A Word About: Methane, Marsh Gas and Miller's Experiment

A Word About

Methane, Marsh Gas and Miller’s Experiment

Methane is commonly found in nature wherever bacteria decompose organic matter in the absence of oxygen, as in marsh, swamps, or the muddy sediment of lakes – hence its common name, marsh gas. Is China, methane has been collected from the mud at the bottom of swamps for use in domestic cooking and lighting. Methane is similarly formed from bacteria in the digestive tracts of certain ruminant animals, such as cows.

The scale of methane productions by bacteria is considerable. The earth’s atmosphere contains an average of 1 part per million of methane. Because our planet is small and because methane is light compared to most other air constituents (O2, N2), one would expect most of the methane to escape from our atmosphere, and it has been calculated that the equilibrium concentration should be vary much less than is observed. The reason for the relatively high observed concentration is that, at the same time that methane escapes from the atmosphere, it is constantly being produced by bacterial decay of plant matter.

In cities, the amount of methane in the atmosphere reaches much higher levels, up to several parts per million. The peak concentrations come in the early morning and late afternoon, a direct correlation with the peaks of automobile traffic. Fortunately, methane, which constitutes about 50% of urban atmospheric hydrocarbon pollutants, seems to have no direct harmful effect on human health.

Methane can accumulate in coal mines, where it is a hazard. Mixed with 5 to 14% of air, methane is explosive. Also, miners can be asphyxiated by it (due to lack of sufficient oxygen). Dangerous concentrations of methane are readily detected by a variety of safety devices, including canaries, which succumb to lower concentrations of methane than are harmful to humans and alert miners to the presence of a hazard.

Methane was probably one of the main components of the earth’s atmosphere in its early years. Also, hydrogen is the most common element in the solar system (it constitutes about 87% of the sun’s mass). It is therefore reasonable that, when the planets were formed, other elements should have been present in reduced (rather than oxidized) forms: carbon as methane, nitrogen as ammonia, and oxygen as water. Indeed, some of the larger planets (such as Saturn and Jupiter), which have very strong gravitational fields and low surface temperatures that help retain light molecules, still have atmospheres that are rich in methane and ammonia.

A now-famous experiment carried out in 1955 by Stanle L Miller (working in the laboratory of H. F. Urey at Colombia University) supports the idea that life could have arisen in a reducing atmosphere. Miller found that when mixture of methane, ammonia, water, and hydrogen were subjected to electric discharges to simulate lightning, some organic compounds were formed (amino acids, for example) that are important to biology and necessary for life. Similar results have since been obtained using heat or ultraviolet light in place of electric discharges (it seems likely that the earth’s early atmosphere was subjected to much more ultraviolet light than it is now). When oxygen was added to there simulated primeval atmosphere, no amino acids were produced-strong evidence that the earth’s original atmosphere did not contain free oxygen. Miller’s experiment provided the model for much work in the branch of science now called chemical evolution, the study of chemical events that may have taken place on earth or elsewhere in the universe leading up to the appearance of the first living cell.

During the years since Miller's experiment, ideas about the chemistry of life’s origin have become more precise and sophisticated as a consequence of much experimentation and of exploration in outer space. For example, it seems likely that the main carbon source in the earth’s early atmosphere was carbon dioxide, not methane as assumed by Miller, and that hydrogen was present mainly as water rather than as hydrogen gas. (A fascinating and lucid account of our current know;adge is contained in “The Chemistry of Life’s Origin,” an article by James P. Ferris in Chemical and Engineering News, August 27, 1984, p. 22).