Riddle...

[kaladorn]

Originally posted by Evil Dr Ganymede:
Was that an example of a world you were having trouble with? Hrm. The '23' atm/hyd combo there plus the temperature is a bit odd. If the pressure's low, water can't exist on the surface. But if the temperature's between 0 and 100 then it can't exist as ice. I'd guess it'd want to sublimate instead and exist as water vapour. Though are the poles cold enough to allow the temperature to drop below freezing? If so, you can have ice caps. Otherwise, I'd say the hydrographics there would have to be in liquid form under the surface if they're to exist anywhere - if that can't work then just say the hydrographics percentage is zero, because there's no other way for it to exist.

This is part of the problem - the hyudrographic stat is created and the other stats aren't constrained directly to make that feasible. Or conversely, the hydrographic stat isn't automatically adjusted as a consequence of things like 'no water or no ice can exist here'.

It'd be nice, perhaps in T5, if they were to come up with a planet generation system that did one of the above two things. Consistency would be a good thing...

I have WBH, and it was a pretty good book, though the temperature calculation is actually fairly wonky. GT: First In is by far and away the most realistic SF RPG world generator out there - the calculations are all realistic there.

Assuming your world is at 1.6 AU from the star, then according to FI it should have a blackbody temperature of 287 K. Taking into account the feeble greenhouse effect and the albedo, it should have a final temperature of about 282 K (9 C) at hex rows 4-5. That's rather colder than your temperature of 37.7 C! Though FI does use different stellar luminosities. In fact, WBH uses the fourth roots of the luminosities stated in CT Book 6, and the latter are in the right ballpark.

The WBH temp calculation is a bit screwy since it uses the fourth root of the luminosities, the Orbit Factor (an indeterminate fudge factor, determined by [O = 374.025/(square root of orbital distance in AU)] ), and directly applies the energy absorption and greenhouse effect. It's not particularly accurate, whereas First In's is very accurate

.

I'm a tad confused, Dr. E.

You say FI is more accurate/realistic, yet the second paragraph above seems to suggest CT book 6 or WBH might have better use/statistics for Stellar Luminosity. Care to clarify this minor confusion for me?

 

[Tanuki]

Originally posted by kaladorn:
You say FI is more accurate/realistic, yet the second paragraph above seems to suggest CT book 6 or WBH might have better use/statistics for Stellar Luminosity. Care to clarify this minor confusion for me?

Well....

The CT B6 numbers are problematic. The luminosity numbers may be okay across the board, but the mass numbers are NOT. It looks like someone ran the mass/luminosity equation back to generate mass numbers for non-main-sequence stars. The results are totally bogus. Best to completely ignore the class VI stars, too.

I _think_ CT B6 is okay for main sequence stars, but I wouldn't trust it for anything else.

I believe there's something wonky about the temperature equation as well. Certainly the one in the first edition was messed up.

 

[EvilDrGanymede]

Originally posted by Evil Dr Ganymede:
Was that an example of a world you were having trouble with? Hrm. The '23' atm/hyd combo there plus the temperature is a bit odd. If the pressure's low, water can't exist on the surface. But if the temperature's between 0 and 100 then it can't exist as ice. I'd guess it'd want to sublimate instead and exist as water vapour. Though are the poles cold enough to allow the temperature to drop below freezing? If so, you can have ice caps. Otherwise, I'd say the hydrographics there would have to be in liquid form under the surface if they're to exist anywhere - if that can't work then just say the hydrographics percentage is zero, because there's no other way for it to exist.

Actually, I should point out that I'm wrong here - Very Thin atmospheres can sustain liquid water on the surface, though the boiling point varies between 319 (0.1 atms) and 349 K (0.4 atms). I was getting confused with Mars' UWP in Book 6, which lists it as having a Very Thin atmosphere when in fact today it has what would be classed in Traveller as a Vacuum (Type 0) atmosphere. Which right there shows you that the classification scheme is rather flawed, since Mars surface most certainly isn't under anything resembling a vacuum (though the pressure is generally too low to be considered 'Trace' - that definition starts at 10 millibars, and Mars' surface pressure only gets that high at the deepest points on the surface).

Originally posted by kaladorn:
This is part of the problem - the hyudrographic stat is created and the other stats aren't constrained directly to make that feasible. Or conversely, the hydrographic stat isn't automatically adjusted as a consequence of things like 'no water or no ice can exist here'.

I think it's more sensible to base it on size (2d-7+size) than on atmosphere, at least. But hydrographics is a bit of a nightmare anyway - as best as I figure it from the Book 3 definition, it represents the total amount of liquid exposed on the surface of the planet. So a Europa-like world with an ocean buried under a thick ice shell does not have a hydrographic % of 'A', because none of its is exposed on the surface - despite the fact that it does have a global ocean beneath the ice.

I'm certainly finding as I bash away at making a realistic world generation system that Hydrographics is proving to be as annoyingly complicated as atmospheres

.

I'm a tad confused, Dr. E.

This much is Obvious, since you're not calling me 'Dr G'

You say FI is more accurate/realistic, yet the second paragraph above seems to suggest CT book 6 or WBH might have better use/statistics for Stellar Luminosity. Care to clarify this minor confusion for me? [/QB]

Sure - the CT luminosities are more in the right ball park than FI (from what I can figure, at least). To be honest, the stellar data in both books is a bit wonky, at least compared to the more up-to-date stellar structure models from the scientific papers I've been using. Come to think of it, perhaps it's not fair to pass judgement on either of them at this stage, particularly considering that neither really takes stellar evolution properly into account (I've got a JTAS article planned on that).

However, FI is definitely rather more realistic in the other aspects of planetary design - orbit placement, base temperature, even minimum molecular weight retained by the planet (tucked away in the sidebar on page 65).

 

[kaladorn]

Originally posted by Evil Dr Ganymede:
I'm certainly finding as I bash away at making a realistic world generation system that Hydrographics is proving to be as annoyingly complicated as atmospheres

.

Quite.

</font><blockquote>quote:</font><hr />I'm a tad confused, Dr. E.

This much is Obvious, since you're not calling me 'Dr G'
</font>[/QUOTE]I assumed Evil was your given name and was being familiar....

Sure - the CT luminosities are more in the right ball park than FI (from what I can figure, at least). To be honest, the stellar data in both books is a bit wonky, at least compared to the more up-to-date stellar structure models from the scientific papers I've been using. Come to think of it, perhaps it's not fair to pass judgement on either of them at this stage, particularly considering that neither really takes stellar evolution properly into account (I've got a JTAS article planned on that).

Okay.... but where does that leave me?

However, FI is definitely rather more realistic in the other aspects of planetary design - orbit placement, base temperature, even minimum molecular weight retained by the planet (tucked away in the sidebar on page 65).

That last sounds a bit like the 2300 AD's method of determining atmosphere - by what would be retained by the planet based on gravity, I think it was.

 

[EvilDrGanymede]

Originally posted by kaladorn:
I assumed Evil was your given name and was being familiar....

Doesn't work though - Dr E would be Dr Evil, which is an entirely different character

.

Okay.... but where does that leave me?

In limbo? . Nah, seriously... I guess you're best off using whatever is consistent with the system you use (ie use FI if you use First In, Book 6 if you use CT). Chances are nobody is ever going to get it right in an RPG anyhow. Though I might come close .

That last sounds a bit like the 2300 AD's method of determining atmosphere - by what would be retained by the planet based on gravity, I think it was.

Exactly like that. Though even this isn't simple. It's also based on radius and temperature - of the top of the atmosphere (the exosphere) specifically - and there's no easy way to figure that out. For example, the MMW retained t depends on atmospheric composition to an extent - earth's exosphere temperature is about 3 or 4 times higher than its blackbody temperature, Venus' exosphere temperature is a bit less than blackbody temperature, and Mars' exosphere temperature is a little bit higher than its blackbody temperature.

 

[kaladorn]

Evil Dr. G., I concede you are correct and I have abused your nomenclature. I shall render you due title in future.

As to other matters, I look forward to seeing your work.

And I know enough physics to know that when the words 'blackbody temperature' enter the conversation (similarly in math the words 'eigenspace' or 'Hamiltonian Transform' or in Chemistry the dreaded 'Gibbs Energy'), it is time for me to pull the Ejection Handle. I've seen others do this when I talk about 'polymorphism' or 'super pipelining' so it is just an acknowledgement of when things are about to go into the 'Here Be Dragons' territory....

 

[EvilDrGanymede]

Originally posted by kaladorn:
Evil Dr. G., I concede you are correct and I have abused your nomenclature. I shall render you due title in future.

Thank you

And I know enough physics to know that when the words 'blackbody temperature' enter the conversation (similarly in math the words 'eigenspace' or 'Hamiltonian Transform' or in Chemistry the dreaded 'Gibbs Energy'), it is time for me to pull the Ejection Handle. [/qb]

Blackbody temperature is nowhere near as complicated as those other things (I don't know what those are, and the sight of eigen-anything makes me run away screaming). 'Blackbody Temperature' is simply the temperature that a perfectly absorbing (ie. black) body would have (in this case, at a given distance from a star). That's all it is.

 

[Pompe]

Atmosphere determination is not a matter of simply determining MMW. That is bound to be faulty for several reasons, such as but not limited to:

-loss depends on the structure of the atmosphere, not just the molecular composition.

-the planet has lost atmosphere not just at its current temperature and solar infall, but at a time when the surface was much hotter and the star had a more influential solar wind.

-the planet has lost or gained atmosphere because of impacts. Worlds with low escape velocity will lose more than they will gain, while the opposite goes for bigger worlds.

-the mass of the atmosphere depends on impacts and outgassing. In the latter case, it is certainly a matter of planetary chemical makeup but also of planetary mass/volume.

 

[EvilDrGanymede]

Originally posted by Pompe:
[QB] Atmosphere determination is not a matter of simply determining MMW. That is bound to be faulty for several reasons:

Yes, but at least it gives you a ballpark figure to start with (i.e. it's better than just wildly guessing). For example, it appears that it is impossible for size 1 worlds to hold onto any common atmospheric gases (CO2, N2, O2, CH4) on a significant (billion year) timescale if they are warmer than about 230K. From which we can say that any worlds that do have such atmospheres are either very recently terraformed, or just plain wrong.

-the planet has lost atmosphere not just at its current temperature and solar infall, but at a time when the surface was much hotter and the star had a more influential solar wind.

If you track the stellar evolution, you can take this into account to an extent.

-the planet has lost or gained atmosphere because of impacts. Worlds with low escape velocity will lose more than they will gain, while the opposite goes for bigger worlds.

-the mass of the atmosphere depends on impacts and outgassing. In the latter case, it is certainly a matter of planetary chemical makeup but also of planetary mass/volume.

True, but the mass of the atmosphere is never really considered explicitly, beyond whether or not it's 'Thin, Standard, or Dense' - assuming that is, you don't go by the utterly rigid (and incredibly constricting IMO) interpretation of the rules in CT book 3 that state that these labels are indicative of the breathability of the atmosphere (the oxygen pressure) and not the actual atmospheric pressure at the surface.

I'm certainly happy to assume that if a Dense atmosphere is indicated, then that shows the world had a more outgassing or more volatile-rich impacts than one with a Standard atmosphere (or retained more of its original gases, or whatever).

 

[Pompe]

Yes, but at least it gives you a ballpark figure to start with (i.e. it's better than just wildly guessing). For example, it appears that it is impossible for size 1 worlds to hold onto any common atmospheric gases (CO2, N2, O2, CH4) on a significant (billion year) timescale if they are warmer than about 230K. From which we can say that any worlds that do have such atmospheres are either very recently terraformed, or just plain wrong.

That last I agree with. However, a MMW value is an estimate which is in itself subject to assumptions and models. If one simply wish to disallow worlds of a certain size to have certain atmospheres, I don't know if one - at least with Traveller's atmosphere codes - need to involve an MMW.

If you track the stellar evolution, you can take this into account to an extent.

It isn't just the stellar evolution, it is the planetary evolution as well. An example.

One commonly uses MMW values based on the current temperatures of worlds. That is not necessarily interesting. Almost all rocky worlds would have started with about the same temperatures for their early life, many hundreds of K. If that is the time when most of the atmosphere is lost, the present day temperature is of less interest - even misleading - when calculating a MMW or for that matter, how dense the atmosphere is in general. Essentially, it all could boil down to planet size/mass.

Later temperatures could have other atmosphere effects not related to MMW, though.

True, but the mass of the atmosphere is never really considered explicitly, beyond whether or not it's 'Thin, Standard, or Dense' - assuming that is, you don't go by the utterly rigid (and incredibly constricting IMO) interpretation of the rules in CT book 3 that state that these labels are indicative of the breathability of the atmosphere (the oxygen pressure) and not the actual atmospheric pressure at the surface.

The mass of the atmosphere is on its own a very important factor when calculating pressure. If you have the planetary radii, the planetary gravity and the atmosphere mass you can tell what the surface pressure is - and then apply an appropriate code.

So factors influencing the mass will end up directly affecting the pressure, and therefore the atmosphere type. It may not be explicitly mentioned, no, but if one tweaks the system in order to make it more realistic I think it is good to know what lies behind the resulting numbers. Otherwise it is simple to overestimate the atmospheric density of small worlds and underestimate it of large ones.