FOUR/3 Atmospheric Composition

STEP ONE: Roll on 4.3.1 to find basic composition.

STEP TWO: Consult 4.3.2 and remove the gasses which aren't retained by a world of this size and temperature.

STEP THREE: Modify the composition based on the UV infall and other effects as described in 4.3.3.

STEP FOUR: Determine the fractions of the remaining gasses by checking 4.3.4.

STEP FIVE: Determine the atmospheric pressure on 4.3.5.

Table 4.3.1 Basic Composition

1d10  Base Temperature (°K)
≤50    51-150    151-240    241-400    ≥401
1-4    H2    N2, CH4    N2, CO2    N2, CO2    N2, CO2
5-6    He    H2, He, N2    CO2    CO2    CO2
7-8    He, H2    N2, CO    N2, CH4    N2, CH4    NO2,SO2
9    Ne    He, H2    H2, He    CO2, CH4, NH3    SO2
10    Special    Special    Special    Special    Special

Table 4.3.2 Retained Gasses

MWR = 0.02783 x T / V2

... where: T is the base temperature (in °K)
V is the escape velocity (in earths)

Any gas with a heavier molecular weight than this value is stable, while any lighter will escape over time. Lighter gasses that are constantly renewed may still be part of the atmosphere, but in general you should remove any gasses not permanently retained. Worlds where the main gasses (above) cannot be retained will have trace atmospheres, therfore do not roll on 4.3.4.

Name    Molecular
    Boiling Point
(at 1 atm)
H2 (hydrogen)     2    20
He (helium)     4    4
CH4 (methane)     16    109
NH3 (ammonia)     17    240
H2O (water)     18    373
Ne (neon)     20    27
N2 (nitrogen)     28    77
CO (carbon monoxide)     28    82
NO (nitrogen monoxide)     30    121
O2 (oxygen)     32    90
H2S (hydrogen sulphide)     34    214
Ar (argon)     40    87
CO2 (carbon dioxide)     44    195
NO2 (nitrogen dioxide)     46    294
SO2 (sulphur dioxide)     64    263

Table 4.3.3 Atmospheric Modifications

UV Infall: If the primary has high enough UV input, ammonia, methane, hydrogen sulphide and water in the atmosphere will break down and the hydrogen may escape. Thus remove these gasses as a major part of the atmosphere if the primary is ...

  • Spectral class O, and T > 150°K
  • Spectral class B, and T > 150°K
  • Spectral class A, and T > 150°K
  • Spectral class F, and T > 180°K
  • Spectral class G, and T > 200°K
  • Spectral class K, and T > 230°K
  • Spectral class M, and T > 260°K

Vulcanism: Volcanic activity replenishes atmospheres. This will primarily modifiy the atmospheric pressure, but a dead world will not typically have sulfur dioxide or hydrogen sulfide.

Life: If a world has life, such life may affect the atmosphere. Methane and ammonia may be replenished, or carbon dioxide (partially) replaced by oxygen. Worlds with liquid water may have oxygen-based life on a roll of 1-3 (on 1d10), replacing carbon dioxide (or part of it) with oxygen. Such worlds should be atleast 1GY old.

Table 4.3.4 Atmospheric Pressure

1d10    Pressure
≤2    M x 1d10 x 0.01
3-4    M x 1d10 x 0.1
5-7    M x 1d10 x 0.2
8    M x 1d10 x 0.5
9    M x 1d10 x 2
≥10    M x 1d10 x 20

... where M is the mass of the planet (in earths).

Modification to initial roll
-1 if dead vulcanism
+1 if extreme vulcanism
-1 if one (but not all) the main gasses are removed in 4.3.2.

Table 4.3.5 Atmospheric Composition

1d10    Main Component (%)
1-5    4d10 + 50
6-8    2d10 + 75
9-10    1d10/2 + 95

This table gives the relative size of the atmosphere's main component (ie. the first listed gas).


The most common basic gasses to form a majority of planetary atmospheres are nitrogen and carbon dioxide. Very dense atmospheres are generally rich in carbon dioxide. Very hot atmospheres may have parts of sulphur trioxide, sodium and other more exotic elements. High-G worlds may retain helium and hydrogen to a large degree - this is especially important in the outer system where there was an abundance of these gasses to begin with. (In the inner system the proto-planets were not rich in helium and hydrogen) Ammonia tend to mix with any present water - this is especially typical on cool nitrogen/methane worlds (T between 200°K and 240°K). Carbon dioxide can be bound up by geological activity and also locked up in water. Cold water under normal and high pressure can dissolve carbon dioxide on a 1-1 basis. Interestingly, warm water does not dissolve carbon dioxide nearly as well. Ice-ball planets which lose atmosphere continuously tend to lose mass as well. They can keep a very thin atmosphere for as long as there are gasses to lose.


There are many variants of special atmospheres. They may signify an unusual amount of some rarer gas, perhaps because alien life utilise it, or an unusual mix of gasses.

Nitrous Oxides & Sulphur Compounds: These may be available on hot volcanic worlds or as smaller parts on very volcanic worlds. Such atmospheres would be hostile. Sulphur-rich environments may be able to sustain very alien life. (See Chapter Five) These compounds would add to greenhouse effect.

Halogens: The atmosphere has an important (typically less than a few percent, but still far more than normal) of chlorine, fluorine, bromine or perhaps iodine. These elements are highly reactive and much of them may be in liquid acid form. Halogens could also theoretically support life. Halogen compounds could add to the greenhouse effect strongly. A world rich in chlorine or fluorine would be a very strange and deadly (to Earth life) place.

Hydrogen: Explosive combined with oxygen and reactive. Hydrogen-rich atmospheres are called reducing but most terrestrial worlds don't retain hydrogen.

Carbon Monoxide: Also mainly found in reducing hydrogen-rich atmospheres. Carbon monoxide is very unhealthy.

Noble Gasses: Helium, neon and argon are the most common noble gasses. Argon is most common on terrestrial worlds where it may amount to a pair of percent or even more in rare low-pressure worlds, but neon and helium are common in the universe. These gasses do not react with other materials.

Water Vapour: Water vapour can also be a significant part of an atmosphere, but this is fairly rare as UV infall break up the water and the hydrogen escapes. Still, on massive ocean worlds water could be a major part of the atmosphere.

Very Dense: This is a world dead in volcanism which still has a very dense atmosphere, perhaps due to extreme infall or history of dense atmosphere. It could also be a world which retains an atmosphere it probably should have lost under 4.3.2 - perhaps it is on the border to keep/lose some of the gasses, but still rolls for normal atmospheric composition and pressure.


A lot of gasses are found in the atmosphere, but most only in minuscule fractions. Several of these gasses may be important in cloud formation (sulphuric acid, ammonium compounds, various photochemical organic smog compounds etc) however, so their importance should not be underestimated. Small amounts of certain fairly common gasses are enough to kill humans.


In a young system, before UV has broken down molecules, atmosphere has escaped and volcanism have settled down, atmosphere tend to have basic building blocks like methane and ammonia even within the Inner Zone. Planets may have denser atmospheres than they can retain over a longer time.


Pressure may vary significantly over a year or even day on worlds with sparse atmospheres, if some atmospheric gas freezes out during cold periods and vaporises during warmer ones. Carbon dioxide could have this effect on cool worlds, and thus pressure could vary by perhaps 50% over time, regularly. We know that the atmosphere on Mars varies like this.


UV infall breaks down molecules, and if part of the molecule can get lost into space (hydrogen, generally) the compound cannot be recreated. This is what happened to water on Venus, and to the ammonia and methane on Earth. The ozone layer (which needs free oxygen to reach a decent size) can protect from some of this radiation, but this is only a slowing-down. UV radiation also serves to create the more complex compounds and organic smog. Worlds orbiting brown dwarves (which have very low UV radiation) would not be affected much at all.


Volcanic activity replenishes the atmosphere. Bound water, carbon dioxide and other compounds can be brought back into the atmosphere. Thus, a world without volcanism is slowly losing atmosphere and faces a shortage of critical building blocks for life.


The presence of life tends to influence the atmosphere. Most importantly right now, it can create free oxygen. This takes time, however, and all forms of life do not need or produce oxygen. A young world may have oxygen-producing life but the oxygen is removed by geological processes. (On Earth, it took perhaps 3 GY to produce free atmospheric oxygen). Thus, worlds with low tectonic activity may get oxygen-rich faster. Some miniscule amount of oxygen is also likely to be present in any carbon dioxide or water-rich atmosphere. Other types of life may sustain methane and/or ammonia levels, or perhaps sulfur or nitrogen oxides. Oxygen levels also vary over time ... Earth's oxygen level were higher hundreds of millions of years ago, for instance. The amount of free oxygen may vary depending on how abundant and advanced local life is, but in general a world with large warm oceans could produce more oxygen. This is easiest to simulate by tweaking the generated oxygen level upwards or downwards. For Earth-like worlds this can be a guideline

Oxygen % = (T - 240) / 200 x h x (1d10 + 5) x 10

... where: T is the surface temperature calculated from FOUR/5 (not the base temperature)
h is the hydrosphere

Oxygen levels will be discussed further in chapter EIGHT, and you may consider deciding oxygen levels by the more detailed rules there. Free oxygen will react with hydrogen, ammonia, carbon monoxide and methane, thus limiting the extent of such gasses. Free oxygen is also important in the creation of any serious ozone (O3) layer. Life based upon the more exotic types of molecular building stones, such as sulfur, silicone or halogens does not necessarily produce free oxygen.


Cometary infall on planets can serve to provide more gasses or shift the balance of existing ones.


The atmospheric composition may change distinctly over time, not only because of outgassing, freezing and overheating but also because of life and geological processes. If erosion becomes less effective gasses generated from tectonic activity will build up, for instance. This can form a regulatory process to keep worlds form freezing over. More on long-term climate change is in Part II.


The scale height is a measure of how extended an atmosphere is. Heavy gasses and low temperatures give a more compact atmosphere, while hot and light atmospheres are much deeper. Scale height is calculated from

H = kT / mg

... where: k is Boltzmann's constant (1.38 x 10-23 J/degree)
T is the temperature of the atmosphere
m is the mean molecular mass of the gas (4.3.2.shows molecular weight, to get the mass multiply by 1.66 x 10-27)
g is surface gravity (in earths)

Scale height is an approximation, as the temperature tends to vary within an atmosphere too.


Pressure decrease with altitude, and on some worlds this may be important, especially if they have distinct topography. Liquid water needs a certain pressure, and that high levels of oxygen and nitrogen can be dangerous. To calculate this, we need the scale height (H) from before. At a certain point above sea level the pressure is

P = p x 2.718(-h/H)

... where: p is the base atmospheric pressure
h is the altitude
H is the scale height

Sea level may be rather uninteresting on a world without oceans. In such cases, use the lowest land as reference.


The mass of an atmosphere (in Earth atmospheric masses) is related to pressure as follows:

M = 2.46 x 10-8 x p x R2 / g

... where: p is the base atmospheric pressure
R is the radius of the planet (in km)
g is the surface gravity (in Earths)


Pressure: High atmospheric pressure is not healthy to humans. Very dense atmospheres are not breathable due to the pressure alone.

Oxygen: To humans and Earth's animal life, oxygen is absolutely necessary. Oxygen pressure should be less than 0.3 atm and more than 0.05 atm. Too little oxygen and brain damage due to oxygen depravation and troubles breathing will occur. Too high oxygen will destroy eyes and lungs and send people into fits. High oxygen levels are also increasing flammability and attack materials. Plants, bacteria and alien forms of animal life may not need free oxygen.

Nitrogen: Is necessary to plant life and bacteria, but not in the same huge amounts as oxygen is to animals. High levels of nitrogen is unhealthy to humans - anything beyond 2.0-3.0 atm of nitrogen induces nitrogen narcosis and dense nitrogen atmospheres can be outright dangerous. The long-term effects of living in a higher-than-normal nitrogen environment are unknown.

Carbon Dioxide: Carbon dioxide is not lethal in small amounts, but more than 0.05 atm of CO2 can lead to unconsciousness and higher amounts to suffocation.

Methane: Methane is flammable in any larger amounts (above 0.06-0.08) and can cause explosions.

Hydrogen: Like methane, flammable. Hydrogen is not toxic, but it will not be found in larger amounts with free oxygen.

Ammonia: Toxic to humans in even small concentrations.

Helium, Noble Gasses: These are not toxic.

Other Gasses: Virtually all other gasses are dangerous to humans. Carbon monoxide and halogens are toxic in small concentrations.