Architect of Worlds – Step Twenty-Three: Determine Atmospheric Mass and Pressure
In this step, we determine the atmospheric mass of the world under development. The atmospheric mass is measured relative to that of Earth – a world with surface gravity of exactly 1, and atmospheric pressure of exactly 1 “atmosphere” at the surface, will have an atmospheric mass of 1.
Some worlds will have a Trace atmosphere – enough to provide climate and weather effects on the world’s surface, but not enough to support any form of complex life. Still other worlds will have no atmosphere at all (or at least no atmosphere that can be detected without sensitive instruments). In both of these cases, the atmospheric mass will effectively be zero.
Atmospheric mass depends on a large number of factors: the world’s blackbody temperature and M-number (determined in Step Nineteen), its prevalence of water (determined in Step Twenty), whether it has undergone a runaway greenhouse event (also determined in Step Twenty), the degree of ongoing vulcanism (determined in Step Twenty-One), and the presence and strength of a magnetic field (determined in Step Twenty-Two).
Once a world’s atmospheric mass has been fixed, the pressure of the atmosphere at the surface (sea level or some other convenient “datum”) can also be determined.
Procedure
Begin by building a list of the likely major components of the world’s atmosphere. Refer to the following table, which lists a number of volatile compounds which might make up a large and stable portion of an atmosphere.
| Atmospheric Components Table | ||
| Possible Major Component | Maximum M-Number | Minimum Blackbody Temperature |
| Molecular Hydrogen (H2) | 2 | 20 K |
| Helium (He) | 4 | 5 K |
| Molecular Nitrogen (N2) | 28 | 80 K |
| Carbon Dioxide (CO2) | 44 | 195 K |
For each item on the Atmospheric Components Table, check to see whether the world under development has an M-number that is no higher than the one given in the table, and a blackbody temperature that is no lower than the one given in the table. If the M-number is too high, that potential component of the atmosphere will undergo thermal escape. If the blackbody temperature is too low, that component will tend to “freeze out” and form liquid or solid layers on the surface. Either way, that volatile will not be available to make up a substantial atmosphere.
Make a list of the atmospheric components that meet both conditions, and then refer to the following three cases.
First Case
This case holds if one or more of molecular hydrogen, helium, or molecular nitrogen meet both conditions from the table.
In this case, roll 3d6 and modify the result as follows:
- +6 if the world has Massive prevalence of water
- +6 if the world has undergone a runaway greenhouse event
- +6 if the world has a Molten Lithosphere
- +4 if the world has a Soft Lithosphere
- +2 if the world has an Early Plate Lithosphere
- -2 if the world has an Ancient Plate Lithosphere
- -4 if the world has a Solid Lithosphere
- -2 if the world has a Moderate Magnetic Field
- -4 if the world has a Weak Magnetic Field
- -6 if the world has no Magnetic Field
If the modified dice roll is 0 or less, then the world will have a Trace atmosphere, with an atmospheric mass of zero. Otherwise, multiply the modified dice roll by:
- 10 if the world has undergone a runaway greenhouse event
- 1 if the world has blackbody temperature less than 125 K and Massive prevalence of water
- 0.1 otherwise
The final result is the world’s atmospheric mass. Feel free to adjust this result by up to half of the multiplier.
Second Case
This case holds if the first case does not, but carbon dioxide meets both conditions from the table.
In this case, the world will automatically have a Trace atmosphere, with an atmospheric mass of zero.
Third Case
This case holds if neither the first case nor the second case is in effect (that is, none of the volatiles listed on the table meet both conditions).
In this case, the world will automatically have no significant atmosphere, and an atmospheric mass of zero.
Surface Atmospheric Pressure
To determine the atmospheric pressure at a world’s surface, multiply the atmospheric mass by its surface gravity.
Examples
Arcadia IV has blackbody temperature of 281 K, an M-number of 5, Extensive water with no runaway greenhouse, a Mature Plate Lithosphere, and a Strong Magnetic Field. Major components of the atmosphere will include both molecular nitrogen and carbon dioxide, so the planet falls squarely into the first case. Alice rolls an unmodified 3d6 and gets a result of 9, so Arcadia IV has an atmospheric mass of 0.9. Since the planet has surface gravity of 1.05, the atmospheric pressure at sea level is abut 0.95, very comparable to that of Earth.
Arcadia V has a blackbody temperature of 226 K, an M-number of 6, Moderate water with no runaway greenhouse, a Mature Plate Lithosphere, and no magnetic field. Major components of the atmosphere will include molecular nitrogen and carbon dioxide, so this planet also falls into the first case. Alice rolls 3d6-6 (modified due to the lack of a magnetic field) for a result of 7, so Arcadia V has an atmospheric mass of 0.7. The planet has surface gravity of 0.82, so atmospheric pressure at the surface is about 0.57.
3 thoughts on “Architect of Worlds – Step Twenty-Three: Determine Atmospheric Mass and Pressure”
As expected, I’m really enjoying the latest bunch of posts on “Architect of Worlds”. A couple of questions/suggestions on this one:
1. It’s always nice to have examples of the kinds of celestial bodies we are talking about. You could mention Earth’s Moon or Mercury as an example of no atmosphere (obvious as it may be), Mars as an example of a trace atmosphere, and Earth or Saturn’s moon Titan as an example of a more massive atmosphere.
2. There is a +6 modifier on the roll if the world has Massive prevalence of water, but there are no modifiers (say, +4 and +2, respectively) for Extensive and Moderate prevalence of water. Is there a particular reason for having a modifier only for Massive Prevalence of water?
3. I think there should be a modifier for planets in close orbits around very low-mass stars, since such low-mass stars are very active early during their lifespan and atmospheric erosion will therefore be strong. However, I’m not sure where that modifier should set in – we know that larger red dwarfs of, say, 0.35 solar masses have much lower flare activity than an extremely low-mass star of 0.10 solar masses. I’m also not sure how big the modifier should be. I’ll look at some papers on the subject if I have time.
1. Not a bad idea. I’ll consider that when I come back to revise this section.
2. The Massive water (only) modifier is there to catch a few special cases – notably super-Earths that are likely to have rather dense atmospheres, and Titan-class objects that do likewise even if they don’t have significant magnetic fields. Earthlike worlds with less than Massive water need to be more reliant on their magnetospheres to retain the air.
3. I looked at ways to model the behavior of energetic stars, and it quickly turned into a snarl. It doesn’t appear to be a simple matter, just how active a given low-mass star is likely to be at any given point in its lifespan. So instead I’m going (at least for now) with a model that assumes *all* young planets are likely to lose their primordial atmospheres quickly, either to a red dwarf primary’s flares or to the primary’s T-Tauri stage. The later atmosphere exists as a balance between the loss to the stellar wind and replenishment via outgassing. I’m making the (very loose!) assumption that all stars are about equally hostile after the first billion or so years, and the governing factor is the strength of the planetary magnetosphere. Probably an oversimplification, but it should work okay as a first approximation, and if I can find the need to tweak it from there I can.
Make sense?
2. I thought you might be trying to cover the case of Titan. Thanks for the clarification, and for elaborating on the other cases as well.
3. Yes, that makes sense. I can’t think of a reasonably straightforward way of incorporating highly-active low-mass stars into the model, either. So, treating the atmosphere as a balance between the remnants of the primordial atmosphere (if there are any) and replenishment via outgassing is a very good idea.
Even if a particular planet has little geologic activity, you could always postulate that it kept most of its primordial atmosphere because it formed at a bigger distance from its low-mass host star and migrated inwards later on.
It’s all a matter of speculation, anyway. For every paper arguing that planets in the habitale zone of M dwarfs will very probably be desiccated and sterilised, you can find another one which argues that, no, these planets could still retain atmospheres and liquid water oceans that are more substantial than Earth’s. There seems to be a lot of back and forth in the scientific literature.