Why don't the gases in the atmosphere separate out according to mass?
Category: Earth Science Published: September 19, 2023
The various gases in earth's atmosphere do indeed partially separate out into layers according to molecular mass, as can be seen in the figure below. This figure shows the composition of the atmosphere as a function of the height above the ground. In this figure, the label for each curve indicates the gas that is represented by that curve and also the approximate mass of one molecule or one atom of that gas (as the case may be) in unified atomic mass units. For instance, the blue curve represents atomic oxygen (O) and each oxygen atom has a mass of about 16 u. As you can see in this figure, the most massive common gases (which are molecular nitrogen, molecular oxygen, and argon) have their highest concentrations in the lower to middle atmosphere. The intermediate-mass common gases (which are atomic oxygen and atomic nitrogen) have their highest concentrations in the middle to upper atmosphere. The lightest common gases (which are helium and hydrogen) have their highest concentrations in the uppermost atmosphere.
The lowest layer of earth's atmosphere is called the troposphere and it extends from the ground to about 15 km high. The troposphere contains most of earth's clouds, weather phenomena, and convection systems. Furthermore, the troposphere contains mostly nitrogen molecules (N2), oxygen molecules (O2), and argon atoms (Ar). However, they are continuously forced to aggressively mix together, as I'll explain later, so that there is effectively zero separation of these gases according to molecular mass in the troposphere. As a result, the relative proportions of these gases is constant in the troposphere, as you can see in the figure above.
The layer above the troposphere is the stratosphere, which extends from about 15 km to 50 km high, and the layer above that is the mesosphere, which extends from about 50 km to 90 km high. The various layers of the atmosphere are shown in the figure directly above. The stratosphere is where the highest-flying airplanes and weather balloons travel. The mesosphere is where most meteors light up and disintegrate as they fall. By comparing the two figures above, you can see that the troposphere, stratosphere, and mesosphere all have about the same proportions of molecular nitrogen, molecular oxygen, and argon. Therefore, in all three of these layers the gases are homogenously mixed and do not separate according to molecular mass. This is because of the aggressive churning of the air in these three layers. For this reason, these three lowest layers of the atmosphere (plus the lower edge of the thermosphere) are collectively called the homosphere. Note that the homosphere contains the exception of the ozone layer, which is not homogenously mixed throughout the entire homosphere. However, this is because of chemical reactions and not because of a separation of gases according to molecular mass.
Things get more interesting in the thermosphere, which extends from about 90 km to 800 km. Thermal convection and other aggressive mixing effects are extremely weak in the thermosphere. As a result, the gases in the thermosphere are indeed able to partially separate according to molecular mass, as you can see in the first figure above. However, there is not a complete separation of the gases according to molecular mass. This is because of molecular diffusion, as I'll explain later. The lower part of the thermosphere consists mostly of approximately equal amounts of molecular nitrogen (N2) and atomic oxygen (O). The middle part of the thermosphere is almost completely made up of atomic oxygen (O and O+). The upper part of the thermosphere consists mostly of approximately equal amounts of atomic oxygen (O and O+) and helium (He and He+), as well as some hydrogen (H and H+). Note that in the middle of the thermosphere, the ionized form of the atoms becomes as common as the neutral form. Above the middle thermosphere, most of the atoms are in the ionized form.
The outermost part of the atmosphere is the exosphere, which extends from about 800 km to about halfway to the moon, or about 190,000 km. The exosphere experiences effectively zero aggressive mixing so that the gases partially separate out according to mass. The very lowest part of the exosphere is mostly ionized helium (He+). Right above this very lowest part (which is not visible in the first diagram above) there are about equal amounts of ionized helium (He+) and ionized hydrogen (H+). The rest of the exosphere (which is not visible in the first diagram above) consists almost entirely of ionized hydrogen (H+). Note that in many contexts it is useful to treat the thermosphere and exosphere as part of outer space.
The data for the two figures above were obtained from the NRLMSISE-00 model. These data sets represent the state of the atmosphere above Canyon, TX on January 1, 2023 at 6:00 pm. Although the state of the atmosphere varies from hour to hour, from day to day, and from location to location, these variations are small so that that the data will always look generally similar to the figures above. Note that the first figure above does not include carbon dioxide (CO2), neon (Ne), methane (CH4), krypton (Kr), water vapor (H2O), or ozone (O3). The reason for this are that these atoms and molecules are too rare at most heights to show up in this figure. For instance, even though ozone molecules are at a relatively high concentration in the ozone layer, they still only make up 0.001% of the atmosphere in this layer, which is far too small to be visible in the first diagram above.
The separation of the gases in the atmosphere according to molecular mass is only a partial separation for three reasons: (1) the lowest three layers of the atmosphere are continuously being aggressively mixed around, (2) chemical reactions change the gases, and (3) molecular diffusion causes some mixing, even in the thermosphere and exosphere. Let's look at these mechanisms more closely.
1. In the lowest three layers of the atmosphere-the troposphere, the stratosphere, and the mesosphere-there is almost zero separation of the gases according to molecular mass because everything is continuously being aggressively mixed together by the bulk motion of air. As an analogy, think of a big glass bottle filled with sand, water, and oil. If you leave the bottle alone and wait for everything to settle, gravity will cause the matter to separate into different layers according to mass, with the sand on the bottom, water in the middle, and oil on the top. However, if you continually stir the contents of the bottle, the sand and water and oil will get mixed together faster than it can separate out into layers, and will therefore remain homogenously blended. Similarly, the ongoing stirring of the atmosphere prevents the gases from separating according to molecular mass. The stirring of the troposphere, the stratosphere, and the mesosphere is driven by several effects that involve the bulk motion of large volumes of air. The rotation of the earth, thermal convention effects, the daily cycle of sunlight, and the complex interaction of various air masses all contribute to creating large air currents in the homosphere that constantly mix up the gases.
2. Another mechanism that works against the tendency of gravity to separate the gases according to molecular mass is the fact that some matter in the atmosphere is changing form. In other words, one particular molecule in the atmosphere does not stay forever the same type of molecule. A bit of sunlight, or a cosmic ray, or a violent collision with some other bit of matter can rip apart a molecule. Additionally, molecules can chemically react when they collide and form new molecules. In this way, a molecule with a higher mass can become two molecules with lower masses, and the opposite can happen too. This tends to make new molecules appear at heights that do not correspond to their molecular mass.
The most significant example of this is the ozone layer. The ozone layer is a region of the stratosphere extending from about 15 km to 35 km high where there is a relatively high concentration of ozone molecules (O3). A high concentration of ozone occurring at this particular range of heights is not due to mass effects, but is rather due to the fact that ozone is created in this layer. Ultraviolet sunlight strikes oxygen molecules (O2) in this layer, splitting them apart. The free oxygen atoms (O) then react with other oxygen molecules (O2) to form ozone molecules (O3).
3. The other main mechanism that tends to blend different gases together is molecular diffusion. As the molecules of a gas randomly move around and bump into each other, they tend to collectively get pushed outwardly away from each other. This is why a gas expands to occupy its container. This is also why different gases in a bottle tend to naturally blend together. The bumping-around effect also causes the molecules in a gas to have a wide range of speeds which are spread out over a Maxwell-Boltzmann distribution. As a result, in the atmosphere, some of the molecules of a particular gas have enough speed to fly high above where they would otherwise be if there where only gravitational effects. Molecular diffusion is the dominant mixing effect in the atmosphere above about 100 km, which involves the exosphere and most of the thermosphere. That is why all of the curves above 100 km in the first figure above look similar to Maxwell-Boltzmann distribution curves.
In summary, the different gases in the atmosphere do indeed partially separate out according to molecular mass because of gravity, but this separation is only partial because of the aggressive mixing of the gases, because of chemical reactions, and because of molecular diffusion. Aggressive mixing is the dominant mechanism in the homosphere (i.e. the troposphere, the stratosphere, the mesosphere, and the lower edge of the thermosphere) and molecular diffusion is the dominant mechanism in the heterosphere (i.e. the exosphere and most of the thermosphere).