In this step, we will work out the implications of several of the preceding steps for the composition of the world’s atmosphere. In most cases, this will depend solely on the atmospheric class (from Step Twenty-Three). However, in the case of a Class III (Earth-type) atmosphere, we will also use quantities developed in Steps Twenty-Five and Twenty-Six to determine some of the details. This will be useful in determining whether a given Earth-like world’s atmosphere is comfortably breathable for humans – or for invented aliens.

Procedure

Apply the appropriate case from the following.

Class I Atmosphere

A Venus-type atmosphere will be composed almost entirely of carbon dioxide, very hot and at immense pressures. It will also contain a small portion of nitrogen, and traces of other compounds such as sulfur dioxide. Exposure to such an atmosphere will be almost instantly fatal to human life.

Class II Atmosphere

A Titan-type atmosphere will be composed almost entirely of nitrogen, with a small portion of methane and traces of other compounds, such as carbon dioxide and various hydrocarbons. The atmosphere itself is likely to be suffocating to human life, but not actively toxic.

Class III Atmosphere

We will deduce the details of an Earth-type atmosphere from other elements of its surface conditions.

Begin by making a note of the total atmospheric mass as determined in Step Twenty-Three. Then, after determining the partial atmospheric mass taken up by each of the following components, subtract that quantity from the total atmospheric mass, so as to keep a running tally of how much of the atmospheric mass remains to be accounted for.

Also, make a note of the total greenhouse effect as determined in Step Twenty-Five. After determining how much of this greenhouse effect can be attributed to each atmospheric component, subtract that quantity from the total greenhouse effect, keeping a running tally of how much of the greenhouse effect remains to be accounted for.

Water Vapor

A world’s atmosphere will contain a significant amount of water vapor (H₂O) if it has Moderate, Extensive, or Massive prevalence of water, and its average surface temperature is at least 251 K. If either of these conditions fails to hold, skip this component.

To determine the atmospheric mass taken up by water vapor, refer to the following table:

Water Vapor Table
Average Surface Temperature	Multiplier
251-260 K	0.001
261-270 K	0.002
271-280 K	0.003
281-285 K	0.004
286-290 K	0.005
291-295 K	0.008
296-300 K	0.010
301-305 K	0.014
306-310 K	0.019
311-315 K	0.025
316-320 K	0.035
321 K or higher	0.050

Multiply the total atmospheric mass by the multiplier from the table for the world’s average surface temperature. Then multiply the result by 0.8 (for Extensive prevalence of water) or by 0.3 (for Moderate prevalence of water). The result is the partial atmospheric mass of water vapor for the world.

To compute the greenhouse effect caused by this water vapor, evaluate the following:

Here, G is the greenhouse effect due to water vapor in kelvins, and M is the partial atmospheric mass of water vapor as determined above. Round G to the nearest kelvin (minimum 0).

At this point, round M to the nearest thousandth, then subtract M and G from the running tally of atmospheric mass and greenhouse effect before moving on to the next component.

Methane

A world’s Class III atmosphere may contain methane (CH₄) in trace amounts if it has life. Even very primitive life forms generate some methane, in tiny amounts compared to the whole volume of a world’s atmosphere. However, methane is a very effective greenhouse gas, so even traces of it can cause measurable warming.

A world’s atmosphere will contain a significant amount of methane if it has undergone abiogenesis, either in deep hydrothermal vents or on the surface (see Step Twenty-Six). If this condition fails to hold, skip this component.

To estimate the greenhouse effect caused by methane, evaluate the following:

Here, G is the greenhouse effect due to methane in kelvins, and M is the total atmospheric mass for the world. Round G to the nearest kelvin (minimum 0).

At this point, subtract G from the running tally of greenhouse effect before moving on to the next component. The actual atmospheric mass of methane will always be insignificant.

Ozone

A world’s Class III atmosphere will contain ozone (O₃) in trace amounts if it has significant free oxygen. Ozone is formed in the upper atmosphere when molecular oxygen (O₂) is exposed to the primary star’s ultraviolet radiation. Ironically, the “ozone layer” then blocks much of that ultraviolet from reaching the world’s surface. The presence of an ozone layer may be a requirement before multicellular life can safely colonize exposed land.  As with methane, ozone is a very effective greenhouse gas, so even traces of it can cause measurable warming.

A world’s atmosphere will contain a significant amount of ozone if it has undergone an Oxygen Catastrophe (see Step Twenty-Six). If this condition fails to hold, skip this component.

To estimate the greenhouse effect caused by ozone, evaluate the following:

Here, G is the greenhouse effect due to ozone in kelvins, and M is the total atmospheric mass for the world. Round G to the nearest kelvin (minimum 0).

At this point, subtract G from the running tally of greenhouse effect before moving on to the next component. The actual atmospheric mass of ozone will always be insignificant.

Carbon Dioxide

All of the remaining greenhouse effect on this world will be attributable to carbon dioxide (CO₂) in the atmosphere. Carbon dioxide is a common component of planetary atmospheres, generated through volcanic activity or the oxidation of organic matter. Carbon dioxide is not a very strong greenhouse gas, but it often exists at much higher concentrations than either methane or ozone, so it is usually the largest or second-largest contributor to a world’s greenhouse effect.

To estimate the partial atmospheric mass of carbon dioxide, evaluate the following:

Here, M is the partial atmospheric mass of carbon dioxide, and G is the remaining greenhouse effect in kelvins after the contributions from water vapor, methane, and ozone have been accounted for. Round M off to the nearest thousandth.

At this point, all of the world’s greenhouse effect has been accounted for. Subtract M from the running tally of atmospheric mass before moving on to the next component.

Molecular Oxygen

If the world has photosynthetic life, then a portion of the atmosphere will be made up of free molecular oxygen (O₂).

If the world has photosynthetic life, but has not gone through the Oxygen Catastrophe, then estimate the amount of molecular oxygen by rolling 3d6, multiplying by 0.002, and then multiplying by the total atmospheric mass. Round the result off to the nearest thousandth.

If the world has gone through the Oxygen Catastrophe, then estimate the amount of molecular oxygen by rolling 3d6+15, multiplying by 0.01, and then multiplying by the total atmospheric mass. Round the result off to the nearest thousandth.

In either case, the result will be the partial atmospheric mass of molecular oxygen. Subtract that figure from the running tally of atmospheric mass before moving on to the next component.

Helium

A world whose M-number (computed in Step Nineteen) is 4 or less will have helium (He) in its atmosphere, left over from the world’s original formation. To estimate the amount of helium, roll 3d6, multiply by 0.025, and then multiply by the total atmospheric mass. Round the result off to the nearest thousandth.

The result will be the partial atmospheric mass of helium. Subtract that figure from the running tally of atmospheric mass before moving on to the next component.

Argon

Almost every world with a Class III atmosphere will have some argon (Ar) in its atmosphere, almost all of it generated by the decay of radioactive isotopes in the lithosphere. To estimate the amount of argon, roll 3d6, multiply by 0.001, and then multiply by the total atmospheric mass. Round the result off to the nearest thousandth.

The result will be the partial atmospheric mass of argon. Subtract that figure from the running tally of atmospheric mass before moving on to the final component.

Molecular Nitrogen

The remainder of the world’s atmosphere will be made up of molecular nitrogen (N₂), possibly with a few traces of other, more exotic non-greenhouse gases. Make a note of the remaining atmospheric mass, rounding off to the nearest thousandth. The result will be the partial atmospheric mass of molecular nitrogen.

Feel free to list all of the atmospheric components determined above, in order by partial atmospheric mass. This list will give you or your readers a quick way to determine how congenial the atmosphere is for human use.

Converting Partial Atmospheric Masses to Partial Pressures

To determine the partial pressure (in atmospheres) for any atmospheric component at a world’s surface, multiply the partial atmospheric mass for that component by the world’s surface gravity.

Evaluating Human Breathability

In order for humans to comfortably breathe an atmosphere, it must meet all of the following criteria:

• Molecular oxygen at a partial pressure of 0.190 to 0.240 atmospheres. Below this range, a respirator is required. Above the range, a filter or “reducing respirator” will be needed.
• Carbon dioxide at a partial pressure no higher than 0.015 atmospheres. Above this range, humans will suffer respiratory difficulties and may need a filter.
• Molecular nitrogen at a partial pressure no higher than about 4 atmospheres. Above this range, humans will suffer from nitrogen narcosis and eventually lose consciousness. A filter or reducing respirator will be required.

The other likely components of a Class III atmosphere are unlikely to have any toxic or suffocating effect, as long as the above criteria are all met.

Class IV Atmosphere

A Mars-type atmosphere will be composed almost entirely of carbon dioxide, with traces of nitrogen, argon, and other compounds. The atmosphere itself will be extremely thin and suffocating to human life, almost as bad as high vacuum, but it will not actively toxic.

Class V Atmosphere

A Luna-type atmosphere is vanishingly thin and may be composed of whatever particles of the interplanetary medium happen to be captured in the world’s gravity well at the moment. It will be an effective vacuum, quickly fatal to exposed human life.

Examples

Both Arcadia IV and Arcadia V have Class III atmospheres, so Alice prepares to work through the process for each of them. She is hoping that the atmosphere of Arcadia IV will turn out to be human-breathable. The atmosphere of Arcadia V will almost certainly not be, since the planet has no native life, but she will see what results occur for the sake of completeness.

Arcadia IV

This planet has a total atmospheric mass of 0.900, and a total greenhouse effect of 32 kelvins. Alice begins to work through the list of potential atmospheric components.

• Water Vapor: Alice refers to the Water Vapor table and determines that the multiplier will be 0.005. She multiplies the total atmospheric mass of 0.9 by 0.005 and then by 0.8 (for the world’s Extensive water) for a partial atmospheric mass of 0.0036. Using the equation to determine the greenhouse effect due to water vapor, she finds that it comes to 21 kelvins (rounded off). She rounds the partial atmospheric mass of water vapor off to the nearest thousandth and corrects her running tallies. She now has atmospheric mass of 0.896 and greenhouse effect of 11 kelvins to account for.
• Methane: Arcadia IV definitely has life, so there will be traces of methane in the atmosphere. Using the equation, she estimates that 2 kelvins of the greenhouse effect will be due to this greenhouse gas. She corrects her running tallies, and now has atmospheric mass of 0.896 and greenhouse effect of 9 kelvins to account for.
• Ozone: Arcadia IV has gone through the Oxygen Catastrophe, so there will be traces of ozone in the atmosphere. Using the equation, she estimates that 1 kelvin of the greenhouse effect will be due to this greenhouse gas. She corrects her running tallies, and now has atmospheric mass of 0.896 and greenhouse effect of 8 kelvins to account for.
• Carbon Dioxide: The 8 kelvins remaining of greenhouse effect implies a partial atmospheric mass of 0.004 of carbon dioxide, about the same amount by mass as found in Earth’s atmosphere. Alice updates her tally of atmospheric mass, finding that she still has 0.892 to account for.
• Molecular Oxygen:As already noted, Arcadia IV has already gone through the Oxygen Catastrophe. Alice rolls 3d6+15 for a total of 28, multiplying this by 0.01 and then 0.9 to get a partial atmospheric mass of 0.252. She updates her tally of atmospheric mass, finding that she still has 0.640 to account for.
• Helium: The planet’s M-number is 5, so there will be no significant helium in the atmosphere. Alice skips this component.
• Argon: Alice rolls 3d6 for a total of 16, multiplies that by 0.001 and then 0.9, and rounds the result off to 0.014 for the partial atmospheric mass of argon. She updates her tally of atmospheric mass and still has 0.626 to account for.
• Molecular Nitrogen: The last 0.626 of atmospheric mass will be composed of molecular nitrogen and traces of other non-greenhouse gases.

Alice multiplies all of these partial atmospheric masses by the planet’s surface gravity of 1.05, and arranges them in order from largest to smallest:

• Molecular Nitrogen: 0.657 atmospheres
• Molecular Oxygen: 0.264 atmospheres
• Argon: 0.015 atmospheres
• Carbon Dioxide: 0.004 atmospheres
• Water Vapor: 0.004 atmospheres

The atmosphere of Arcadia IV looks reasonably close to human-breathable, although it actually has a little too much oxygen in the mix. Humans living there may be able to breathe without assistance for a while, and may even find the air exhilarating, but they are likely to suffer various forms of oxygen toxicity over the long term. The oxygen-rich atmosphere might also encourage fires.

Arcadia V

This planet has a total atmospheric mass of 0.700, and a total greenhouse effect of 17 kelvins. Alice begins to work through the list of potential atmospheric components.

• Water Vapor: The planet’s average surface temperature is only 221 K, well below the minimum to have any significant water vapor in its atmosphere. Alice skips to the next component.
• Methane: Arcadia V has no native life, and so no significant amount of methane.
• Ozone: Arcadia V has no photosynthetic life and so no ozone.
• Carbon Dioxide: Apparently the planet’s 17 kelvins of greenhouse effect are entirely due to carbon dioxide in the atmosphere. That amount implies a partial atmospheric mass of 0.033. Alice updates her tally of atmospheric mass, finding that she still has 0.667 to account for.
• Molecular Oxygen:No photosynthetic life, and so no free oxygen in the atmosphere.
• Helium: The planet’s M-number is 6, so there will be no significant helium in the atmosphere.
• Argon: Alice rolls 3d6 for a total of 9, multiplies that by 0.001 and then 0.7, and rounds the result off to 0.006 for the partial atmospheric mass of argon. She updates her tally of atmospheric mass and still has 0.661 to account for.
• Molecular Nitrogen: The last 0.661 of atmospheric mass will be composed of molecular nitrogen and traces of other non-greenhouse gases.

Alice multiplies all of these partial atmospheric masses by the planet’s surface gravity of 0.82, and arranges them in order from largest to smallest:

• Molecular Nitrogen: 0.542 atmospheres
• Carbon Dioxide: 0.027 atmospheres
• Argon: 0.005 atmospheres

As expected, the atmosphere of Arcadia V is completely unbreathable, and in fact the high partial pressure of carbon dioxide would be mildly toxic to any human who made the attempt. Visiting humans will need full respirators.