Methanogenesis, Mesospheric Clouds, And Global Habitability
Research Staff: Rudolf F. Pueschel
Hyperthermophilic methanogens can exist in a deep hot biosphere up to 110°C, or 6 km deep. Thus, they are part of microbial life, in mass and volume probably comparable with all surface life. This life is widespread at depths in the crust of the earth, just as life has recently been identified in numerous ocean vents and geysers. This life is not dependent on solar energy and photosynthesis for its energy supply, and is essentially independent of surface conditions. The methanogens are capable of obtaining energy for growth by converting carbon dioxide (CO2) and molecular hydrogen (H2) into methane (CH4) and water (H2O). Some methanogenic bacteria are also capable of transforming acetate into CH4 and CO2. This chemical energy scheme inside earth is simpler than photosynthesis on the surface and therefore is more likely to have preceded it, suggesting that life possibly originated deep inside earth.
Geologic (microbial plus abiogenic, thermally stable to 300°C or 300 km depth) CH4 is transported upward, attested to by its association with helium, to contribute to subterrestrial petroleum pools and methane-hydrates beneath the sea floor. Near or at the surface, geologic CH4 mixes with other natural CH4 from wetlands, termites, open ocean, marine sediments and wild fires, and with anthropogenic CH4 from rice paddies, animals, manure, landfills, waste water treatment, biomass burning and coal mining. Some of this CH4 is emitted into the atmosphere at current annual rates of ~500 Tg (teragrams), of which ~ 200 Tg are natural and ~ 300 Tg are man-made. The atmospheric lifetime of CH4, a greenhouse gas 20 times more effective than CO2 in raising global temperatures, is approximately ten years. It is removed from the atmosphere mainly by reactions with the hydroxyl radicals (OH) in the troposphere to form CO2, but also by dry soil and by conversion to H2O in the stratosphere and mesosphere. The current trend in anthropogenic CH4-increase is a contemporary analogue of a "methane burp" some 55 million years ago, when three trillion tons of CH4 out of 15 trillion tons that had formed beneath the sea floor were released into the atmosphere within a few thousand years. The consequence was a greenhouse effect that raised the atmospheric temperatures by 9-12°C which, in turn, allowed modern mammals, the ancestors of horses, cows, deer, apes, and humans, be brought to global dominance. What prevented this CH4-induced greenhouse effect from running away?
CH4, in contrast to CO2 and other greenhouse gases, has the unique property of being partly converted to H2O by cosmic radiation in the mesosphere. H2O, in turn, condenses into visible clouds in the presence of cloud nuclei at temperatures that fall below the saturation temperature at given concentrations. Clouds, by virtue of their albedo, prevent sunlight from reaching the lower atmosphere and the earths surface and thus lower the global temperature. The current CH4-doubling over the past century resulted in an increase in upper level H2O from 4.3 ppmv to 6 ppmv. This 30% increase in H2O vapor yielded a tenfold increase in brightness of polar mesospheric clouds because of a strong dependence of the ice particle nucleation rate on the water saturation ratios. Models show that at a given temperature the optical depth of mesospheric clouds scales as [H2O]b with b varying between 4 and 8. Radiative transfer tools applied to mesospheric cloud particles suggest that an optical depth of approximately one, or 1000 times the current mesospheric cloud optical depth, would result in tropospheric cooling of about 10°C. Assuming b = 6, a thousandfold increase in optical thickness would require a three-fold increase of H2O, or a 20-fold increase of CH4. At the current rate of anthropogenic emissions this is expected to occur within the next millennium. This timescale is also commensurate with what has been assumed for the CH4-burp 55 million years ago.
A link with Astrobiology is based on the possibility that subsurface life may be widespread among other planetary bodies of the solar system, because many of them have equally suitable subsurface conditions, while having totally inhospitable surfaces. One may even speculate that such life may be widely disseminated in the universe, since planetary type bodies with subsurface conditions similar to earth may be common in other solar-type systems. Thus, research into the connection between CH4 and climate has a strong resonance with the origin of life. This is emphasized by the recent reclassification of prokaryotes into bacteria and archaea. Hyperthermophilic methanogens belong to the domain of archaea which must have had an early origin in the evolution of life, judging by these organisms simple genetic system. Furthermore, because so many strains of archaea are hyperthermophilic, they could have originated at a high temperature deep inside the earth. Thus, it seems that archaea represent a global evolution of an early form of life that depended on the chemical energy that the earth delivered. Archaea would thus be the product of a long evolution in a large, connected, and long-lived habitat. They may be the earliest inhabitants (and even today the principal inhabitants) of the deep hot biosphere beneath the surface of the earth.
Several types of investigations could be undertaken to prove the above described concepts. First, primordial hydrocarbons (chondrites) could be collected in space and subjected to an extreme environment similar to the earths interior in a deep hot biosphere chamber. Any modification of those hydrocarbons in the direction of CH4, and possibly amino acid formation, would suggest that conventional petroleum can be of abiogenic origin, and that life could have originated at high temperatures inside the earth, respectively. Second, accurate measurements of the rate of gas seepage, particularly over regions where natural gas is produced commercially, might yield data that would be difficult to explain by the biogenic theory of hydrocarbon formation, if the volume and seepage rate of gases entering the atmosphere in such regions turned out to be so great that the gas reservoirs underneath should have been exhausted naturally a long time ago. Determination of the C13/C12 isotope ratios would separate the fraction of gas that is of anthropogenic origin from the natural one. Third, microbial samples could be drawn from oil wells and other bore holes at temperatures and depths in which the methanogens typically exist. These samples then could be cultured at the same extreme environmental conditions and the production rate of CH4 determined. Few, if any, microbiology laboratories are equipped with a deep hot biosphere chamber. Further, cloud chamber experiments should be conducted at mesospheric concentrations of CH4 and H2O at 120°C temperature and simulated cosmic radiation in a properly designed cloud chamber.
Point of Contact: Rudolf F. Pueschel, 650/604-5254, rpueschel@.mail.arc.nasa.gov