What Energy Sources Would There be for Underground Life?

Thomas Gold, 1997

Microbial life could only flourish if there was a supply of the element carbon and a chemical energy source, a "food" for them. The heat that surrounds each microbe can supply no energy; energy can be derived only from the flow of heat from a hot body to a colder one, and the microbes in the rocks are far too small for any temperature differences across their bodies to arise. ("You can sit in a hot tub as much as you like, but you will still need to eat.")     Hydrocarbons are a chemical energy source, but only in the presence of oxygen, so that it becomes possible for the microbiology to mediate the energy-giving process of oxidizing them. On the surface of the Earth this is easy, the atmosphere provides virtually unlimited amounts of free oxygen. But where is the oxygen deep down in the pores of the rocks where we find oil?

The rocks contain oxygen in abundance, only most of it is bound too tightly, and it would take more energy to free this oxygen than could be obtained by the oxidation of the hydrocarbons. There are just a few commonly occurring substances in the rocks that have sufficiently loosely bound oxygen to allow the oxidation of hydrocarbons to be an energy source. Highly oxidized iron is one of them, sulfates (oxidized sulfur compounds) are another. Microorganisms can then feed on the combination of hydrocarbons with some oxygen they can take off these substances. One must then expect to see the accumulation at least of the solid end- products of some or all of these processes in hydrocarbon-rich areas. Search for Life on Other Planetary Bodies.

The search for sub-surface life on other planetary solid bodies such as the Moon, Mars, and many asteroids and satellites of the major planets, will now become a high priority item in planetary research. The surface conditions on the other solid planetary bodies are all quite different from those we have here, where the conditions are remarkably favorable for the development of surface life. But the sub-surface conditions will be similar to ours on most of these bodies, though depth dependence of pressure and temperature will be different. The possibility of developing life in them may then be not too different from the circumstances here. Hydrocarbons on them are known, and sub-surface liquid water can be expected on many of them. The rocks will contain some oxidized components that will serve as oxygen donors. The scene would be set for the existence of microbiology there. The recommendations I made specifically for Mars (in the paper mentioned above) included the search for evidence of microbial life in the carbonaceous Martian meteorites that had been found in Antarctica (a search that is still in progress now). For future interplanetary missions that could return a sample back to Earth, I thought that it would be best to go to locations where material is exposed now, that must once have been at some depth. The outstanding case is the floor of the deep "Vallis Marineris," where massive landslides have exposed material that must once have been at a depth well into the liquid water domain.

What are the Solid Products of this Microbial Activity?

The liquid or gaseous products will generally escape in short times and would not be maintained in a small meteorite on a long space flight. Where iron oxides served as the oxygen donors, the end product will be iron in a less oxidized state in which it is magnetic. Magnetite is the most common form. A further removal of oxygen, such as the step to metallic iron, requires more energy than is available in the reaction. Where sulfur oxides were the oxygen donors, one must expect to see just sulfur or unoxidized sulfur compounds such as hydrogen sulfide or metal sulfides. The product of the oxidation of the hydrocarbons will be carbon dioxide and water, and in many rocks this will react with oxides of calcium or magnesium to make solid carbonates. Those are the carbonate cements that fill up small pore spaces, and must have been transported by a liquid before precipitating.

From:  http://j_kidd.tripod.com/b/103.html