Chemistry on the early Earth

It is very likely that the atmosphere, surface and oceans of the early Earth were very different from their present condition. We have very little direct evidence, and this has been an area of considerable disagreement.

As the Earth was forming, there was still a lot of debris in the Solar System, so the Earth was subjected to a much greater rate of bombardment than it is today. As the conditions became less violent, the Earth cooled enough for water to form on the surface. The continents were beginning to form. There was considerable volcanic activity, which introduced large quantities of hydrogen sulfide (H2S) into the atmosphere.

There was no free oxygen, O2. The oxygen in today's atmosphere is the result of plant life. (Remember, there is a symbiotic relationship between animal and plant life. Plants consume CO2, which is given off by animals, and through photosynthesis convert sunlight into energy, and give off O2 as a by-product. The O2 is then used by the animals.) The absence of free oxygen also meant that there could be free iron (Fe) on the surface. Today any free Fe would react with O to make iron oxide (rust).

For a long time it was thought that the atmosphere contained a lot of hydrogen in various forms, especially molecular hydrogen (H2), methane (CH4), ammonia (NH3) and water vapor (H2O). More recently, it has been suggested that there was more carbon monoxide (CO) and carbon dioxide (CO2) than methane, and more nitrogen (N2) than ammonia. These more recent ideas also suggest that there was not very much molecular hydrogen. There is some disagreement over how much phosphorous was available. We will see below that P is an important component in some biotic molecules.

There is also some disagreement over what the temperature was on the early Earth. Estimates range from freezing to boiling. The Sun gave off 25% less energy 4 Gyr ago than it does now. However, there was no ozone layer to absorb solar ultraviolet, so a larger fraction would have penetrated to the surface.

There may have been frequent lightning in the early Earth's atmosphere. The effects of such lightning were simulated in a laboratory at the University of Chicago, in the early 1950s, by a graduate student, Stanley L. Miller, under the supervision of his advisor Harold Urey. (Urey had won the 1934 Nobel Prize in Chemistry for his discovery of deuterium.) The Miller-Urey experiments helped chemists understand how the first prebi-otic molecules may have formed in the Earth's atmosphere. To simulate the effects of lightning, they carried out their experiment in a sealed glass tube with electrodes and produced repeated electrical discharges.

Miller and Urey started with a mixture of methane, ammonia and hydrogen. The effects of evaporation and condensation of the early oceans were simulated by recycling water through the system. They ran the experiment for a few days at a time and then analyzed what had been produced. After some runs they found simple organic molecules important as building blocks of life, including amino acids. At the time they did these experiments, it was thought that the early atmosphere contained large amounts of methane, ammonia and hydrogen. As we said above, there is now some disagreement over how abundant those molecules actually were. Alternative means have been investigated in which the simple amino acids formed in various clay deposits.

More recently, it has been suggested that molecules like those that came out of the Miller-Urey experiments could have been formed in the interstellar medium as part of the molecular cloud from which the Sun (and Solar System) formed. As we saw in Chapter 14, a rich collection of interstellar molecules has been found, including some simple prebiotic molecules. It had been thought that, even if these molecules were made in interstellar space, they would not survive the process of star formation, at least not in the inner Solar System. However, we know that some of this interstellar material has been preserved, as comets in the Oort cloud or the Kuiper belt. Some comets that passed close to the early Earth could have left some of this material behind, for it to sink into the atmosphere, and then eventually find its way to the surface.

There is some evidence that such materials might survive a meteoritic impact. Remember, meteors are the debris of comets, left in their orbits. The most famous example is the Murchison meteorite (mentioned in Chapter 26), which fell in 1969, in Murchison, Australia. It was found to contain a number of amino acids which are not likely to have been made on Earth.

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