Earth's atmosphere is a complex self-regulating system that provides a protective envelope in which life thrives. Our atmosphere is not only intimately intertwined with life, it shares some attributes in common with the living, such as organization, homeostasis and evolution. It is difficult to imagine that any other planet, even of the estimated billions out there, comes even close to our own unique world. But that is the key: there are an estimated 10 billion planets that should exist within the habitable zone of their star, just within the Milky Way alone. When we think of those numbers it becomes hard to imagine that there is not any other world out there supporting life, and that there are perhaps many different kinds of worlds that support many different kinds of life.
Astrobiologists explore the theoretical possibilities of what alien life could look like. They use the immense variety of life that has evolved on Earth over the eons as a baseline comparison or a starting point to explore how different atmospheres, different gravities and different chemistry-based physiologies could, in theory, work. There is the possibility that life might indeed exist right now on another planet or moon right here within our own solar system. These are exciting questions and this is an exciting time to be curious about our atmosphere. As you will see in this article and the next two following articles, an understanding of our atmosphere, not just understanding it as it is now but how it evolved from a deadly soup of chemicals it once was billions of years ago, is key to exploring such possibilities. In this article we focus on how Earth's atmosphere took shape and evolved. As we do so, consider what kind of environments life requires and how life itself impacts those environments.
1: Where the Raw Material of the Atmosphere Came From
4.6 billion years ago, Earth started out as a ball of molten rock surrounded by a thin envelope of hydrogen, helium and a few other gases, much like all the other rocky inner planets did. This is an artist's rendering of how Earth and the other rocky planets looked as they were forming:
About half a billion years before that, a supernova exploded in the Milky Way, spewing heavy elements into a nearby cloud of hydrogen gas and interstellar dust. Under its own gravity, this mixture condensed in toward itself, growing hotter and hotter until at the center, material became so compressed and hot it ignited into a ball of ongoing nuclear fusion, and our Sun was born. The Sun's ignition was itself a gigantic explosion, blowing most of the dense cloud of dust around it, called the accretion disk, away, with lightest elements blowing furthest outward. By this time, clumps of this dust had already begun to clump together under the attractive force of gravity, a process called accretion.
Earth was simply the result of this accretion of the elements that were most abundant within its zone of the Sun's accretion disk (shown above). Inner planets formed mostly from heavier elements (metals) and more distant planets formed from lighter elements (ices and gases). This 48-minute NOVA video shows how Earth was made from this gas and dust:
2: Setting Up Conditions Favourable For Producing and Maintaining a Complex Atmosphere
Molten-ball Earth didn't simply settle down and cool to form our present-day atmosphere. A series of events had to take place to set the stage for its development. The most important event of all was the formation of an enveloping magnetic field. Without it, all of Earth's atmosphere would have been stripped away by the Sun's intense solar wind soon after it formed. Another important event was the formation of an atmosphere very different from what we have today. This first atmosphere was the cradle of all life on Earth, and yet it is not anything we would want to breathe - we would immediately be both poisoned and asphyxiated by it. Let's look at these events in more detail.
Earth didn't start out with a magnetic field. It was simply an unorganized amalgam of rocky fragments of rocky meteorites, metallic fragments of metallic meteorites and icy fragments of comets, all material from the accretion disk. To create a magnetic field, Earth required organization into an inner core of swirling molten conductive metal.
This sets up a dynamo, shown below, which creates a large magnetic field:
That kind of density (and composition) stratification took time. As Earth grow in mass, it eventually became large enough to create internal heat sufficient to melt entirely so heavy molten metals could gradually sink through lighter material into the center of the planet, a process called the iron catastrophe. Meanwhile, Earth's first atmosphere, technically a tenuous exosphere, the result of outgassing mostly of the two lightest elements, hydrogen and helium, from its molten rock, had no magnetosphere to protect it so it blew away with the intense solar wind estimated to be a hundred times greater then. Eventually, the magnetosphere, a large protective magnetic envelope that deflects incoming solar wind, developed and offered some protection:
In this diagram the solar wind flows from left to right.
Over the next 200 million years, Earth eventually cooled enough to form an inner molten metallic core and solid lighter rocky crust. This young Earth was still extremely hot for three reasons: It had a greater abundance of radioactive elements then, it was constantly bombarded by all the debris that littered the early solar system, and it experienced intense gravitational stresses from other planets and moons developing nearby jostling around each other in unstable orbits. Earth, surrounded by orbiting solar debris and a young moon (not shown) might looked something like this at that time:
By about 4 billion years ago, Earth had differentiated into a its present-day structure, with an iron-rich metallic core, a less dense magnesium-silicate mantle and a relatively thin light crust composed mostly of silicates (rocks). Lighter water was also present on Earth by this time and a new atmosphere was forming above that, shown here:
It was still not a peaceful time, however. Large meteorites continued to rain down and gigantic volcanoes, spewing toxic chemicals, littered the surface, while the young Sun blasted Earth with intense ultraviolet radiation. The surface of Earth may have looked something like this:
Credit: David A. Aguilar (CfA)
The Moon was much closer to Earth then, as shown in this image.
3: Earth's First Atmosphere Was Nothing Like Today's Atmosphere
Gravitational stresses led to extreme volcanism, and this is where Earth's first atmosphere came from. Contemporary volcanoes, however, do not expel the kinds of gases that most researchers believe must have been present in Earth's first atmosphere. Volcanoes today release lots of water vapour, carbon dioxide and sulphur dioxide but they do not release ammonia (CH3) or methane (CH4), two reduced gases, in any appreciable amount. As a result, modern volcanic gases cannot create a reducing atmosphere and a reducing atmosphere is essential for producing the most fundamental building material of life, organic compounds. An organic compound may be strictly defined as any molecule that contains carbon. In this article, we will define an organic compound much more narrowly - it must contain a carbon hydrate, a carbohydrate in other words, consisting of carbon, oxygen and hydrogen, examples of which are shown here:
(copyright chemistryland.com, an excellent primer on organic chemistry)
These compounds, for example carbohydrates, lipids, proteins and nucleic acids, are all molecules associated with life processes. In living organisms they are synthesized inside cells, which are reducing environments.
In a reducing atmosphere, there is no oxygen but plenty of hydrogen. Scientists have been challenged with trying to figure out what Earth's earliest volcanoes spewed out and they have attempted to recreate plausible early Earth conditions in which organic compounds could be produced. The Miller-Urey experiment, shown below, conducted in 1952, provided evidence that organic compounds could be synthesized from inorganic precursors under conditions that are thought to resemble Earth's primitive atmosphere.
(copyright: YassineMrabet (Wikipedia))
Recent refinements of this experiment have further confirmed these findings by showing that a large variety of different amino acids are formed in an environment rich in methane, water, ammonia, carbon monxide and hydrogen that is energized by an electric current (recreating abundant volcanic lightning).
To further confirm these findings, scientist Bruce Fegley recently turned to chondrites, primitive meteorites (shown below), for answers.
(copyright: H.Raab (User:Vesta)(Wikipedia)
Chondrites are stony chunks of rock that were never modified by melting or differentiation in any way since the solar system formed. They are accretions of solar dust that are composed of exactly the same material that comprised early Earth. All he needed to do was to heat them up and collect all the gases that are released, as minerals inside them react with each other and decompose. For example, when the mineral calcium carbonate is heated up it decomposes into carbon dioxide gas. When this chondrite-outgassing mixture is exposed to an electric current, recreating Earth's early lightning-rich atmosphere, a reducing mixture is created and simple organic compounds form. These findings imply that the composition of Earth's outgassing, through volcanoes, has changed significantly over time, and that these changes contributed to Earth's atmospheric evolution.
The hydrogen budget, the redox budget in other words, of Earth's early atmosphere is essentiall to understanding its early composition, and critical to figuring out how organic compounds, precursors to life, formed. A reducing environment is also critical for making amino acids, as shown here:
Organisms use amino acids to build proteins. There are many different kinds od amino acids, each of which contains an NH2 base group, a -COOH group and a hydrogen atom, all attached to a carbon atom. Ammonium ions are required to make amino acids and these can only exist in a reducing environment.
4: From Organic Molecules to Organized Structures
Nucleic acids also formed under these atmospheric conditions and they could catalyze the construction of the first proteins out of amino acids. Nucleic acids (RNA and DNA) are polymers of nucleotides. The basic structure of a nucleic acid is shown here:
Some researchers believe that RNA was the first nucleic acid to be formed. An example of pre-messenger RNA (a chain of nucleotides) is shown here:
Each nucleotide is composed of a nucleobase (shown as green) and a phosphate-sugar back backbone (blue).
RNA's first function may have been to act as an enzyme catalyzing the polymerization of amino acids into proteins. Later, it evolved the ability to store, transmit and duplicate genetic information. Ultraviolet light, abundant on early Earth, causes RNA to polymerize, while it breaks down other organic chemicals that could potentially break down RNA. The first simple RNA-mediated protein chains could be considered primitive life forms in the sense that different forms could compete with each other, with the ones that can most efficiently catalyze their own replication having a selective advantage over the others. Some viruses still use RNA as their genetic material. There is some question as to whether viruses are a life form or not, and the same criteria could be applied to the ancient RNA polymers. Neither have a cell structure. Nor do they have their own metabolism.
The experiments of Sidney Fox have shown that organic compounds can spontaneously aggregate together and surround themselves with a membrane-like structure under conditions similar to those on early Earth. Slightly more complex structures are called protobionts. They exhibit some properties associated with life such as simple reproduction, metabolism and excitability as well as the maintenance of an internal environment. Nanobacteria could be examples of protobionts.
The most primitive non-disputed living organism is the prokaryote; its body plan is shown here:
Most researchers believe that more complex DNA evolved from RNA within some kind of protected environment, as DNA is sensitive to damage by UV radiation. Prokaryotes lack a cell nucleus or any organelle. They are simple sacks that reproduce using a free-floating DNA-protein complex and carry out metabolism across their membrane. Many can form aggregate communities and it is through this kind of social aggregation that more complex eukaryotic organisms may have evolved. Single cell eukaryotes evolved into the complex multicellular life that exists (along with the simpler forms mentioned) on Earth today.
5: What Drove the First Life Into Being? - A Perplexing Entropy Question
That organic compounds can form into more complex molecules and arrangements is all but verified by the experiments mentioned above and others. The question of why they do so is left for us to explore.
All physical processes are driven by entropy. Reactions always favour the movement toward a lower energy state. According to the second law of thermodynamics, the entropy of a closed system therefore tends to increase.
Let's first explore what a chemical reaction is, so we can probe the thermodynamics of life. Any chemical reaction can be spontaneous, requiring no input of energy, or non-spontaneous, requiring energy input such as electricity, light or heat. A chemical reaction may require a catalyst, such as an enzyme, in order to increase the reaction rate, and this is the case for almost all biochemical reactions that make up the metabolic pathways of living organisms. Most chemical reactions are reversible, with each direction competing with the other and differing in reaction rate. The direction of a reaction depends on many factors, external conditions such as heat or the concentration of reactants available, for example. Most reactions eventually establish a point of equilibrium at which reactions in each direction balance out. At this point the Gibbs free energy of the system is zero. Gibbs free energy is a thermodynamic term, which means the ability to do non-mechanical work. In a closed system, a reaction tends toward a lowest possible energy state, in other words. A reaction will spontaneously proceed if it is exergonic, that is, it releases energy. A reaction can also consume energy, and in doing so it decreases the entropy of the system. In the case of many of the synthesis reactions described here, electrical energy is converted into chemical bond energy. In doing so, the entropy of the product, for example an amino acid, is lower than that of the total entropy of the reactants, ammonia and acetic acid for example. A more ordered arrangement is created. If we extrapolate to a living organism, its highly ordered arrangement of molecules is maintained through the processes of metabolism and all those reactions involved require an input of energy (from the Sun or food for example). When an organism dies, metabolism ceases and the molecules of its body return to a disorganized state through decomposition. According to the rules of thermodynamics, the entropy of the remains increases until a new overall (lowest energy) equilibrium point is reached.
This is how the thermodynamics of living organisms works. It describes the behavior of the various components of that system very well. Yet you may still question the thermodynamics of life from a larger holistic perspective. Its an excellent question, and when we try to describe the intricate relationship between Earth's atmosphere and life on Earth, we may very well begin to wonder if there must be some divine input involved. Can the intricate relationship between Earth and life by explained by science? The answer to this question will necessarily impact how you ask questions about life in the universe in general, and about the universe itself. As a scientific explorer, I recommend that as you grapple with these questions, keep an open mind and use your developing skills of deduction, reasoning and researching to guide you. We have explored how the chemistry of synthesis obeys the second law of thermodynamics. Yet it may still seem that the general self-organizing nature of life breaks this fundamental law. This organizing nature is called emergent behaviour.
You may recall from my article on atmospheric structure that small pockets of air of different densities can and do organize into more complex systems such as thunderstorms. This is called an emergent property of a complex system. The formation of our Sun and planets from a giant cloud of dust is another example. We can even say that the entire universe is increasing in order and decreasing in entropy, at least from our viewpoint. And viewpoint is the key to understanding the entropy of systems. If we consider the self-organization of life to be a more open system to include energy inputs, we can see that electrical energy from lightning as well as solar energy were expended in order to increase the complexity of organic molecules into systems capable of interactions with each other and of self-replication. Once this occurred, the process of natural selection can be used to explain how further complexity evolved. But let's step back for a moment - It is precisely the point where a system of biochemical reactions acquires the capacity to interact with other systems, and to replicate, to be alive in other words, that many people continue to wonder about. What force could drive this acquirement?
The energy of the Sun is behind the development of complex weather systems. Gravitational energy is what ultimately organizes dust into stars and planets. And perhaps it is possible to say that the universe continues to draw on the energy of the Big Bang as its overall complexity continues to increase. There is a great deal of discussion going on among experts and laypeople alike about how life may have developed as an emergent property of matter and the question is not only ultimately still open, but it invites philosophical and religious debate. As scientists continue to refine experiments that attempt to recreate the self-organization of non-living organic aggregates into primitive but living cells (and continue to refine how life is defined), some day perhaps we may be satisfied that all the processes leading up to the living Earth as it is today are driven entirely by physical forces in the universe. However, we may be left with yet another unsettling question: How did a universe with an inherent capacity to bring forth life come to be? With that, I will leave the topic of how life formed, having barely scratched the surface of a deeply fascinating puzzle.
Protobionts, viruses and viroids (much simpler than even viruses and perhaps one of Earth's best examples of biological organization at the knife edge of life) give us possible clues as intermediate steps toward increasing organization, and where non-living becomes living. Eventually, very simple life forms called prokaryotes evolved. The oldest fossilized example dates back 3.5 billion years, just 1 billion years after Earth itself formed. It is a fossil of cyanobacteria inside ancient rocks in Western Australia:
(Image from University of Berkeley website)
This is an Australian fossil of filamentous cyanobacteria that is about 850 million years old. These simple organisms hold one of the most important keys to understanding how Earth's atmosphere evolved, as we will discuss next.
Molecular biomarkers in rocks (oxygen bound up in limestone, iron and other minerals) about 2.4 billion years old indicate photosynthesis by this time was well spread.
6: Photosynthesis Makes Earth's Atmosphere Unique
The evolution of photosynthesis is what really changed Earth's atmosphere into what we have today. Early Earth was colonized by many different kinds of prokaryotes, a group which is divided into two domains - bacteria and archaea:
(copyright: Bacterial/Prokaryotic Phylogeny Webpage (Wikipedia)
Prokaryotes have been found in every kind of habitat on Earth. One kind of prokaryote, cyanobacteria, while very tiny as individual organisms, multiplied into great numbers and changed the entire atmosphere of Earth. These organisms evolved from the simple prokaryote bag-of-chemicals plan into a more elaborate system of enclosed membranes that can carry out photosynthesis. They are tiny factories that use the Sun's energy to manufacture carbohydrates. Neither chloroplasts nor chlorophyll had yet evolved. Cyanobacteria then and today do not have any distinct organelles like the more advanced eukaryotes do. These organisms instead used a bluish pigment called phycocyanin to capture sunlight energy. In doing so, they sequestered carbon dioxide into carboydrates and released oxygen gas as a byproduct. As oxygen built up, the atmosphere changed from a reducing one to an oxidizing one. All the surface minerals on Earth were eventually oxidized, saturated with oxygen in other words, changing Earth's geology and resulting in thousands of new minerals. Once minerals were saturated, oxygen gas began to accumulate in the atmosphere. When the atmosphere was reducing one, iron and other metal ions would have been dissolved in seawater. When oxygen became abundant, it oxidized these ions, resulting in insoluble iron oxide compounds, which fell out of solution as sediments. Most of the iron mined today for example, comes from these ancient sea bottom sediments. These deposits are generally no more than 2 billion years old, indicating that it took a long time for the atmosphere to become oxidized, about 1.5 billion years from the first fossil evidence of cyanobacteria.
Cyanobacteria released oxygen as a poisonous waste gas. These bacteria did not poison themselves because they evolved protective enzymes that could eliminate the DNA-damaging hydroxyl radical that forms during the production of oxygen. Anaerobic bacteria, bacteria that require an oxygen-free environment, had also evolved by this time and had colonized much of Earth. When free oxygen began to accumulate in the atmosphere, most (but not all - these organisms were very successful in finding various new niche environments including our own bodies) of these organisms died, precipitating Earth's first major extinction event.
As oxygen gas accumulated, Earth's atmosphere underwent a global redox reaction. Meanwhile, highly resourceful cyanobacteria continued to etch out new ecological niches. They now inhabit almost every habitat on Earth, even existing as endosymbionts inside lichens, plants, protists and sponges, providing energy for the host organism. Cyanobacteria contributed approximately 10% of today's oxygen level during the Precambrian period, and it probably fluctuated wildly. Today they continue to contribute significantly to the atmospheric oxygen pool. These very simple organisms can live as single cells and as colonies of cells, which can form filaments, sheets and hollow balls. They are important primary producers in ocean food webs. Some filamentous colonies can even differentiate into several different cell types, each one adapted to a different living environment. Normally photosynthetic, these cells can differentiate into tough spore-like cells and it is these cells that can fix nitrogen gas into ammonia, nitrates and nitrites, as well as survive long harsh periods such as glaciation events.
Nitrogen fixation paved the way for the next explosion of life on Earth - plants. Plants improved upon the simpler kind of photosynthesis cyanobacteria use by evolving chloroplasts, highly efficient solar energy capturing systems. In fact, there is evidence that chloroplasts evolved from an ancient endosymbiotic relationship with cyanobacteria. Plants significantly increased the level of oxygen in the atmosphere as they evolved and colonized the planet.
Atmospheric oxygen oxidized methane (a strong greenhouse gas) into carbon dioxide (a weaker greenhouse gas), triggering the Huronian glaciation event beginning about 2.5 billion years ago, the first and most extreme of a series of global glaciation events, this one lasting up to 400 million years. By this time, a protective ozone layer was forming as oxygen was broken down high up in the atmosphere. Volcanic activity continued to pump out various greenhouse gases, ultimately rewarming the surface and bringing an end to the glaciation. Glaciation cycles continued, but life survived the extreme conditions and eventually some aquatic organisms grew complex enough and, thanks to the ozone layer protecting them from deadly UV radiation, were to colonize land. An explosion of plant life caused oxygen's atmospheric level to spike around 550 million years ago at about 35% (today oxygen makes up about 21% of our atmosphere). These high oxygen levels may have contributed to an explosion of new organisms called the Cambrian explosion as well the massive sizes of amphibians, dinosaurs and the first insects to follow.
7: Earth's Atmosphere - Unique AND Ordinary?
What is perhaps most intriguing about the history of Earth's atmosphere is that there is nothing very unusual about the physical processes that provided the material and environment for life to organize and form. Yet how life began to create its own environment does seem extraordinary. This leads us to a pressing question: If life started here on Earth, why didn't it start on other planets in our solar system? They all had the same raw materials (at least Venus and Mars formed at similar distances from the Sun did they not?) And further still, shouldn't other protostars have formed of similar ancient star debris and shouldn't other planets just like Earth exist out there? These questions will be explored in upcoming articles.
8: Where Did All The Nitrogen Come From?
Earth's atmosphere contains more nitrogen than any other gas. Where did it come from? Surprisingly for a gas that makes up almost 80% of Earth's atmosphere, no one is entirely sure, but some researchers believe it may have been formed through transmutation of atmospheric carbon and oxygen between about 3.8 and 2.5 billion years ago, caused mostly by neutrino bombardment from the young Sun and from violent volcanic activity. Others compare nitrogen to oxygen and conclude that atmospheric nitrogen is so abundant simply because it is not easily incorporated into rocks and it is very stable, so it has accumulated gradually over the eons.
If ammonia was significantly present in Earth's early atmosphere, nitrogen could have come from its decomposition in the presence of UV radiation (which would have bombarded the young Earth). When life evolved on Earth, nitrifying bacteria could act on ammonia to produce nitrites for plant growth. Other denitrifying bacteria could add nitrogen gas to the atmosphere. Nitrogen is an almost entirely inert gas so it is generally not taken back out of the atmosphere by binding with other elements. This means that even small contributions if continuous, could add up to all the nitrogen in the atmosphere today. Because nitrogen is inert, it is difficult to estimate when it began to make a significant contribution to the atmosphere, as there are few if any chemical markers in ancient rocks, for example, to use. Other bodies in our solar system, particularly Titan, have significant nitrogen atmospheres but as we will see in a future article, the mechanisms responsible are likely far different.
When we consider how Earth's atmosphere evolved, the co-evolution of life must be taken into account in order to explain many of the complex changes that have occurred. A constantly evolving complex interrelationship between life and the atmosphere exists in which one both depends on, and alters, the other. Although this article focuses on the atmosphere, I hope I have provided a starting point from which to explore the mystery of life in general as well.
We will continue to build on our questions about life by comparatively focusing on the atmospheres of Venus, Mars and a very interesting moon, Titan, with the goal of further deepening our appreciation for the unique atmosphere of Earth, next.