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In the beginning there was no oxygen. Four billions years ago, the air probably contained about one part in a million of oxygen. Today, the atmosphere is just less than 21 percent oxygen, or 208500 parts per million. However this change might have come about, it is pollution without parallel in the history of life on Earth. We do not think of it as pollution, because for us, oxygen is necessary and life-giving. The above lines are taken from a wonderful book Oxygen, The Molecule that made the World by Nick Lane (Oxford University Press, 2002), a popular scientific saga of life and death on Earth. You may find some parts of the book speculative, but it is popular science writing at its very best, enjoyable yet rigorous. Read more below. |
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The original source of oxygen in the atmosphere was not biological photosynthesis, however, but a chemical equivalent. Few processes show more vividly the importance of the rate of reaction, and the difference that life can make. Solar energy, especially the ultraviolet rays, can split water to form hydrogen and oxygen without the aid of a biological catalyst. Hydrogen gas is light enough to escape the Earth’s gravity. Oxygen, a much heavier gas, is retained in the atmosphere by gravity. On the early Earth, most of the oxygen formed in this way reacted with iron in the rocks and oceans, locking it permanently into crust. The net result was that water was lost, because after it had been split, the hydrogen seeped into space and the oxygen was consumed by the crust instead of accumulating in the air. Over billions of years, the loss of water through the effect of ultra- violet radiation is thought to have cost Mars and Venus their oceans. Today, both are dry and sterile, their crust oxidized and their atmospheres filled with carbon dioxide. Both planets oxidized slowly, and never accumulated more than a trace of free oxygen in their atmospheres. Why did this happen on Mars and Venus, but not on Earth? The critical difference may have been the rate of oxygen formation. If oxygen is formed slowly, no faster than the rate at which new rocks, minerals and gases are exposed by weathering and volcanic activity, then all this oxygen will be consumed by the crust instead of accumulating in the air. The crust will slowly oxidize, but oxygen will never accumulate in the air. Only if oxygen is generated faster than the rate at which new rocks and minerals are exposed can it begin to accumulate in the air. Life itself saved the Earth from the sterile fate of Mars and Venus. The injection of oxygen from photosynthesis overwhelmed the avail- able exposed reactants in the Earth’s crust and oceans, allowing free oxygen to accumulate in the atmosphere. Once present, free oxygen stops the loss of water. The reason is that it reacts with most of the hydrogen split from water to regenerate water, so preserving the the oceans on Earth. James Lovelock, father of the Gaia hypothesis and a rare scientific mind, estimates that today, with oxygen in the air, the rate of hydrogen loss to space is about 300 thousand tons per year. This equates to an annual loss of nearly 3 million tons of water. Although this may sound alarming, Lovelock calculates that at this rate it would take 4.5 billion years to lose just 1 percent of the Earth’s oceans. We can thank photosynthesis for |
this protection. If ever life existed on Mars or Venus, we can be sure that it never learnt the trick of photosynthesis. In a very real sense, our existence today is attributable to the early invention of photosynthesis on Earth, and the rapid injection of oxygen into the atmosphere through the action of a biological catalyst. |
See also a couple of very instructive figures from the Oxygen by Nick Lane.
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Krešimir J. Adamić
