Haze aerosols in the atmosphere of early Earth: manna from heaven

An organic haze layer in the upper atmosphere of Titan plays a crucial role in the atmospheric composition and climate of that moon. Such a haze layer may also have existed on the early Earth, providing an ultraviolet shield for greenhouse gases needed to warm the planet enough for life to arise and evolve. Despite the implications of such a haze layer, little is known about the organic material produced under early Earth conditions when both CO(2) and CH(4) may have been abundant in the atmosphere. For the first time, we experimentally demonstrate that organic haze can be generated in different CH(4)/CO(2) ratios. Here, we show that haze aerosols are able to form at CH(4) mixing ratios of 1,000 ppmv, a level likely to be present on early Earth. In addition, we find that organic hazes will form at C/O ratios as low as 0.6, which is lower than the predicted value of unity. We also show that as the C/O ratio decreases, the organic particles produced are more oxidized and contain biologically labile compounds. After life arose, the haze may thus have provided food for biota.     

Early martian environments: the Antarctic and other terrestrial analogs.

The comparability of the early environments of Mars and Earth, and the biological evolution which occurred on early earth, motivates serious consideration of the possibility of an early martian biota. Environments which could have contained this early martian life and which may presently contain evidence of this former life include aquatic, ice, soil, and rock habitats. Several analogs of these potential early martian environments, which can provide useful information in searching for extinct life on Mars, are currently available for study on Earth. These terrestrial analogs include the perennially ice-covered lakes and sandstone rocks in the Polar Deserts of Antarctica, surface of snowfields and glaciers, desert soils, geothermal springs, and deep subsurface environments.     

Why O2 is required by complex life on habitable planets and the concept of planetary "oxygenation time"

Life is constructed from a limited toolkit: the Periodic Table. The reduction of oxygen provides the largest free energy release per electron transfer, except for the reduction of fluorine and chlorine. However, the bonding of O2 ensures that it is sufficiently stable to accumulate in a planetary atmosphere, whereas the more weakly bonded halogen gases are far too reactive ever to achieve significant abundance. Consequently, an atmosphere rich in O2 provides the largest feasible energy source. This universal uniqueness suggests that abundant O2 is necessary for the high-energy demands of complex life anywhere, i.e., for actively mobile organisms of approximately 10(-1)-10(0) m size scale with specialized, differentiated anatomy comparable to advanced metazoans. On Earth, aerobic metabolism provides about an order of magnitude more energy for a given intake of food than anaerobic metabolism. As a result, anaerobes do not grow beyond the complexity of uniseriate filaments of cells because of prohibitively low growth efficiencies in a food chain. The biomass cumulative number density, n, at a particular mass, m, scales as n (> m) proportional to m(-1) for aquatic aerobes, and we show that for anaerobes the predicted scaling is n proportional to m (-1.5), close to a growth-limited threshold. Even with aerobic metabolism, the partial pressure of atmospheric O2 (P(O2)) must exceed approximately 10(3) Pa to allow organisms that rely on O2 diffusion to evolve to a size approximately 10(3) m x P(O2) in the range approximately 10(3)-10(4) Pa is needed to exceed the threshold of approximately 10(2) m size for complex life with circulatory physiology. In terrestrial life, O(2) also facilitates hundreds of metabolic pathways, including those that make specialized structural molecules found only in animals. The time scale to reach P(O(2)) approximately 10(4) Pa, or "oxygenation time," was long on the Earth (approximately 3.9 billion years), within almost a factor of 2 of the Sun's main sequence lifetime. Consequently, we argue that the oxygenation time is likely to be a key rate-limiting step in the evolution of complex life on other habitable planets. The oxygenation time could preclude complex life on Earth-like planets orbiting short-lived stars that end their main sequence lives before planetary oxygenation takes place. Conversely, Earth-like planets orbiting long-lived stars are potentially favorable habitats for complex life.     

A preliminary survey of non-lichenized fungi cultured from the hyperarid Atacama Desert of Chile

The Atacama Desert is one of the driest environments on Earth, and has been so for over 200,000 years. Previous reports have suggested that surprisingly low numbers of culturable bacteria, counted as biomass or species diversity, are present in Atacama sands collected from the most hyperarid regions. In previous studies, the presence of eukaryotic organisms was not discussed. In this report, we describe a method of direct plating onto rich media that resulted in culturing a range of fungi from Atacama samples. All fungi identified in this preliminary survey are spore-forming saprobes that are readily dispersed by wind, a likely mechanism that accounts for their presence in the central Atacama Desert.     

Testing the H2O2-H2O hypothesis for life on Mars with the TEGA instrument on the Phoenix lander

In the time since the Viking life-detection experiments were conducted on Mars, many missions have enhanced our knowledge about the environmental conditions on the Red Planet. However, the martian surface chemistry and the Viking lander results remain puzzling. Nonbiological explanations that favor a strong inorganic oxidant are currently favored (e.g., Mancinelli, 1989; Plumb et al., 1989; Quinn and Zent, 1999; Klein, 1999; Yen et al., 2000), but problems remain regarding the lifetime, source, and abundance of that oxidant to account for the Viking observations (Zent and McKay, 1994). Alternatively, a hypothesis that favors the biological origin of a strong oxidizer has recently been advanced (Houtkooper and Schulze-Makuch, 2007). Here, we report on laboratory experiments that simulate the experiments to be conducted by the Thermal and Evolved Gas Analyzer (TEGA) instrument of the Phoenix lander, which is to descend on Mars in May 2008. Our experiments provide a baseline for an unbiased test for chemical versus biological responses, which can be applied at the time the Phoenix lander transmits its first results from the martian surface.     

The possible origin and persistence of life on Enceladus and detection of biomarkers in the plume

The jets of icy particles and water vapor issuing from the south pole of Enceladus are evidence for activity driven by some geophysical energy source. The vapor has also been shown to contain simple organic compounds, and the south polar terrain is bathed in excess heat coming from below. The source of the ice and vapor, and the mechanisms that accelerate the material into space, remain obscure. However, it is possible that a liquid water environment exists beneath the south polar cap, which may be conducive to life. Several theories for the origin of life on Earth would apply to Enceladus. These are (1) origin in an organic-rich mixture, (2) origin in the redox gradient of a submarine vent, and (3) panspermia. There are three microbial ecosystems on Earth that do not rely on sunlight, oxygen, or organics produced at the surface and, thus, provide analogues for possible ecologies on Enceladus. Two of these ecosystems are found deep in volcanic rock, and the primary productivity is based on the consumption by methanogens of hydrogen produced by rock reactions with water. The third ecosystem is found deep below the surface in South Africa and is based on sulfur-reducing bacteria consuming hydrogen and sulfate, both of which are ultimately produced by radioactive decay. Methane has been detected in the plume of Enceladus and may be biological in origin. An indicator of biological origin may be the ratio of non-methane hydrocarbons to methane, which is very low (0.001) for biological sources but is higher (0.1-0.01) for nonbiological sources. Thus, Cassini's instruments may detect plausible evidence for life by analysis of hydrocarbons in the plume during close encounters.     

Remote sensing for organics on Mars.

The detection of organics on Mars remains an important scientific objective. Advances in instrumentation and laboratory techniques provide new insight into the lower level detection limit of complex organics in closely packed media. Preliminary results demonstrate that algae present in a palagonite medium do exhibit a spectral reflectance feature in the visible range for dry mass weight ratios of algae to palagonite greater than 6%--which corresponds to 30 mg algae in a 470 mg (just optically thick (< 3 mm) layer) palagonite matrix. This signature most probably represents chlorophyll a, a light harvesting pigment with an emission peak at 678 nm.     

Surface-atmosphere interactions on Titan compared with those on the pre-biotic Earth.

The surface and atmosphere of Titan constitute a system which is potentially as complex as that of the Earth, with the possibility of precipitation, surface erosion due to liquids, chemistry in large surface or subsurface hydrocarbon reservoirs, surface expressions of internal activity, and occasional major impacts leading to crustal melting. While none of the above have been observed as yet, the composition, density and thermal properties of Titan's atmosphere make it uniquely suited in the outer solar system as a place where such processes may occur. The one attribute of the Earth not expected on Titan is biological activity, which has had a profound effect on the evolution of the Earth's surface-atmosphere system. The earliest environment of Titan could have been warm enough for liquid ammonia-water solutions to exist on or near surface; pre-biotic organic processes may have taken place in such an environment. After a few hundred million years surface ammonia-water would have disappeared. Therefore, study of Titan through the Cassini-Huygens mission, planned for launch in 1997, primarily affords the opportunity to understand planet-wide surface-atmosphere interactions in the presence of fluids but in the absence of life. More speculative is the possibility that endogenic and exogenic heating continue to provide short-lived environments on Titan wherein pre-biotic organic processes in the presence of water happen.     

In situ microbial detection in Mojave Desert soil using native fluorescence

We report on the use of a portable instrument for microbial detection in the Mojave Desert soil and the potential for its use on Mars. The instrument is based on native fluorescence and employs four excitation wavelengths combined with four emission wavelengths. A soil dilution series in which known numbers of Bacillus subtilis spores were added to soil was used to determine the sensitivity of the instrument. We found that the fluorescence of the biological and organic components of the desert soil samples studied can be as strong as the fluorescence of the mineral component of these soils. Using the calibration derived from B. subtilis spores, we estimated that microbial content at our primary sampling site was 10(7) bacteria per gram of soil, a level confirmed by phospholipid fatty acid analysis. At a nearby site, but in a slightly different geological setting, we tested the instrument's ability to map out microbial concentrations in situ. Over a ～50 m diameter circle, soil microbial concentrations determined with the B. subtilis calibration indicate that the concentrations of microorganisms detected varies from 10(4) to 10(7) cells per gram of soil. We conclude that fluorescence is a promising method for detecting soil microbes in noncontact applications in extreme environments on Earth and may have applications on future missions to Mars.