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Part 1 : General Considerations, and Oxygen

(Author's Note: Thad Coons's 'Planetology 101' series was an inspiration for this set of articles.)

Atmospheres can be divided by pressure and composition.


The pressure of a gas is equivalent to the force its component molecules impart to the walls of its container. This is a function of the kinetic and potential energy of the molecules (in a gravitational field), which is a function of temperature.

The following expression can be derived:

Py = P0e(-ngy/8312T)


There are several units of pressure measurement. The two units from the S.I. system are the pascal (Pa) and the bar.

One pascal is equal to one newton of force applied to an area of one square metre. It's a pretty small unit. Most pressures are quantified in terms of kilo- or megapascals.

One bar is equivalent to one standard atmosphere (atm) of pressure. This is equal to 101.3 kPa, 760mmHg or Torr, or 14.5 psi. The latter two measurements are sometimes encountered in physiology or industry and variously refer to the force exerted by a column of mercury of the given height, or pounds force per square inch in 'Imperial' units.

In Traveller, atmospheres are divided up in the following manner based on pressure (the following values are in atmospheres) :

Atmosphere Classification Classic Traveller 'First In'/GURPS Space
Trace < 0.09  
Very thin < 0.42 < 0.5
Thin < 0.70 < 0.8
Standard < 1.49 < 1.2
Dense < 2.49 < 1.5
Very dense > 2.5 >= 1.5
superdense (e.g. Venus)   > 10??

A 'high-pressure nervous syndrome' has been described. It appears to be a function purely of ambient pressure rather than the gas mix. Symptoms and signs include tremors, drowsiness and decreased alpha activity on the electroencephalogram [EEG] (consistent with drowsiness!).


The chemical makeup of a planet's atmosphere is a complex function of the energy delivered by the primary, the planet's gravity and geology, the compounds available on the planet and their properties, and the activity of native life (if any).

The velocity of gas molecules is a function of temperature which depends predominantly on the energy provided by the local star. If the gas molecules can break free of the planet's gravitational pull, they will be lost.

Classically, gases obey Maxwell-Boltzmann statistics; that is, the molecules of a given mass of gas have a range of velocities - a speed distribution.

Temperature is an index of the most probable velocity and the standard deviation of the distribution. Colder gas molecules are more closely clustered around the most probable velocity with a smaller standard deviation than the same gas at a higher temperature.

For a given gas, the most probable velocity is equal to:  v (m/sec) = sqrt(16628T/n), where T is absolute temperature and n molecular mass of gas concerned (H2 = 2, O2 = 32, etc.)

So at 288K (15C) : velocity of an oxygen molecule = 387 m/sec, velocity of a hydrogen molecule = 1547 m/sec.

Escape Velocity (m/sec) = sqrt(2 * G * R), where
G is surface gravity in m/s2 (1G=10m/s2)
R radius in metres.

Surface gravity can be determined as per the world generation rules you are using. It is a function of planetary mass and size.

E.g. For the earth, escape velocity = sqrt(2 X 10 X 6.4 X 106) = 11314 m/sec.

The residence time of a substance in the lower atmosphere is based on the Boltzmann distribution. A loose rule of thumb looks like this:

Escape velocity* Residence time
3 days
4 centuries
5 ~1 billion [109] years
6 10s of billions of years - "forever"

* - as a multiple of most probable velocity.

So in the case of the earth, escape velocity is almost 30x the most probable velocity for oxygen at 15 C, and 7x that of hydrogen.

However, hydrogen has largely escaped Earth's atmosphere because it can reach an altitude where it can get enough of a thermal 'boost' to exceed escape velocity.

What hydrogen remains makes up most of the highest layers of the atmosphere - the exosphere (above 600km altitude) where pressures are on the order of 10-30 atm - rarefied stuff.

Compounds available are restricted generally to those from the 'Cosmic Top 30' list of elements. This still gives us a lot of room to move in: H2, He, N2, O2, F2, Cl2, Ne, Ar and compounds of B, C, Si, P and S.

Some definitions:

Melting point: the temperature at which a substance changes state from solid to liquid (typically at one atmosphere ambient pressure).

Boiling point: the temperature at which a substance changes state from liquid to gas (typically at one atmosphere pressure).

Critical temperature: the temperature at or above which a gas cannot be returned to a liquid state.

Partial pressure: the overall pressure exerted by a gas mixture is the sum of the pressures exerted by the components of the mixture. Each of these pressures is a 'partial pressure' and is proportional to the number of gas molecules present. All partial pressures are quoted at 20 degrees Celsius unless otherwise noted.

Ideal gas: one that obeys the ideal gas law: Pressure X Volume = number of molecules X absolute temperature X constant

Real gases obey a slightly modified (van deWaal's law) version of the ideal gas law (molecules have finite size and exert attractive forces on each other).

(Saturated) Vapour pressure: the pressure exerted by the gaseous component of a substance in equilibrium with its liquid component at a given temperature.

For example, consider water:

Temp(C) -10 0 5 10 15 20 25 30 37 50 75 100 200
SVP(mmHg) 2 4.6 6.5 9.2 12.8 17.5 23.8 31.8 47 92.5 289.1 760 11200

It can be seen that boiling point is attained when vapour pressure equals atmospheric. Note that the vapour pressure of water at 37 degrees is 47mmHg (0.06atm). So at this ambient pressure or lower, a person's body fluids will begin to boil.

Some form of pressure suit is therefore required for survival in atmospheres rated 'trace' or less. Vapour pressure curves (pressure vs temperature) are the same general shape (roughly exponential) but obviously vary greatly between substances.

Humidity: Absolute humidity is the amount of water vapour per unit mass or volume of air at a given temperature. Relative humidity is the ratio of the amount of water vapour present to the amount of water vapour present at saturation for a given temperature, and is the quantity weather reporters talk about.

Recall that living things will change the atmosphere at first from photosynthesis or its local equivalent, and perhaps later with agriculture and industrial activity.

In Traveller terms:

The list of atmospheres given above refer to those that contain oxygen plus other (inert) gases. Dense/high, thin/low and ellipsoid subtypes could have a similar composition.

Exotic, corrosive and insidious atmospheres must be made up largely of less benign substances (the subject of subsequent posts).


Data : Formula O2, molecular weight 32, melts at -219C, boils at -183C, critical temperature -118.35C.

This highly reactive substance is essential to the survival of most species found in Charted Space; not very surprising, given that oxygen is the third most common element in the universe! Production is typically the result of photosynthesis - the conversion of carbon dioxide to sugars. Extreme volcanic activity can liberate oxygen from silicon and other oxides which make up most of the mass of rocky (and more dense) planets.

Oxygen is unlikely to be present in an atmosphere in amounts in excess of 0.26 atmospheres as it will cause dry vegetation to ignite and burn at this level. Beyond 0.30 atmospheres partial pressure, typical carbon-based life will readily combust if exposed to flame.

Human oxygen requirements average 0.21 atmospheres pressure ; some groups have adapted to pressures as low as 0.11 atm (equivalent to an elevation of 5500m on Earth - where the highest permanent settlements are).

A compressor is therefore a minimal requirement for atmospheres rated 'very thin' or less.

Lack of oxygen (hypoxia)

  1. Nastiness

    The human body has negligible oxygen stores breathing 0.21 atm air, some 450mL in an adult. Resting oxygen consumption is 250mL/minute, so this supply will last roughly two minutes.

    Reserves can be increased a factor of seven by breathing 1.0 atm oxygen and 'washing out' all the nitrogen from the lungs.

    The effects of sudden pressure drops (on a non-acclimated person, if acclimation is possible):

    Loss of consciousness in Oxygen Pressure (atm) Altitude equivalent (m)
    (air mix - 21% oxygen)
    5 minutes 0.09 6100
    2 minutes 0.08 7500
    1 minute 0.07 8000
    20 seconds 0.03 13700
    (100% oxygen)
    5 minutes 0.1 ~14000
    20 seconds 0.07 ~17000

    Breathing inert gas mixtures that don't contain oxygen will cause unconsciousness in 20-30 seconds. Without oxygen ('hypoxic hypoxia'), death follows from brain anoxia within 4 to 5 minutes of the onset of unconsciousness.

  2. Mountain sickness

    This is a pathological adaptation to altitude. (Very likely with atmospheres rated 'thin' - adaptation is possible during the week of jump). Most people will pass a lot of urine for the first two to three days, some will notice that they are hyperventilating [The depth of ventilation doubles with every 0.007 atm fall in the partial pressure of oxygen!], and that's about it.

    Onset is 8-24hr after arrival, and acute mountain sickness lasts 4-8days. Symptoms and signs include impaired judgement, drowsiness, dulled pain sensibility, irritability, excitement, disorientation, headache, anorexia, nausea, vomitting, tachycardia and hypertension.

    Severe cases of mountain sickness lead to potentially lethal flooding of the lungs and brain swelling - pulmonary and cerebral oedema. This is much more likely with vigorous activity or rapid 'decompression'.

  3. Features of chronic adaptation.

    The main physiological changes seen are an increased red cell mass (to maximise oxygen carrying capacity), enlarged hearts (blood is more viscous, so it's harder to pump around) and chests (more work of breathing being done - relative hyperventilation compared to 'low-landers'). Life expectancy is reduced compared to 'low landers' due to the long term effects of excessive strain on the right heart - pulmonary hypertension.

  4. Short-term adaptation.

    The ascent of Everest without supplemental oxygen represents the limit of adaptation for a few days. The mountaineers who have performed this task are incredibly fit. The most trivial exertion is equivalent to running a marathon.

    Resting pulse rates run at 160 per minute or more with respiratory rates of 40-50 per minute. At sea level this degree of exertion would herald imminent cardiorespiratory collapse.

    Preparation usually consists of staged ascents from a base camp at the 5500m level, having spent a couple of weeks becoming acclimated to the pressure at that altitude.

Oxygen excess (hyperoxia)

  1. Up to 1 atm pressure

    Used routinely in medicine with appropriate precautions, oxygen is a very useful drug. It is not without its side-effects, however:

    • drying of mouth, nose and lungs (without humidification)
    • reduced lung capacity; at one atmosphere absolute pressure, 100% oxygen will cause decreased lung volume within eight hours in previously healthy volunteers. Marked diminution (15-20%) in vital capacity (lung volume) is evident within 24 hours from absorption atelectasis (all oxygen removed from alveoli, which therefore collapse).
    • chest pain - occurs within six to eight hours of 1.0 atm therapy. Usually dull and associated with inspiration.
    • Within 48 hours, histological if not frank clinical evidence of pulmonary oedema (lung flooding with inflammatory fluid) is present.
    • Premature infants are at especial risk of also developing retrolental fibroplasia of the lens of the eye from ingrowth of blood vessels, if exposed to 100% O2 for this length of time.
  2. Hyperbaric

    High partial pressures of oxygen leads to central nervous system stimulation, which culminates in fitting. Warning symptoms include nausea and vomitting, muscle twitching, anxiety, vertigo, respiratory changes or altered level of consciousness, sweating or behavioural changes, and visual or auditory changes.

    • The likelihood of fitting is a function of partial pressure and duration of exposure. Increased uptake (e.g. exercise) is another important consideration.
    • At a partial pressure of oxygen of 4 atmospheres, fitting will occur within 30 minutes in 50% of people. The dose response-curve rises steeply beyond this.
    • Hyperbaric chambers typically run with oxygen partial pressures in the neighbourhood of 3 atmospheres. Exposure should be limited to less than 5 hours with O2 less than 3 atm pressure. Symptoms are uncommon with 5 minute air breaks for every 45 mins O2 (incidence less than 0.2%).


Data : Formula O3, molecular weight 48, melts at -193C, boils at -111C.

A much more reactive species than O2, ozone is commonly found in the upper regions of standard atmospheres, where it helps attenuate stellar ultraviolet radiation. It is typically produced when O2 absorbs ultraviolet light or electrical energy; atomic oxygen (O) is formed which then combines with other O2 molecules to form ozone.

Ozone is a potent antimicrobial, and readily causes lung damage like 1.0 atm oxygen (lung inflammation and pulmonary oedema).

Level* Comments
80 Average level required to obtain 'good air quality' index
160 Average level required to obtain 'standard air quality' index
200 Abnormal lung function possible following 8 hours of exposure
240 Abnormal lung function following 1 hour of exposure
400 Increased incidence of asthma attacks, etc. with any exposure
800 Increased risk for those with cardiorespiratory disease
1000 Healthy persons at risk. Minimise exertion, stay indoors.
1200 Premature death of ill and elderly.

*Ozone level is expressed in microg/m3 air at 1 atm and 25C; a level of 80 microg/m3 air is equal to a partial pressure of 3.1 X 10-5 atm. Lung function abnormalities at levels of 200 microg/m3 can persist for a week or more.

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