In order to have a planet house life, an atmosphere is absolutely essential, unless the sentient species is capable of enclosed domes, which even then they’d still have an atmosphere within that enclosed dome. This brings us to the fourth part of building a planet: Atmospheres and natural cycles.
Atmospheres are an envelope of various gases that are held in place by gravity; pressure is used to describe the weight of the atmosphere that’s held down by the gravity of a planet. (There’s various terms to measure pressure, but for our purposes the reasoning behind these terms isn’t necessary for the discussion: I can provide resources for those interested in how measuring pressure works, but for now, we’ll use the unit of atmospheres (atm) when appropriate for the discussion.) Since atmospheres are composed of several types of gases, often the idea of partial pressures is used to describe the total pressure times the percentage of a specific constituent within an atmosphere. For example: on earth, oxygen makes up 21 percent of the atomsphere (or in decimal units: .21). The total atmospheric pressure of Earth is 14.7 psi (pounds per square inch). So the partial pressure of oxygen is: (.21) x (14.7) = ~3.1 psi. An interesting side note on this discussion: if you had an all oxygen atmosphere within a spacecraft, the spacecraft can be lighter and simpler than if a more varied atmosphere is used due to the ship needing to hold in a much smaller pressure from the oxygen. This tidbit of information may be useful in discussions of spacecraft, but we can discuss that in a later post if anyone is interested. (Also note, from here on out, I will focus on terrestrial planets. If people are interesting, I can have a separate post concerning gas giants.)
An important point to note about volatiles is that the molecules within a gas move freely, often colliding with other molecules. These collisions cause the molecules to careen off each other, and if the molecule is moving fast enough in the right direction, it might exceed the escape velocity and escape into space. All atmospheres will experience some ‘leakage’ of gas particles to space, but the percentage and composition of those that escape depend upon the weight of the molecule, the surface temperature of the planet, and the escape velocity of the planet.
Escape velocity is determined by the mass of your planet and it’s radius. It can be computed by the following equation: Escape velocity = square root(2GM/r), where G is the gravitational constant, M is the mass of your planet, and r is the radius. If your planet is too small in radius and mass, the escape velocity would be too low, allowing a lot of the crucial ingredients of an atmosphere to escape into space, thus leaving your planet with little to no atmosphere whatsoever (Mercury and the Moon are good examples of this in our solar system). This concept can be further visualized in the following graph:
This graph shows how escape velocity and temperature affects what particles a planet can keep in its atmosphere, and if a planet can keep an atmosphere at all. It also shows how the large mass and extremely high escape velocities of gas giants allows for them to keep more of the lighter volatiles than terrestrial planets. Temperature plays a role simply in the sense that the higher the temperature of air molecules, the faster they tend to go, for as you can see in the above graph, the velocities of the escaping gas increases with surface temperature. Also, lighter gases tend to escape more easily, as seen in the above graph, due to their lesser mass, which allows them to achieve higher velocities than a heavier gas. This is why for terrestrial planets, hydrogen – one of the lightest gases in the universe – is rarely if ever retained in the planet’s atmosphere.
Terrestrial planets will often produce atmospheres (or else thicken already formed ones) depending on the volcanic and tectonic activity of its surface. During planetary formation, volatiles, often in their icy/solid form, will become trapped within the mantle of a forming planet. If the planet is warm enough, it will undergo significant tectonic activity, and the volcanic eruptions – sometimes termed ‘outgassing’ can release these volatiles into the atmosphere. Whether these volatiles are retained in the atmosphere once again depends on the escape velocity of the planet and it’s surface temperature. The surface temperature also plays a role in the tectonic activity of the planet, thus forming a cycle. If the escape velocity is enough to retain much of the volatiles spewed forth from tectonic activity, then what’s called a ‘secondary atmosphere’ will form. This way of creating an atmosphere is the generally accepted theory of how the Earth’s atmosphere came to be. This is also how atmospheres can differ from planet to planet – this process produces a different composition of volatiles depending on the composition of your planet.
However, it isn’t quite enough for a planet to be able to hold an atmosphere. As we’ve established, any atmosphere can leak gases over time due to the collisions of molecules, especially in the upper most parts of an atmosphere. Since gases can ‘leak’ out into space due to such a process, then how does a planet ‘buffer’ its atmosphere in order to replenish what was lost in such leakage? This is done through various cycles on the planet itself, in which the atmosphere interacts with crust of the planet; the most common cycle is the carbonate/silicate cycle. However, in order for this cycle to exist, the planet must be large enough to keep the activity in its crust moving, which, as noted before, is powered by its internal heat. If the planet is too small, then the internal heat will be lost far too quickly and tectonic activity soon stalls, and then the atmospheres is not replenished and is thus diminishes over time to leakage.
Another point to consider is the composition and pressure of a planet’s atmosphere isn’t static over time. Over time, it can be modified by various things such as chemical reactions with the surface of the planet, life on the planet, photodissociation at the outer edge of the atmosphere, as well as changes over geologic time due to altering composition of crust from the lost of lighter elements and/or the sun’s changes which can influence the temperature of a planet over time. Life itself can also alter the composition within a crust and the atmosphere. I’ll discuss each of these in turn:
Photodissociation is a process where solar ultraviolet light can break up molecules at the outer edges of the atmosphere; for example, water molecules that drift into the outer edges of the atmosphere can be broken up into oxygen and hydrogen by this light from the sun; however, hydrogen is much lighter than oxygen, and so can achieve the escape velocity far easier, causing it to be lost to space. This effect is often how hydrogen bearing molecules can be lost over time. In order to avoid this effect, a planet needs to have a ‘cold trap’ where molecules like water can be frozen out before it gets high enough to be destroyed by solar UV, but without this effect, even an ocean’s worth of water can be lost in a fraction of geologic time; for Venus in particular, this had devastating consequences. Another consequence of this effect is that ammonia and methane are more easily broken up than other hydrogen-bearing molecules, and thus are easily broken up by solar UV. In the case of our earth, this was a good consequence, because it allowed our type of life to evolve more readily.
Another important factor is greenhouse gases such as ammonia, methane, carbon dioxide, and chlorofluorinated hydrocarbons (CFCs). Certain gases are transparent to visible light, but are nearly opaque to infrared light, which is what is radiated by the ground once it’s warmed by the visible light rays from the sun. These gases that are opaque to infrared light absorbs it, which traps the energy and warms the atmosphere. Although ammonia, methane, carbon dioxide, and CFCs are the most potent gases when it comes to trapping energy, water vapor can play a rather large role as well.
Water vapor is not a free parameter due to the cycle water vapor goes through on a planet with liquid water on its surface. At a given temperature for liquid water to exist, there must be a certain partial pressure of vapor over it, and this partial pressure increases with temperature. To demonstrate: when the temperature is higher, more water evaporates thus increasing the water vapor within the air, which equalizes the partial pressure in the water cycle. When the atmosphere cools, the water vapor content drops as liquid water condenses out in the form of clouds (which can release their condensation through various types of precipitation). This water cycle is one of the many cycles that influence the weather of the planet. Now examine a scenario where an ocean planet experiences a raise in temperature – various situations can cause this, some being the presence of other greenhouse gases that trap heat and thus causing a temperature increase or the sun evolving from one phase of its life into another thus causing its radius to be larger and closer to the planet in question: with the higher temperature, the more water evaporates from the oceans; however, water vapor helps trap energy from sunlight, which in turn increases the temperature of the atmosphere. This, in turn, causes more water to evaporate, which in turn increases the temperature, and so on and so on. This positive feedback amplifies the effect of the other greenhouse gases within the atmosphere, thus increasing the temperature more.
How is this effect mitigated to avoid too high of a positive feedback? By the condensation of clouds which reflect sunlight incredibly well, which in turn can help lower the temperature of the atmosphere and allow more clouds to form, thus cooling everything down and reversing the positive feedback. The problem here is when the temperature gets too high, then this amplification of other greenhouse gases can throw this cycle out of whack and cause the positive feedback to grow out of control – this scenario is often called a “runaway greenhouse effect.”
To return to an earlier point, the thickness and composition of an atmosphere over time is controlled by reaction with the crust and oceans on a planet; I’ve already discussed the ocean’s effect, so let’s take a look at the crust. The carbonate-silicate cycle is what regulates the carbon dioxide content of the atmosphere through atmospheric interaction with a planet’s crust. I’ll use Earth as an example for clarity: on earth nearly all of its carbon dioxide is locked in the crust within limestone. Limestone mostly consists of calcium carbonate, which precipitates easily in a water solution if dissolved calcium is present along with dissolved carbon dioxide, thus producing limestone. Calcium is one of the most abundant element in Earth’s crust, and so it tends to be present to some degree somewhere on the planet. The more carbon dioxide in the air, the more it can be dissolved into any exposed water – this carbon dioxide solution is fairly acidic allowing for more efficient weathering of a planet’s crust. If carbon dioxide becomes too plentiful in the atmosphere – causing the temperature to warm – efficient weathering can release a lot of calcium, which then over time brings the carbon dioxide levels down. Tectonic processes in turn raise new rock for weathering over time, and crustal rocks newly formed are cycled through various tectonic processes into the earth, so that through volcanic activity the carbon dioxide can be spewed out into the atmosphere again, which resets the cycle. Over time, this cycle tends to keep the atmosphere of Earth at a fairly constant temperature.
This cycle can also happen due to the effects of life. Limestone can also be created from biological debris such as coral reefs, shells, and other lifeforms that are cemented with carbonates formed inorganically. So limestone isn’t just made inorganically as I just described in the above paragraph, but is also formed through the effects of life. This helps contribute to the cycle, spurring it forward, and it also helps guard against excess water acidity since limestone has drawn the carbon dioxide from the water to form itself.
Another biological process that effects carbon dioxide in the atmosphere is plants on the Earth’s surface: photosynthesis. Plants captures carbon dioxide within the leaves of the plant and through the process of photosynthesis, release oxygen. Animals on the surface of Earth in turn take in oxygen and breathe out carbon dioxide, and thus an organic cycle of carbon dioxide and oxygen is formed.
All three process are at work on Earth, and can also be at work for your planet if you utilize oxygen breathing organisms. (If you decide not to, the composition of the atmosphere and the cycles that will appear over time will be slightly different from Earth’s cycles, but I’ll save this for a different post.)
One of the last topics I’ll discuss in regards to atmospheres is how it thins the further one travels from the surface of the planet. This thinning of atmosphere can effect how high the denizens of your world can climb and how high they can fly without technological help. It’s also something inherent within all atmospheres; the larger and thicker an atmosphere is, the more slowly it thins as one travels away from the surface. How slowly or how quickly an atmosphere thins is called the scale height, which depends, as I said, with how large the atmosphere is as well as the composition of the atmosphere. Atmospheres with heavier molecules tend to have smaller scale heights than those with lighter molecules. The thinning of an atmosphere happens at a near exponential rate: p = p(0) exp( -gh/H), where p is pressure at an altitude h, p(0) is pressure at sea level, g is gravitational pull in terms of Earth (so for Earth this is 1), and H is scale height. This equation can be used to calculate how thin the atmosphere is at certain heights on the planet, for example – at the tops of mountains, which in turn can help you determine if any denizen on your planet would have enough air and pressure to survive. Another example is if you have a spot located far below sea level that contains no water, the pressure of the atmosphere can be calculated to determine what the environment would be like at that depth. If the basin contains water, the calculation would differ greatly due to the pressure of water being far greater than the pressure of air, so the actual pressure at the bottom of the ocean includes both.
The last point I’ll make is that an ozone layer is absolutely necessary in a planet if it wishes to harbor life. Ozone is a fairly active and toxic form of oxygen where it has three molecules bonded together instead of two. Ozone is what absorbs the ultraviolet rays from the sun, thus preventing these rays from reaching the surface of the planet. Why is this so necessary? Because UV is energetic enough to break apart molecules, especially those necessary for life. In order for life to survive, there must be a mechanism that blocks these rays from hitting the surface, and ozone is one of the main molecules for the job. Thus some variation of an ozone layer has to exist for life to survive; ozone isn’t the only molecule for the job, but it is the most studied one due to is presence on Earth. Also note, if ozone is used, then oxygen has to be present in the atmosphere since it’s the reactions of oxygen molecules that form the regular form we can breathe (two oxygen atoms bonded together) and the ozone variety.
If you have any questions or would like clarification on any of this, feel free to comment below! For my next world-building post, I can discuss weather, oceans, and land. Maybe then I’ll be ready for the flora and fauna.