A Unique Blend

In a ground test of the Apollo 1 spacecraft in 1967, three astronauts perished in a horrible fire that broke out and rapidly spread through the capsule. This disaster required major changes in the design of future Apollo capsules, including reducing the oxygen content from 100% to the 21% present in our atmosphere.

You might think that a pure oxygen atmosphere would be better than just 21% since that is the only gas our bodies utilize when we breathe. However, a pure or nearly pure oxygen atmosphere would be disastrous. Not only would fires quickly rage out of control but experiments have shown that the cells of living organisms would be damaged if exposed to excessive oxygen for a long time.

Bulking up with Nitrogen

We tend to take for granted the healthfulness of the air around us as long as we get the oxygen we need. We don’t pay much attention to the other elements. Yet the bulk of the atmosphere consists of nitrogen (78%), and the remaining 1% is a mixture of trace materials. What is all that nitrogen doing in the atmosphere?

Nitrogen is in a diatomic form, meaning that each molecule of nitrogen is made of two tightly bound atoms of nitrogen. We write this chemically as N₂. Diatomic nitrogen is chemically inert, which means that it rarely interacts with anything. So the majority of the earth’s atmosphere doesn’t do anything except provide bulk and take up space. This is ideal, because it provides the proper amount of air pressure to support life without interfering with basic life processes. For example, just the simple exchange of gases in breathing requires a certain amount of air pressure. If all that inert nitrogen weren’t present, breathing would be difficult.

Avoiding the Greenhouse Effect

The oxygen is diatomic as well (O₂). Together, more than 99% of the earth’s atmosphere is diatomic, and that’s also a good thing! Other types of gases contribute to the greenhouse effect. Too much of them would turn our planet into an unbearable hothouse.

No other known planets have atmospheres that are diatomic. The most common gases on the other planets are polyatomic (molecules containing three or more atoms per molecule). Examples of polyatomic gases are carbon dioxide (CO₂), water vapor (H₂O), ozone (O₃), and methane (CH₄). We call these “greenhouse gases” because they have the same effect as windowpanes in a greenhouse.

As sunlight passes through the panes, the interior heats up, but the glass prevents the heat from escaping. Polyatomic gases have the same effect on a planet. Sunlight heats up the surface, but polyatomic gases trap the heat in the lower atmosphere so that it can’t escape. This explains why the surface of Venus is very hot; its thick atmosphere consists almost entirely of CO₂.

photos: NASA

Avoiding Radical Temperature Swings

The earth’s atmosphere is very different from any other planet’s. The small amount of polyatomic gas produces just enough greenhouse effect to keep the temperatures from getting too cold at night, as heat escapes. It also blocks much of the infrared radiation coming from the sun, preventing the earth from heating up too quickly during the day. If the earth didn’t have any polyatomic gases, then the temperature would fluctuate well over 100° F (55.5° C) every day.

Scientists understand this danger very well, so they have begun looking for extrasolar planets that might have diatomic atmospheres. They know that no other atmosphere will work. No luck, so far. It seems we live on a unique planet, specially created to be inhabited.