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Facts of the Matter Richard Brill |
O, what a
versatile element
To stop the air would hurt them."
"Pericles" Act 1, Scene 1
It occurs in the atmosphere in the form of a molecule of two chemically bonded oxygen atoms and also occurs as ozone, an unstable and highly reactive form that consists of a three-atom molecule.
Oxygen comprises only about 20 percent of Earth's atmosphere, but it is just the right amount. A few percentage points less and we would suffocate. A little more and even small fires would burn with abandon, and many substances would spontaneously burst into flame.
Oxygen is present in Earth's atmosphere because of plants. In the beginning, Earth's atmosphere was very different from today, and unsuitable for today's advanced life forms. As early as 3.5 billion years ago, cyanobacteria began the process of photosynthesis.
The oxygen concentration gradually increased until it reached a critical point where there was enough to react with iron-bearing minerals to create the earliest known red beds, layers of iron oxide-rich rock that were deposited as sediment around 2 billion years ago.
Eventually an equilibrium was reached in the atmosphere between oxygen input from photosynthesis and removal by respiration, which produces carbon dioxide.
The equilibrium between oxygen and carbon dioxide does not mean that there are equal amounts in the atmosphere. In fact, carbon dioxide is only a tiny fraction of 1 percent of all of the mixture of atmospheric gases.
Throughout geologic time, the rock record shows periods when the equilibrium between the two gases shifted one way or the other.
There are periods in the history of Earth where predominantly red sediments formed in an oxygen-rich environment. At other times, sediments were gray, indicating an oxygen-poor environment. The cause of these fluctuations is subject to debate, but most are related to mass extinctions, events that wiped out a large percentage of Earth's plant and animal species.
There have been at least 10 mass extinctions throughout geologic time, including the one 65 million years ago that marked the demise of dinosaurs. Because the fossils in the rocks change so abruptly across layers, mass extinctions often mark the boundaries between major geologic periods.
Mass extinctions are thought to be due to drastic environmental changes following impacts from meteors or a comet, or large outpourings of lava.
Oxygen's role is not limited to the air that we breathe. It plays a crucial role in just about all of Earth's materials, chemically combined with other elements.
One-third of all the atoms in water are oxygen atoms, but by weight, water is nearly 90 percent oxygen. A water molecule would not look much different from an oxygen atom, other than being slightly distorted in shape.
Oxygen is the most abundant element in Earth's crust; more than 70 percent of all atoms in Earth's crust are oxygen atoms. By weight, oxygen is more than half, and by volume it is more than 90 percent.
The oxygen atom is relatively large, and so rocks are mostly made of oxygen atoms with the small spaces between them filled with other atoms. It is the size of those atoms and the strength of their bond with oxygen that gives each mineral in the rocks its unique characteristics.
The list of common minerals that contain oxygen is a long one and includes quartz, calcite and apatite, from which glass, chalk and coral, and teeth, respectively, are made.
Oxygen is a highly reactive substance chemically and is quite corrosive. It would be poisonous to today's aerobic life forms were it not for enzymes that mediate the rate at which organic compounds are oxidized. Today, most organisms use oxidation as a source of energy through respiration.
Oxygen in the atmosphere plays a different but equally important biological role in the form of its alter ego, ozone.
The intense ultraviolet (UV) light from the sun is destructive to many molecules since the energy of UV photons is in the same range as the energy of organic chemical bonds. In the seas of early Earth, organisms were protected from UV rays by water, which is very nearly opaque to UV, but could not survive direct sunlight.
Sometime around 600 million years ago, the oxygen content of the atmosphere reached a level about 10 percent of today's level, which launched a revolution in evolution that would allow life to exist and eventually flourish on land.
That revolution was made possible by the production of ozone that could block the sun's deadly UV.
Fortunately for us, the chemical bond that holds the two atoms of oxygen together in gaseous oxygen is also vulnerable to the destructive power of UV radiation.
Some oxygen molecules are broken apart by the UV photons into isolated oxygen atoms. Some of these lone atoms combine with diatomic oxygen to form ozone.
The process also works in reverse. UV can disassemble ozone molecules, ejecting a single oxygen atom that can combine with another oxygen atom to form another molecule of ozone, or with another single oxygen atom to form a molecule of oxygen gas.
Ozone and molecular oxygen amounts eventually reach equilibrium because a higher concentration of ozone increases the rate that it is decomposed.
Rogue chemicals, such as fluorocarbons, can affect the equilibrium, increasing the rate of ozone decomposition and altering the equilibrium between oxygen and ozone.
Ironically, ozone is a hazardous substance because of its extreme oxidizing potential.
In the lower atmosphere, nitrogen oxides emitted from petroleum-fueled engines cause ozone to form in the presence of sunlight, causing photochemical smog.
Earth is likely not the only planet with large amounts of oxygen, but it is the only one we know of with oxygen in the atmosphere. It not only makes life possible for land-dwelling plants and animals, but in the atmosphere it is a signature of the presence of life, one that we are actively searching for on extraterrestrial worlds.