History of Earth's Atmosphere I: _{The origin of the modern atmosphere}

Measuring the gases in Earth’s atmosphere

It’s tempting to think of air as empty, but in fact, we live and breathe inside a 100 kilometer-thick layer of gas molecules called the atmosphere. These gases are held near Earth’s surface by gravity. Greek philosophers and medieval thinkers originally believed that this blanket of air consisted of a uniform substance, but 19th century scientists like John Dalton used a series of experiments to determine that it is actually a mixture of many different gases, primarily nitrogen, oxygen, and argon (see our module on The Composition of Earth's Atmosphere). In addition to these major components, the atmosphere also contains very small amounts of other gases that prove especially useful in understanding the origin and history of our atmosphere.

For example, Earth’s atmosphere contains around 20 parts per million (abbreviated as ppm) of neon gas, the same gas used to produce the bright glow of restaurant signs and cinema marquees. In 1919, a chemist at Cambridge University named Francis Aston became interested in neon, although not for its red glow. He had been working on a new measurement technique known as mass spectrometry that allowed him to separate elements based on their masses and to identify very small amounts of rare atoms. Neon was the first element he studied (Aston, 1919).

Mass spectrometry relies on the fact that ions, or charged atoms, are influenced by magnetic fields. To make a measurement with a mass spectrometer, samples are first ionized (meaning they are stripped of an electron, leaving them with a positive charge) and then shot at high speed between powerful magnets, a process that deflects their path according to their ratio of mass to charge (see Figure 1 for a diagram). The paths of light ions like oxygen bend the most, while heavy ions like uranium are least affected. The result is that a mass spectrometer separates these different ions from the initial substance, collecting and counting them with a detector called a Faraday collector, and allowing precise measurements of the concentrations of individual ions.

Using this new technique, Aston discovered something interesting about neon: There wasn’t much of it on Earth. He discovered this somewhat by accident while trying to establish the average composition of Earth’s atmosphere, ocean, and crust. He did this by measuring representative samples of each (Aston, 1924a), an effort which earned him the Nobel Prize in chemistry along the way. Using the concentrations he measured, Aston calculated the total amounts of more than 100 elements on Earth from hydrogen to bismuth, and noticed that their abundances followed the gradually decreasing trend shown in Figure 2.

In general, there appeared to be more light elements like oxygen and silicon and fewer heavy ones like lead. But neon and the other noble gases including helium, argon, krypton, and xenon did not fall along the curve. Aston had no way of knowing how much neon there was supposed to be on our planet, but he noticed that this gas and others like it were present in much lower concentrations than would be predicted by the trend.

At the time, Aston could not explain this observation, but he speculated that perhaps “in the hurly-burly of” the early solar system, “the Earth’s share of inert gases [had] been lost to the sun” (Aston, 1924b). This hypothesis would require additional measurements from outside of Earth’s atmosphere to test.

Measuring the composition of the universe

Aston’s work raised the question of whether the entire universe lacked neon, or just Earth. To answer it, scientists began looking at other cosmic objects like stars and planets. But how? The vast distances of space made it impossible to collect samples directly. Instead, scientists had to find other ways to assess the composition of distant bodies from afar.

At the same time that Aston was working at Cambridge, physicist Cecilia Payne was working on her PhD at Harvard University, studying the nature of light given off by stars. She knew that gases like neon emit light when they are “excited” by an electric current, or for example, by the energy of a burning star. During excitation, that energy allows electrons around a neon atom’s nucleus to temporarily jump to higher energy orbitals (see our module Atomic Theory II: Ions, Isotopes, and Electron Shells). However, they can’t stay there long, and soon fall back into their normal positions, discharging that excitation energy in the process. This energy is often discharged in the form of a photon, or a packet of light, whose wavelength depends on the amount of energy discharged. For neon, excitation causes the gas to emit a wavelength of light that falls in the red portion of the visible spectrum, which accounts for the familiar crimson glow of neon signs.

Because of differences in the amount of energy needed to excite elements into higher orbitals, different elements absorb and emit light at specific frequencies along the electromagnetic spectrum when excited. Sodium emits yellow light, while hydrogen has emission bands in the orange, blue, and purple regions of the visible spectrum. Using this knowledge, scientists can remotely identify elements in other bodies in the universe based on their unique emission spectrum. Before Cecilia Payne published her research results, scientists studying the emission spectrum of the sun thought that it was primarily composed of iron, which has strong emission lines, shown in Figure 3.

Payne, however, was the first to recognize that the brightness of the emissions given off by elements depends not only on their concentration, but also on how excited the atoms are. That means that the brightest colors given off by stars may not represent the most abundant element in them, but the element that is most excited and able to emit more photons. By calculating the theoretical excitation states of different elements and then correcting the emission spectra of stars using this information, Payne showed that the primary component of the sun and other stars was hydrogen, not iron, as previously thought (Payne, 1925). The abundances she determined are shown in Figure 4.

Payne’s new interpretations of emission lines allowed scientists to accurately convert the emission spectra of planetary atmospheres into estimates of the elements they contain. Using her discoveries, astronomers began to scan the universe, tabulating the composition of stars, nebulae, and supernovas. In 1933, Payne and her colleagues succeeded in identifying the emission spectrum of neon in celestial sources (Boyce et al., 1933). They demonstrated that neon and the other noble gases are much more common in stars and nebulae. Throughout the universe, the abundances of most elements – including neon and the other noble gases – fall neatly along the smooth curve of elements predicted by Aston’s measurements (Figure 5). Something must have happened to Earth to account for its lack of neon, but what?

Earth's primordial atmosphere

Aston suspected that the early solar system was an important time for Earth’s atmosphere, and indeed it was. In the “hurly-burly” chaos following the formation of the solar system 4.6 billion years ago, our corner of the universe would have looked strikingly different than it does today. Instead of eight large planets and a sun separated by virtually empty space, a jumble of small rocky masses floated inside a haze of dust and gas. Together, this matter formed a rotating disc, or nebula, around the young sun. Inside the nebula, rocky masses collided with one another, forming increasingly larger masses known as planetesimals that finally grew into the planets we know today. These massive objects attracted gases from the solar nebula through their gravitational pull. During this time, all of the planets except perhaps tiny Mercury would have captured their own cloud of gases, resulting in the formation of what scientists call “primordial” atmospheres.

If all the planets in the solar system retained these primordial atmospheres drawn directly from the solar nebula, the modern atmospheres of all the planets should have similar compositions to one another and to the sun. In fact, Jupiter’s atmosphere does have a similar composition to the sun, but it is very different from Earth’s atmosphere and those of the other rocky planets – Mars, Venus, and Mercury (see Figure 6 for a comparison of atmospheric compositions). This fact was well known by the 1940s, when University of Chicago physicist Harrison Brown became interested in the question of planetary atmospheres. However, at that time, no one knew just why the atmospheres on Jupiter and Earth looked so different.

Like Aston, Brown focused his attention on the noble gas neon, which was more abundant in Jupiter’s atmosphere than Earth’s. To understand just how depleted Earth’s atmosphere was in neon, Brown calculated the ratio of neon to silicon, the second most abundant element in rocks that form Earth’s crust (see our module The Silicate Minerals). In this way, he normalized the amount of neon on Earth to the mass of our planet so that the sheer size of different planets and stars did not skew his results.

Brown then compared the abundance of noble gases in the universe to the abundance on Earth, a number he called the fractionation factor (shown in Figure 7). By doing this, Brown demonstrated that more neon had been lost from Earth than the other noble gases: argon, krypton, and xenon. In fact, he discovered that neon is a billion times rarer on Earth than on Jupiter, while xenon is only a million times rarer on Earth.

Brown considered these results – that neon is extremely rare in Earth’s atmosphere, and that it is rarer than heavy noble gases like argon and xenon – and came up with a potential explanation. He proposed that Earth may have had a primordial atmosphere with a composition rich in noble gases similar to other planets, but that the planet lost this atmosphere extremely early in its history through gravitational escape and collisions with other planetesimals (Brown, 1949). This would explain why Earth has lost more neon than argon (it is lighter and could escape more easily), and why Earth has lost more neon than Jupiter (Earth is smaller than Jupiter, therefore the force of gravity on Earth isn’t as great).

Developing the modern atmosphere

Brown’s work helped explain where the noble gases on Earth came from and why they are so rare. But, as is often the case in science, his answers only led to more questions. For example, in his 1949 paper, he made an important observation:

It would appear that during the process of earth formation, the mechanism was such as to prohibit the retention of an appreciable fraction of any substance that existed at that time primarily in the gaseous state.

In other words, Brown thought that if neon and argon were lost from the primordial atmosphere, all gases present at that time should have been lost as well. If the gases that make up the modern atmosphere (like nitrogen, carbon dioxide, and oxygen) had existed in the early atmosphere, they surely would have escaped along with neon. He then proposed an answer this conundrum: that they must have been added to the atmosphere later as Earth developed its modern (or secondary) atmosphere:

It appears that the earth’s atmosphere is almost entirely of secondary origin, and it was formed as the result of chemical processes that took place subsequent to the formation of the planet.

What are these chemical processes suggested by Brown that can add gases to a planet’s atmosphere, particularly Earth?

Scientists have since identified several mechanisms – both chemical and physical – that can add new gases to a planet’s atmosphere. Some atmospheric gases were probably brought to Earth from space by meteorites and comets. These extra-terrestrial objects have been shown to contain water, carbon dioxide, and other gases like methane and ammonia. Since many of them collided with the Earth over its long history, releasing their contents on impact, they certainly contributed to the buildup of the secondary atmosphere.

Another source of gases is volcanic activity, which alters the atmosphere by continuously bringing gases up from deep within Earth and releasing them through volcanic eruptions (Figure 8). Studies of modern volcanoes and their gas emissions reveal that eruptions release carbon dioxide, sulfur dioxide, water vapor, carbon monoxide, and ammonia. These studies show that volcanoes are important contributors to the atmosphere.

Because of these processes, Earth’s secondary atmosphere probably started out high in water vapor, carbon dioxide, and methane. But Earth is an active planet, and geologic processes continued to alter the composition of the atmosphere over time. For example, some of the gases produced by volcanoes get recycled back into the mantle during the process of subduction (see our module The Rock Cycle).

The formation of oceans on the early Earth was the most important event in the evolution of the secondary atmosphere. As the planet gradually cooled from its early molten state and liquid water began to accumulate, seawater absorbed much of the carbon dioxide produced by volcanoes. A large fraction of this carbon then became locked up for hundreds of millions of years in carbonate rocks, effectively removing it from the atmosphere. On planets without oceans, like Venus and Mars, carbon dioxide is still the main component of the atmosphere, but because of the oceans on Earth, it is just a small fraction here.

All of this happened relatively quickly, geologically speaking; within the first billion years of Earth history, the planet lost its primordial atmosphere and accumulated another. Over the next billion years, the composition of that atmosphere evolved. Eventually, nitrogen gas (N₂) came to be the dominant gas in our atmosphere, where it now accounts for 80% of the air on Earth. It accumulated in the atmosphere because it does not easily form minerals or react with other chemicals. But one gas was still notably absent: oxygen. To learn where this important gas came from, read our module The History of Earth’s Atmosphere II: The Rise of Atmospheric Oxygen.

Earth’s atmosphere in context

Our understanding of how planetary atmospheres evolve has aided scientists now studying other planets. For instance, information gathered by the Curiosity rover suggests that Mars once had a much thicker atmosphere, but lost most of it nearly 4 billion years ago (Mahaffy et al., 2013). Scientists at NASA deduced this by studying the abundance and isotopes of argon in the Martian atmosphere, just as Aston and Brown studied neon on Earth.

Farther afield, astronomers have begun to investigate Titan, a moon orbiting Saturn that has a thick, nitrogen-rich atmosphere. Scientists think that it might have a similar composition to the secondary atmosphere Earth would have had before the evolution of life. This illustrates how studying planetary atmospheres continues to require analyzing our own planet as well as other bodies in the solar system, and how the knowledge we gain improves our understanding of both.