The mantle was probably oxidized early, during and shortly after accretion, and so the early atmosphere of Earth was likely dominated by CO 2 and N 2 , not by CH 4 and NH 3 . CO 2 declined from multibar levels during the early Hadean to perhaps a few tenths of a bar by the mid- to late Archean. Published geochemical constraints on Archean CO 2 concentrations from paleosols are highly uncertain, and those from banded iron formations are probably invalid. Thus, CO 2 could have been sufficiently abundant during the Archean to have provided most of the greenhouse warming needed to offset the faint young Sun. H 2 might have augmented this warming prior to the origin of methanogenic bacteria. Atmospheric CH 4 concentrations increased from at most tens of parts per million (ppm) on prebiotic Earth to hundreds of parts per million once methanogens evolved. CO was an important trace gas on prebiotic Earth because of its high free energy and its ability to catalyze key reactions involved in prebiotic synthesis. Large impacts could have made the atmosphere transiently CO rich, and this may have played a role in the origin of life and in fueling early biological metabolisms.

Abstract The release of greenhouse gases from underground coal-mine fires is a function of temperature and the concentration of O 2 . In a laboratory study on spontaneous combustion, samples of coal, coal refuse, and carbonaceous shale were heated at a controlled rate between ambient temperature and 250 °C. In these experiments, the concentration of O 2 was not limited and the concentration of CO 2 increased with increasing temperature to a maximum of 10%. Carbon monoxide was not detected at temperatures below 100 °C, and the maximum concentration of CO was less than 4%. In field studies, samples of combustion gases were obtained from fires in three abandoned coal mines. These indicated a linear increase in the concentration of CO 2 relative to the decreased concentration of O 2 . At an O 2 concentration of 2%, the CO 2 concentration approached 15%, and CO was detected only when the O 2 concentration was less than 8%. At temperatures over 50 °C, the rate of desorption of CH 4 also increased, but the average concentration in the mine atmosphere was 0.20%. These laboratory experiments and field studies indicate that the rate of gas production is controlled by O 2 concentration and temperature, but physical factors, such as overburden fracturing and differences between surface and subsurface temperature and pressure, control the rate of emission to the atmosphere. In coal-mine fires, both chemical and physical factors control the rate and magnitude of contributions to the atmospheric concentration of greenhouse gases.

Abstract Coal fires are one of the most common geohazards in most coal-producing countries, such as india and China. Combustion can occur spontaneously or due to anthropogenic causes, either within underground coal seams or in exposed layers of coal on Earth’s surface. Once started, coal fires are difficult to extinguish and sometimes cannot be controlled. In addition to burning millions of tons of coal, the fires have an enormous negative impact on the local and global environments. Coal fires produce large quantities of greenhouse gases, such as CO, CO 2 , CH 4 , SO x , and NO x , which have a direct impact on the local and global atmospheric composition. Since the preindustrial era, the concentration of CO 2 , a major greenhouse gas that contributes to global warming, has increased from 280 ppm to 375 ppm. Land subsidence is an associated problem in areas that are affected by coal fires. Coal fires also create operational difficulties in existing mines and endanger human safety. After the first use of remote sensing to study a coal fire in the early 1960s, this technology became a useful and convenient tool for the detection and monitoring of additional coal fires. Several air- and spaceborne thermal remote sensors are available for studying coal fires. Coal-fire–related emissions have not been studied extensively; ground-based methods mainly use CO 2 detection instruments or other indirect calculations (e.g., amount of coal burnt). Few attempts are being made to estimate coal-fire emission using remote sensing.

A handful of investigative teams in several parts of the world are studying abundant biological communities in caves formed by sulfuric-acid speleogenesis. These caves are atypical in terms of origin, chemistry, and ecosystem properties. They prominently display sulfur minerals, characteristic cavity topologies, and notable biological diversity and biological productivity resulting directly from the conditions that produce the caves. Even long-inactive systems still harbor some of these indicators. The microbial and macroscopic ecosystems within sulfuric-acid speleogenetic caves are geologically mediated and maintained. This geological mediation is a theme connecting them with other sulfur-driven ecosystems on Earth, including deep-sea hydrothermal vents, sulfurous near-surface hydrothermal systems, and solfataras. Evidence exists for potentially significant microbial participation in the process of speleogenesis itself. Recent results confirming the high relative abundance of sulfur on Mars, an apparent sedimentary basin with high sulfate concentration, near-surface indicators of ice and water, and trace detection of reduced gases (especially methane) in the Martian atmosphere, possibly deriving from subsurface microbial sources, set the stage for suggesting that sulfuric-acid speleogenetic systems may be useful as astrobiological analogs for hypothetical Mars ecosystems. Unique speleogenetic mechanisms may occur on Mars and could provide subsurface void space suitable for habitation by such hypothetical microbial systems.