I’ve finished designing the first draft of an abbreviated Architect of Worlds design sequence specifically for the roleplaying game Traveller. It should be compatible with any version of Traveller that uses the standard UWP codes, including GURPS Traveller. It’s available at the following link:
Unlike most of my work, this document is not entirely covered by my copyright, and I freely grant permission to share or redistribute it, so long as the attribution is not altered. I’d be interested in hearing from any Traveller referees or players who experiment with it!
Architect of Worlds – Step Twenty-Seven: Determine Details of Atmospheric Composition
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.
Architect of Worlds – Step Twenty-Six: Determine Presence of Native Life
In this step, we will determine whether the world has native life, the nature of that life, and how complex it has become at the present time.
The evolution of complex life forms takes time, and also requires that local conditions be congenial. We will estimate the amount of time it requires for seven specific evolutionary steps to take place, affected by the world’s other properties. In the process, we will sketch out the world’s evolutionary history.
Procedure
Begin by noting the current age of the star system, as established in Step Four. For any of the seven following events, if the total time to the event is greater than the star system’s age, then that event has not yet taken place on the world under development. Any further events that depend upon that one can be ignored.
Abiogenesis (Deep Hydrothermal Vents)
If all of the following conditions are true for the world under development:
Class II, Class III, Class IV, or Class V atmosphere
Moderate, Extensive, or Massive prevalence of water
Molten, Soft, Early Plate, Mature Plate, or Ancient Plate lithosphere
World does not have Fixed Plate Tectonics
Then roll 3d6, multiply by 30 million years, and make a note of the result as the time to deep abiogenesis.
If the age of the star system is greater than this, then the world has native life which derives energy and nutrients from volcanic vents at the bottom of its oceans.
Note that the world does not require exposed liquid-water oceans in order for this form of life to appear. For example, there is speculation that life may exist on Jupiter’s moon Europa, in a large liquid-water ocean beneath the icy surface.
Abiogenesis (Surface Refugia)
If all of the following conditions are true for the world under development:
Class III or Class IV atmosphere
Moderate or Extensive prevalence of water
Average surface temperature is at least 265 K
Lithosphere is Molten, Soft, Early Plate, Mature Plate, or Ancient Plate
World does not have Fixed Plate Tectonics
Then roll 3d6, multiply by 100 million years, and make a note of the result. Also, if the world has undergone deep abiogenesis (see above), roll 3d6, multiply by 75 million years, add the time to deep abiogenesis, and make a note of that final result as well. Take the lesser of these two times and make note of it as the time to life in surface refugia.
If the age of the star system is greater than this, then the world has native life which derives energy and nutrients from sources in shallow water or on exposed land. Very early on, this life will require special conditions that sometimes occur in shallow oceans, on beaches, or in fresh-water ponds. These refugia form the basis for all later expansion of life across a world’s surface.
Special Case: Lifeless Worlds
Note that all of the following evolutionary events are dependent on the presence of life in either the deep oceans or in surface refugia. If a world doesn’t meet the criteria for either of the above events, then it will be barren and effectively lifeless. It’s possible that some hardy form of bacterial life clings to existence, but it will be difficult to detect and will have no significant effect on the world’s environment.
Multicellular Life
If the world has undergone deep abiogenesis (see above), then roll 3d6, multiply by 75 million years, and then add the time to deep abiogenesis. Make a note of the final result as the time to multicellular life.
If the age of the star system is greater than this, then the world’s native life has developed the ability to form multicellular organisms. These organisms will likely be simple and primitive for a very long time, but they will form the basis for all future complex ecosystems based on plant and animal life.
Photosynthesis
Photosynthesis is critical to the development of a complex surface ecology for a world, such as exists on Earth. Early forms of photosynthesis are likely to appear very soon after the development of surface refugia. However, the form of photosynthesis with which we are most familiar – the so-called oxygenic photosynthesis that releases free oxygen as a by-product – takes much longer to evolve.
In any case, the difficulty of developing photosynthesis is strongly dependent on the color of light that a world receives from its primary star.
Stars similar to Sol produce most of their radiation in the visible-light range, which passes easily through the transparent atmosphere of an Earth-like world. Smaller and cooler stars (K-class and especially M-class) produce more of their radiation in the infrared bands, which have more trouble penetrating an atmosphere. Furthermore, the chemical pathways involved in photosynthesis are most effective at certain visible light wavelengths. A world with a small, cool primary star receives less such light, and may take much longer to develop photosynthetic life.
If the world has life in surface refugia (see above), and its primary star is not a brown dwarf, then refer to the following table:
Photosynthesis Table
Primary Star Class
Photosynthesis Development Timescale
A, F, or G0-G7
100 million years
G8-G9
105 million years
K0
110 million years
K1
115 million years
K2
120 million years
K3
130 million years
K4
145 million years
K5
160 million years
K6
180 million years
K7
210 million years
K8
240 million years
K9
270 million years
M0
300 million years
M1
360 million years
M2
480 million years
M3
600 million years
M4
800 million years
M5-M9
1 billion years
Roll 3d6, multiply by the Photosynthesis Development Timescale entry for the spectral class of the world’s primary star, and then add the time to life in surface refugia. Make a note of the final result as the time to photosynthesis.
If the age of the star system is greater than this, then the world’s native life has developed the ability to synthesize food by using sunlight to drive the necessary chemical reactions. Organisms which develop this trait will establish all of the world’s families of plant life.
Oxygen Catastrophe
Although the development of oxygenic photosynthesis opens many new opportunities for a world’s ecology, it can also be immensely harmful for existing life. Life forms which arose in the absence of oxygen will often find it to be actively poisonous. The period of Earth’s history during which free oxygen became prevalent is sometimes called the Oxygen Catastrophe.
After oxygenic photosynthesis develops, oxygen will be released by photosynthetic organisms, but at first that oxygen will tend to be dissolved in the oceans, or it will combine with exposed materials on land. It may take a long time for photosynthesis to build up significant free oxygen in a world’s atmosphere.
If a world has developed photosynthesis (see above), roll 3d6, multiply by 1.5 and by the appropriate entry in the Photosynthesis Table, and then add the time to photosynthesis. Make a note of the final result as the time to the Oxygen Catastrophe.
If the age of the star system is greater than this, then the world’s oceans, land surfaces, and atmosphere have all become saturated with free oxygen and oxygen compounds. Many earlier forms of anaerobic life will likely have been driven into extinction, but new aerobic forms will have arisen which can tolerate or even take advantage of the available oxygen.
Animal Life
Although multicellular life may appear quite early in a world’s history, the development of animals – complex multicellular organisms that are mobile and survive by eating organic matter – may take a very long time. The appearance of free oxygen in the environment can promote the development of animals, but it is theoretically possible for anaerobic animals to evolve as well.
If a world has developed multicellular life (see above), roll 3d6, multiply by 300 million years, and then add the time to multicellular life. If the result is longer than the time to the Oxygen Catastrophe, then subtract half of the difference. Make a note of the final result as the time to animal life.
If the age of the star system is greater than this, then the world will have given rise to complex animal life. Equivalents on Earth would be life forms that first appeared late in the Precambrian period, leading up through the so-called Cambrian Explosion about 540 million years ago. These ancestral forms eventually gave rise to all of the animal phyla that exist today.
Pre-Sentient Life
The earliest animals are driven purely by biological programming, the tropisms and instincts that they evolve to govern their behavior. Eventually, however, animal life will develop more elaborate systems for gathering and processing sensory data. Some animal species will even develop the first potential for self-aware, tool- and language-using minds. We will use the term pre-sentient for such species.
If a world has developed animal life (see above), roll 3d6. Multiply the result by 50 million years if the world has hydrographic coverage of less than 100% (from Step Twenty-Four), or by 100 million years otherwise. Add the time to animal life to compute the final result. Make a note of the final result as the time to pre-sentient life.
If the age of the star system is greater than this, then the world will have given rise to one or more classes of animal life that have advanced nervous systems and may be capable of complex learned behavior. Equivalents on Earth would be the behaviorally complex animals that first appeared late in the Triassic period, about 300 million years ago.
Examples
The Arcadia star system is 5.6 billion years old, so Alice makes a note of this as the endpoint for her generation of the evolutionary history of both Arcadia IV and Arcadia V.
Arcadia IV
Arcadia IV has a Class III atmosphere, Extensive water, and a Mature Plate Lithosphere with Mobile Plate Tectonics. It meets the criteria for deep abiogenesis. Alice rolls 3d6 for a result of 12, multiplying that by 30 million years, and she makes a note that the time to deep abiogenesis is 360 million years.
Arcadia IV also has an average surface temperature of 286 K, so the planet has liquid-water oceans and meets the criteria for surface refugia. Alice rolls 3d6 for a result of 10, multiplying that by 100 million years, and she makes a note that the first result is a full 1 billion years. She also rolls 3d6 for a result of 8, multiplying that by 75 million years and adding the time to deep abiogenesis of 360 million years, for a second result of 960 million years. The time to life in surface refugia is the lesser of the two results, or 960 million years. Apparently, Arcadia IV’s later surface life derives from forms that first evolved near the deep hydrothermal vents.
Meanwhile, Alice rolls 3d6 for a result of 10, multiplying that by 75 million years and adding the time to deep abiogenesis of 360 million years. She makes a note that the time to multicellular life is also about 960 million years. Even while some of the deep-vent life forms were colonizing the planet’s surface, other organisms were beginning to experiment with multicellularism.
Arcadia IV has life in surface refugia, so it is likely to develop photosynthesis. The primary star of the system has spectral class of K2. Alice rolls 3d6 for a result of 14, multiplies that by 120 million years, and adds the time to life in surface refugia of 960 million years. She makes a note that the time to photosynthesis is about 2.64 billion years.
From there, Alice moves on to estimate the time to the Oxygen Catastrophe. She rolls 3d6 for a result of 14, multiplies that by 1.5 and then by 120 million years, and adds the time to photosynthesis. She makes a note that the time to the Oxygen Catastrophe is 5.16 billion years. The development of an oxygen atmosphere on Arcadia IV apparently took noticeably longer than it did on Earth!
To estimate the time of appearance of animal life, Alice rolls 3d6 for a result of 14, multiplies that by 300 million years, and adds the time to multicellular life of 960 million years. Her first estimate for the time to animal life is (by an odd coincidence) 5.16 billion years, or almost exactly the same time as the Oxygen Catastrophe. She makes a note of that as well.
Arcadia IV has hydrographic coverage of about 70%. Alice rolls 3d6 for a result of 9, multiplies that by 50 million years, and adds the time to animal life of 5.16 billion years. She makes note of the result: 5.61 billion years, or slightly more than the total age of the star system. Alice decides that Arcadia IV does have complex ecosystems of plants and animals, but that the first pre-sentient species are just now beginning to appear. The planet has lots of equivalents to fish, insects, amphibians, and small reptiles, but is only starting to develop the equivalent of dinosaurs or primitive mammals.
Arcadia V
Arcadia V has a Class III atmosphere, Moderate water, and a Mature Plate Lithosphere with Fixed Plate Tectonics. Since it has Fixed Plate Tectonics, it does not meet the criteria for deep abiogenesis.
Arcadia V has an average surface temperature of 221 K, which does not meet the criteria for abiogenesis in surface refugia.
Since all of the evolutionary events that follow are dependent on abiogenesis taking place in at least one of these two locations, Alice quickly concludes that Arcadia V is effectively lifeless. There may be some form of hardy bacterial life, lurking in water deposits under the surface, but this isn’t sufficient to have any significant effect on the planet’s environment.
Architect of Worlds – Step Twenty-Five: Determine Average Surface Temperature
In this step, we will determine the world’s average surface temperature. This quantity is strongly dependent on the world’s blackbody temperature, established in Step Nineteen. However, worlds are not perfect thermal blackbodies, so this estimate will need to be adjusted.
In particular, some of the primary star’s energy input will be reflected away from the world’s surface, making no contribution to its heat budget. This factor is related to the world’s albedo, a measure of its reflectivity. Meanwhile, a world with an atmosphere containing certain gaseous components (the so-called greenhouse gases) will tend to retain some heat, warming the surface in a phenomenon called the greenhouse effect. These two factors are critical to any estimate of a world’s average surface temperature.
Unfortunately, a world’s albedo and greenhouse effect are dependent on a swarm of factors, many of which are poorly understood or beyond the scope of this design sequence. In fact, the values of these factors can change drastically over time on a single world. We will therefore determine a world’s albedo and greenhouse effect at random, or by designer choice within a range of plausible results, and then determine some of the consequences of those random choices in later steps.
Procedure
To compute a world’s average surface temperature, determine its albedo and greenhouse effect, then use these two factors to modify the blackbody temperature.
Albedo
To determine a world’s albedo at random, begin by referring to the following table.
Worlds with Class V atmospheres are a special case. The albedo of these worlds is strongly dependent on whether their surfaces are subject to volcanic activity. Even worlds with Massive presence of water, covered with thick layers of ice, may have cryovolcanoes which constantly refresh the icy surface. Check to see whether any such world falls into any of the following cases.
If the world has a Molten or Soft lithosphere, add 0.5 to the base albedo.
If the world has an Early or Mature Plate lithosphere, add 0.3 to the base albedo.
If the world has an Ancient Plate lithosphere with Mobile plate tectonics, add 0.3 to the base albedo.
If the world has an Ancient Plate lithosphere with Fixed plate tectonics, or it has a Solid lithosphere, and its blackbody temperature is lower than 80 K, add 0.3 to the base albedo.
Finally, roll 3d6, multiply the result by 0.01, and add it to the base albedo. The final result is the world’s actual albedo.
Greenhouse Effect
The greenhouse effect for a given world is measured in kelvins. The procedure for estimating the greenhouse effect depends on its atmosphere type. Apply the appropriate case from the following.
Class I Atmosphere
To determine the greenhouse effect for a Venus-type atmosphere, compute the following:
Here, M is the atmospheric mass of the world, and G is the world’s greenhouse effect in kelvins. Round the result to the nearest integer.
Class II Atmosphere
To determine the greenhouse effect for a Titan-type atmosphere, compute the following:
Here, M is the atmospheric mass of the world, and G is the world’s greenhouse effect in kelvins. Round the result to the nearest integer.
Class III Atmosphere
To determine the greenhouse effect for an Earth-type atmosphere, compute the following:
Here, M is the atmospheric mass of the world, and G is the world’s greenhouse effect in kelvins. Feel free to adjust the result by up to 1.5 times the atmospheric mass in either direction. Round the result to the nearest integer.
Class IV Atmosphere
To determine the greenhouse effect for a Mars-type atmosphere, roll 1d6-4 (minimum 0). The result is the world’s greenhouse effect in kelvins.
Class V Atmosphere
A Class-V (Luna-type) atmosphere is far too thin to create a significant greenhouse effect. The greenhouse effect in this case is always 0 kelvins.
Average Surface Temperature
With the world’s albedo and greenhouse effect established, the average surface temperature can be computed. Evaluate the following:
Here, T is the average surface temperature in kelvins, B is the world’s blackbody temperature, A is the world’s albedo, and G is the world’s greenhouse effect in kelvins.
Examples
Arcadia IV has a blackbody temperature of 281 K, a Class III atmosphere with atmospheric mass of 0.9, and Extensive water.
Alice begins by estimating the planet’s albedo. She rolls 3d6, gets a result of 11, multiplies that by 0.01 and adds it to the base albedo of 0.22. Arcadia IV has an albedo of 0.33, and so is slightly more reflective than Earth. This is most likely due to high-altitude clouds, covering a slightly greater portion of the planet’s surface than on Earth.
Alice then estimates the planet’s greenhouse effect. She rolls 3d6, gets a result of 12, multiplies that by 3 and then 0.9, and gets a final result of 32.4. She decides not to adjust this result and rounds it off to 32 kelvins, indicating a slightly weaker greenhouse effect than that of present-day Earth (a little over 33 kelvins).
Computing the planet’s average surface temperature, Alice gets:
This result is almost identical to Earth’s average surface temperature in the present day (about 287 K).
Arcadia V has a blackbody temperature of 226 K, a Class III atmosphere with an atmospheric mass of 0.7, and only has Moderate water.
To estimate the planet’s albedo, Alice rolls 3d6, gets a result of 14, multiplies that by 0.01 and adds it to the base albedo of 0.19. By an odd coincidence, Arcadia V also has an albedo of 0.33, although this is likely due to extensive sheets of ice and snow on the surface rather than high-altitude clouds.
Alice then moves on to the planet’s greenhouse effect. She rolls 3d6, gets a result of 8, multiplies that by 3 and then 0.7, and gets a final result of 16.8. Again, she decides not to adjust this figure and rounds it up to 17 kelvins. Arcadia V has a noticeably weaker greenhouse effect than Earth.
Computing the planet’s average surface temperature, Alice gets:
Arcadia V is bitterly cold, with surface temperatures averaging 221 K (about -52° C), comparable to winter temperatures in Antarctica on Earth. The planet is a little warmer than Mars, however (average surface temperature about 210 K).
Architect of Worlds – Step Twenty-Four: Determine Hydrographic Coverage
In this step, we will determine how much of the world’s surface is covered by water, either as liquid-water seas and oceans, or as a layer of ice. We will express this hydrographic coverage in terms of a percentage. A world with 0% hydrographic coverage has no significant surface water or ice, while a world with 100% hydrographic coverage has no exposed dry land.
This is largely determined by the prevalence of water, from Step Twenty. However, in some cases the amount of dry land surface will also depend on the world’s geophysical parameters, from Step Twenty-One. Large land masses are unlikely to form unless a world has a strong lithosphere and is geologically active, creating variation in topographical relief faster than it can be worn down by weathering and erosion. Otherwise, the world’s surface is likely to be dominated by shallow oceans or ice sheets.
Procedure
Refer to the following table, and find the row corresponding to the world’s prevalence of water and current lithosphere. To determine the hydrographic coverage at random, roll dice as shown in the third column and apply the result.
As I work on the next few steps in the Architect of Worlds design sequence, I’ve realized that I can save myself a lot of hassle by tweaking a couple of the steps I’ve recently posted.
So, for those of you who are following along and experimenting with the sequence as I post it, here are two minor changes you may want to consider implementing immediately. The next few steps are going to assume these modifications are in effect.
Under Step Twenty-One, add the following right after the “Status of Lithosphere” block:
Special Case: Molten Lithospheres and Prevalence of Water
If the world has a Molten Lithosphere, and its prevalence of water is not Massive, then it cannot currently support liquid-water oceans or ice sheets. Reduce the prevalence of water to Trace. This does not constitute a runaway greenhouse event. If the world’s surface cools in the future, water may appear.
The idea here is to avoid cases where the world’s surface is covered with molten lava and yet somehow has significant liquid (or even solid) water. Of course, if the world has so much water that its surface is going to be covered hundreds of kilometers deep, even a Molten Lithosphere isn’t going to be able to evaporate all of it.
Next modification: under Step Twenty-Three, beginning with the paragraph that starts “Make a list of the atmospheric components that meet both conditions . . .” and ending with the block titled “Third Case,” replace the text with the following:
Make a list of the atmospheric components that meet both conditions, and then refer to the following three cases. In each case, the world will also be assigned an atmospheric class of I through V, which will be relevant in later steps of the design sequence.
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 (the world will have a Class I or Venus-type atmosphere)
1 if the world has blackbody temperature less than 125 K and Massive prevalence of water (the world will have a Class II or Titan-type atmosphere)
0.1 otherwise (the world will have a Class III or Earth-type atmosphere)
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. This will be a Class IV or Mars-type atmosphere.
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. This will be a Class V or Luna-type atmosphere.
Assigning these “atmospheric classes” at this point will make several of the steps, starting with Twenty-Five, much more concise.
That’s all for now. I may be able to post Step Twenty-Four tomorrow, and we’ll see how smoothly the next few steps after that fall together.
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.
Architect of Worlds – Step Twenty-Two: Determine Magnetic Field
In this step, we will estimate whether the world under development has a significant magnetic field.
The possible cases for a world’s magnetic field will be sorted into four categories: None, Weak, Moderate, and Strong, defined as follows.
None: The world has no detectable magnetic field and is completely unprotected from the stellar wind. Examples: Venus, Earth’s moon, Mars, most of the gas giant planets’ major satellites.
Weak: The world has a detectable magnetic field (about 1% as strong as Earth’s), but it offers no significant protection from the stellar wind. Examples: Mercury or Ganymede.
Moderate: The world’s magnetic field is strong enough to offer limited protection against the stellar wind (about 10% as strong as Earth’s). Examples: None in our planetary system.
Strong: The world’s magnetic field is at least comparable to that of Earth, sufficient to provide adequate protection against the stellar wind. Examples: Earth, the gas giant planets.
A world’s magnetic field seems to depend on several items:
The world needs to have a hot, liquid outer core of significant mass, composed largely of iron
There must be convection taking place in that iron outer core, causing rising and falling currents
The world must rotate on its axis
If all three of these conditions hold, the iron outer core forms a dynamo which creates a significant magnetic field. This, in turn, helps protect the world’s atmosphere from being stripped away by stellar wind, and also protects the surface of the world from some harmful radiation. In our own planetary system, only Earth and the gas giant planets have strong magnetic fields.
Note that the third condition – that the world must rotate on its axis – is almost universal. Even a tide-locked world still rotates on its axis, and physical modeling seems to indicate that even slow rotation is enough to support a working dynamo. The existence of strong convective heat transfer through a world’s outer core seems to be the critical factor.
Procedure
To determine the strength of a world’s magnetic field at random, roll 3d6 modified as follows:
+4 if the world has a Soft Lithosphere
+8 if the world has Early Plate Lithosphere or Ancient Plate Lithosphere and also has Mobile Plate Tectonics
+12 if the world has Mature Plate Lithosphere and also has Mobile Plate Tectonics
Refer to the Magnetic Field Table entry for the modified roll.
Magnetic Field Table
Modified Roll (3d6)
Magnetic Field
14 or less
None
15-17
Weak
18-19
Moderate
20 or greater
Strong
Examples
Arcadia IV has a Mature Plate Lithosphere and Mobile Plate Tectonics, so Alice rolls 3d6+12 and gets a result of 25. Arcadia IV has a Strong Magnetic Field.
Arcadia V has a Mature Plate Lithosphere but has Fixed Plate Tectonics. Alice rolls an unmodified 3d6 and gets a result of 13. Arcadia V has no significant magnetic field.
Architect of Worlds – Step Twenty-One: Geophysical Parameters
Before we get started with this step in the design sequence, be aware that the modeling here is even more pragmatic and “rule-of-thumb” than usual. I think the following material will work properly, but it’s going to need some rigorous testing and tweaking before I’m satisfied with it.
In this step, we will determine some of the geological history of the world under development. In particular, we will estimate the world’s internal heat budget, characterize the presence and degree of active plate tectonics, and estimate the level of vulcanism.
A world’s internal heat will normally derive from three different sources:
The primordial heat of the world’s formation
Radiogenic heat derived from the decay of radioactive isotopes
Tidal heat generated by friction due to any tidal forces acting on the world
The structure and behavior of the world’s lithosphere will be strongly determined by the amount of heat remaining in the world’s deep interior. The hotter the world’s mantle and core, the more likely it is that heat will escape to and through the world’s surface, softening or melting surface rocks and possibly giving rise to volcanic eruptions.
Procedure
Primordial and Radiogenic Heat Budget
Begin by estimating the primordial and radiogenic heat budget of the world under development.
Evaluate the following quantity for all worlds:
Here, K is the density of the world compared to Earth, R is the world’s radius in kilometers, M is the metallicity of the star system (as determined in Step Five), and A is the age of the star system in billions of years. HP is a rough measure of the total amount of primordial heat and radiogenic heat a world possesses, on a logarithmic scale. On this scale, Earth had an HP value of about 90 immediately after its formation (and has an HP value of about 54 today).
Tidal Heat Budget
Not all worlds will have a significant budget of internal heat due to tidal friction. Or each world under development, check to see whether the world falls into any of the following two cases. If so, compute the quantity HT according to the formula given.
First Case: Major Satellites of Gas Giants
A major satellite of a gas giant planet only (not a Leftover Oligarch, Terrestrial Planet, or Failed Core) will experience significant tidal heating if and only if:
There is at least one other major moon in the next outward orbit from the gas giant, as established in Step Fourteen, the first case, and
That “next outward” major moon is in a stable resonance with the moon being developed (that is, the ratio of their two orbital radii was derived from the Stable Resonant Orbit Spacing Table in Step Eleven).
In this case, the resonance between the two orbital periods will tend to maintain a small degree of eccentricity in the first moon’s orbit. This in turn will cause tidal forces imposed by the gas giant to increase and decrease slightly during the moon’s orbital period, causing the moon’s body to “flex” and create friction. In our own planetary system, two of the satellites of Jupiter fall into this case (Io and Europa).
If a moon falls into this case, evaluate the following:
Here, M is the mass of the gas giant in Earth-masses, D is the moon’s radius in kilometers, and R is the moon’s orbital radius in kilometers. HT is a rough estimate of the moon’s tidal heat budget, on the same logarithmic scale as HP.
Second Case: Spin-Resonant Planets Without Major Satellites
A Leftover Oligarch, Terrestrial Planet, or Failed Core which has no major satellite may experience significant tidal heating due to its primary star, if and only if the planet is in a spin-orbit resonance with its primary star, as determined in Step Sixteen, and at least one of the two following cases is correct:
The spin-orbit resonance is not 1:1 (that is, the planet is not tide-locked to its primary star), or
Both of the following are true:
There is at least one other planet in the next outward orbit from the primary star, as established in Step Eleven, and
That “next outward” planet is in a stable resonance with the planet being developed (that is, the ratio of their two orbital radii was derived from the Stable Resonant Orbit Spacing Table).
In either case, tidal forces imposed by the primary star will cause the planet’s body to flex slightly during its orbital period, giving rise to internal friction and heat. In practice, the effect is likely to be minimal unless the planet orbits very close to its primary star.
If a planet falls into this case, evaluate the following:
Here, M is the mass of the primary star in solar masses, D is the planet’s radius in kilometers, and R is the planet’s orbital radius in AU. HT is a rough estimate of the planet’s tidal heat budget, on the same logarithmic scale as HP.
Once you have computed HP and (possibly) HT, make a note of the greater of the two– that is, the heat budget associated with the source that is currently providing more internal heat for the world – for use in the rest of this step.
Status of Lithosphere
The lithosphere of a world is the top layer of its rocky structure. A world’s lithosphere usually begins as a global sea of magma, but it will soon cool, forming a solid crust that provides a (more or less) stable surface. Over time, as the world cools, the crust will tend to become thicker and more rigid, eventually forming a single immobile plate that covers the entire sphere.
Note that on a world with Massive prevalence of water, the lithosphere is effectively inaccessible, submerged beneath deep ice sheets or liquid-water oceans. In this case, the actual surface of the world will be atop the water layers (the hydrosphere). Determine the status of the lithosphere in any case since it will still affect several other properties of the world.
The possible cases will be sorted into six categories: Molten, Soft, Early Plate, Mature Plate, Ancient Plate, and Solid. These categories are defined as follows.
Molten Lithosphere: Large portions of the world’s lithosphere are still covered by magma oceans. A thin solid crust may form in specific regions. Active volcanoes are extremely common and may appear anywhere on the lithosphere. Examples: Earth in the Hadean Eon.
Soft Lithosphere: A solid lithosphere has formed, and no magma oceans remain. However, the lithosphere is not strong enough to resist the upwelling of magma from the world’s mantle, so active volcanoes remain very common and continue to appear anywhere on the lithosphere. Examples: Earth in the early Archean Eon.
Early Plate Lithosphere: The lithosphere is becoming strong enough to resist the upwelling of magma from the mantle. The crust is organizing into solid plates. Volcanoes remain common, but (depending on the presence of active plate tectonics) may be limited to certain locations. Examples: Earth in the later Archean Eon.
Mature Plate Lithosphere: The organization of the crust into solid plates is complete, with most or all of the crust now integrated into the system. Some of the crustal plates are now thicker and more durable. Volcanoes are less common. Examples: Earth today.
Ancient Plate Lithosphere: The lithosphere is becoming thick and rigid, and the system of crustal plates is becoming stagnant. Vulcanism is increasingly rare. Examples: Earth billions of years from now, Mars today.
Solid Lithosphere: The lithosphere is solid and completely stagnant. Vulcanism is vanishingly rare or extinct. Examples: Earth’s moon.
To determine the current status of a world’s lithosphere, roll 3d6 and add HP or HT, whichever is greater. Then refer to the Lithosphere Table.
Lithosphere Table
Modified Roll (3d6)
Lithosphere Status
96 or higher
Molten Lithosphere
88-95
Soft Lithosphere
79-87
Early Plate Lithosphere
45-79
Mature Plate Lithosphere
31-44
Ancient Plate Lithosphere
30 or less
Solid Lithosphere
Plate Tectonics
Even if a world’s crust is organized into solid plates, those plates may or may not be able to move and interact in an active system of plate tectonics. In our own planetary system, several worlds show some sign of plate tectonics. However, only on Earth is the entire crust arranged into a clear set of plates that move across the mantle and actively recycle crust material. The decisive factor seems to be Earth’s extensive prevalence of water, which permeates the crustal rocks and reduces friction among the tectonic plates.
Determine the status of the world’s plate tectonics only if its lithosphere is in an Early Plate, Mature Plate, or Ancient Plate status as determined above.
The possible cases will be sorted into two categories: Mobile Plate Tectonics and Fixed Plate Tectonics, defined as follows.
Mobile Plate Tectonics: The crust’s tectonic plates are able to move freely past or against one another. As tectonic plates collide, some of them experience subduction, moving down into the mantle and recycling the crustal material. Orogeny, or the formation of mountain ranges, takes place in such areas as well. Volcanic activity is likely to take place at plate boundaries. Volcanoes may also appear in plate interiors, at the top of magma plumes rising from the deep mantle. Such shield volcanoes will tend to form arcs or chains, as the tectonic plate moves across the top of the plume.
Fixed Plate Tectonics: The crust’s tectonic plates are unable to move freely. Little or no subduction takes place to recycle crustal material. Orogeny is rare. As with Mobile Plate Tectonics, volcanoes are likely to appear at plate boundaries. Shield volcanoes are also possible, but since the tectonic plates are nearly immobile, such volcanoes can grow very large over time.
In general, a world is likely to have Mobile Plate Tectonics if it is younger (and therefore still has a hot mantle and core) and has plenty of surface water to reduce friction among the plates. To determine the status of a world’s plate tectonics at random, roll 3d6 and modify the result as follows:
+6 if the world has Extensive or Massive prevalence of water
-6 if the world has Minimal or Trace prevalence of water
+2 if the world has an Early Plate Lithosphere
-2 if the world has an Ancient Plate Lithosphere
A world will have Mobile Plate Tectonics on a modified roll of 11 or greater, and Fixed Plate Tectonics otherwise.
Special Case: Episodic Resurfacing
If a world has an Early Plate or Mature Plate Lithosphere, and has Fixed Plate Tectonics, then vulcanism will follow an unusual pattern of episodic resurfacing.
In this case, the lithosphere is too strong to permit magma to reach the surface under normal conditions. Since any tectonic plates are fixed in place, subduction and orogeny are very rare. Active volcanoes are also uncommon. However, heat built up in the mantle periodically breaks through, causing massive volcanic outbursts that “resurface” large portions of the lithosphere before the situation restabilizes.
For an Early Plate Lithosphere, these resurfacing events will take place millions of years apart. For a Mature Plate Lithosphere, resurfacing becomes much less frequent, tens or even hundreds of millions of years apart. In our own planetary system, Venus is an example of this case.
Examples
Both Arcadia IV and Arcadia V are planets without major satellites, and both of them are at a significant distance from the primary star, so Alice assumes that tidal heating will be insignificant for both of them. She evaluates HP for both. The star system is 5.6 billion years old and has metallicity of 0.63.
Arcadia IV has density of 1.04 and radius of 6450 kilometers, and so has HP of 41 (rounded to the nearest integer). With a 3d6 roll of 9, the total is 50. Arcadia IV has a Mature Plate Lithosphere.
Arcadia V has density of 0.92 and radius of 5670 kilometers, and so has HP of 34 (rounded to the nearest integer). With a 3d6 roll of 13, the total is 47. Arcadia V also has a Mature Plate Lithosphere. Notice that both of these planets have total scores fairly low in the range for a Mature Plate result, indicating that they have rather “old” geology and may be transitioning to an Ancient Plate configuration.
Arcadia IV has Extensive water, so Alice rolls 3d6+6 for a result of 15. Arcadia IV has Mobile Plate Tectonics, resembling Earth in this respect.
Arcadia V only has Moderate water, so Alice makes an unmodified roll of 3d6 for a result of 7. Arcadia V has Fixed Plate Tectonics and exhibits episodic resurfacing on a timescale of tens or hundreds of millions of years.
Architect of Worlds – Step Twenty: Determine Prevalence of Water
Water is one of the most common substances in the universe. Its special properties will lead it to have a profound effect on the surface conditions of any world, from its initial geological development, to its eventual climate, and finally to the evolution of life. Some worlds may never have much water, others will tend to lose whatever water they begin with, and still others will retain massive amounts of water throughout their lives.
In this step, we will estimate how much water can be found on a given world. The possible cases will be sorted into five categories: Trace, Minimal, Moderate, Extensive, and Massive. These categories are defined as follows.
Trace: No liquid water or water ice remains on the vast majority of the surface. If there is a substantial atmosphere, it may carry traces of water vapor. Small pockets of water ice may remain on the surface, in permanently shadowed craters or valleys, or on the night face of a world tide-locked to its primary star. Small deposits of water may be locked in hydrated minerals deep below the surface. Examples: Mercury, Venus, Earth’s moon, or Io.
Minimal: Liquid water is vanishingly rare on the surface, but large deposits of water ice may exist in the form of polar caps, in sheltered craters or valleys, or on the night face of a tide-locked world. Substantial aquifers or ice deposits may exist close beneath the surface. Hydrated minerals can be found in the world’s interior. Examples: Mars.
Moderate: A substantial portion of the world’s surface, but not a majority, is covered by some combination of liquid-water seas and water ice, depending on local temperature. The liquid-water oceans or ice deposits are up to a few kilometers in depth. Far away from the oceans or ice deposits, water becomes vanishingly rare. Hydrated minerals are common in the world’s interior. Examples: Mars a few billion years ago.
Extensive: Most of the world’s surface is covered by some combination of liquid-water oceans and water ice, up to several kilometers in depth. Water is common in most areas of the surface, even away from the oceans or ice deposits. Hydrated minerals are plentiful far into the world’s interior. Examples: Earth, Venus a few billion years ago.
Massive: The entire surface is covered by some combination of liquid-water oceans and water ice, up to hundreds of kilometers deep. Deeper layers of this world-ocean may be composed of higher-level crystalline forms of water (Ice II and up). Hydrated minerals are plentiful far into the world’s interior. Examples: Europa, Ganymede, Callisto, Titan, some “super-Earth” exoplanets.
The amount of water available on a given world will depend upon its M-number (Step Nineteen), its blackbody temperature (Step Nineteen), its location with respect to the protoplanetary disk (Step Nine), and (in some cases) the arrangement of any gas giant planets elsewhere in the planetary system (Steps Ten and Eleven).
Procedure
Begin by noting which of the following three cases the world being developed falls under, based on its M-number.
First Case: M-number is 2 or less
In this case, the world’s prevalence of water is automatically Massive.
Second Case: M-number is between 3 and 28
In this case, determine whether the world is outside or inside the protoplanetary nebula’s snow line, as determined in Step Nine. If the world’s orbital radius (or that of its planet, in the case of a major satellite) is exactly on the snow line, assume that it is outside.
If the world in this case is outside the snow line, then its prevalence of water is automatically Massive.
If the world in this case is inside the snow line, then roll 3d6, modified as follows:
Subtract the world’s M-number.
Add +6 if there exists a dominant gas giant in the planetary system, it experienced a Grand Tack event, and it is currently outside the protoplanetary nebula’s snow line.
Add +3 if any gas giants in the planetary system are currently outside the protoplanetary nebula’s slow-accretion line.
Take the modified 3d6 roll and refer to the Initial Water Prevalence table:
Initial Water Prevalence Table
Modified Roll (3d6)
Prevalence
-5 or less
Trace
-4 to 3
Minimal
4 to 11
Moderate
12 to 19
Extensive
20 or higher
Massive
If the result on the table is Moderate or higher, and the world’s blackbody temperature is 300 K or greater, then the presence of water vapor in the world’s atmosphere has given rise to a runaway greenhouse event. Make a note of this event for later steps in the design sequence and reduce the prevalence of water to Trace.
Otherwise (the world’s blackbody temperature is less than 300 K) the prevalence of water is as indicated on the table.
Third Case: M-number is 29 or greater
In this case, determine whether any of the three following cases is true:
The world’s blackbody temperature is 125 K or greater.
The world is the major satellite of a Large gas giant, and its orbital radius is no more than 8 times the radius of the gas giant.
The world is the major satellite of a Very Large gas giant, and its orbital radius is no more than 12 times the radius of the gas giant.
If any of these three cases are true, then the world’s prevalence of water is Trace. Otherwise, its prevalence of water is Massive.
Examples
Both Arcadia IV and Arcadia V fall into the second case. The Arcadia system has a dominant gas giant, which underwent a Grand Tack and ended up outside the snow line. The outermost gas giant (at 9.50 AU) is not outside the system’s slow-accretion line (at 14.0 AU). For both planets, therefore, she will roll 3d6, minus the planet’s M-number, plus 6. Her rolls are 13 for Arcadia IV and 10 for Arcadia V, so Arcadia IV has Extensive water while Arcadia V has only Moderate water.
![T=(B\times\sqrt[4]{1-A})+G](https://s0.wp.com/latex.php?latex=T%3D%28B%5Ctimes%5Csqrt%5B4%5D%7B1-A%7D%29%2BG+&bg=ffffff&fg=000&s=0&c=20201002)
![T=(281\times\sqrt[4]{1-0.33})+32\approx286\ K](https://s0.wp.com/latex.php?latex=T%3D%28281%5Ctimes%5Csqrt%5B4%5D%7B1-0.33%7D%29%2B32%5Capprox286%5C+K+&bg=ffffff&fg=000&s=0&c=20201002)
![T=(226\times\sqrt[4]{1-0.33})+17\approx221\ K](https://s0.wp.com/latex.php?latex=T%3D%28226%5Ctimes%5Csqrt%5B4%5D%7B1-0.33%7D%29%2B17%5Capprox221%5C+K+&bg=ffffff&fg=000&s=0&c=20201002)
