The outstanding characteristic of Astrobiology is its interdisciplinary approach, which is absolutely essential for raveling our fundamental goals (as set by NASA Astrobiology Program); how life begins and evolves, whether life exists elsewhere in the Universe, and what the future of life on Earth and beyond is. This special issue Astrobiology specifically demonstrates the direction which this study area is going take by putting together remarkable review articles from the perspectives of space exploration, astronomy, geophysics, astrochemistry, abiotic chemistry, isotope geochemistry, and biogeochemistry. All these science are linked each other in terms of the energy balance concept for habitability.

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Now, many planetary bodies in our Solar System are of strong astrobiological interests, but Mars still continues to be the most feasible planet to look for life. Mars scientific exploration has focused on water, habitability (i.e., climate), and signs of life. This paper reviews major questions about these three issues and discuss an outlook of future Mars missions. The questions addressed in this paper are: (1) Did Mars have warm and wet climate in the past? (2) Is there methane in the Mars atmosphere? (3) Why hasn't organics been found on Mars? Current answers based on a review of recent researches found in the literature are the following. First, we are obtaining more and more data indicating that Mars had wet and warm climate, but the duration of such climate is very unclear. One possibility is that warm and wet climate may have been short lived (perhaps tens of years to tens of thousands of years each time) but occurred many times during the first several hundred million years in the Mars history. Second, the recent criticisms on the reports for methane discovery on Mars that they may have been errors due to telluric ¹³CH₄ absorption lines pose a serious question about these reports, although the criticisms do not necessarily account for all the reported data. Thus, methane on Mars should be regarded as a possibility at present and needs further investigation with much higher fidelity. Third, the discovery of perchlorate on Mars by the Phoenix mission opened up the possibility that organics on Mars surface may have been oxidized by O₂ released by perchlorate during pyrolysis and form chlorocarbons as detected by Viking landers. This oxidation process by perchlorate in pyrolysis may have been the reason why no organics have been found on Mars. Following these progresses, Mars exploration is stepping forward to a sample return mission. Although this will be an extremely exciting mission, it may make Mars exploration programs rather inflexible over the next decade or more. Thus, well-focused very small Mars probes, which can be relatively easily launched by a nation that have not landed on Mars before, such as Japan, may be able to make important contributions to Mars astrobiology.

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Characterizing extra-solar planetary systems is one of the most important research topics in contemporary astronomy. Today, more than 550 planets are known to exist, many of which show quite different characteristics from the Solar system planets in their masses, orbits, and densities. The situation makes us to recognize that it is central to construct the generic model on planet formation and evolution to reproduce the variety of exoplanets, and to place our Solar system in the context of other planetary systems. The key ingredients for the model are physical and chemical properties of protoplanetary disks, rotating structures around young stars where planets form. Although we are still far from being able to understand them, the astronomical observations have provided fundamental knowledge about such as masses, lifetimes, and structural evolution of disks. State-of-the-art instruments allow to directly explore close neighborhoods of the central stars, revealing rich diversity in their structures and signs of planets embedded in disks. In this paper, recent observational efforts of young circumstellar disks are reviewed focusing on infrared photometric and imaging studies.

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Since the first detection in interstellar medium in 1989, solid CO₂ has been found in various lines of sight and regarded as one of the main constituent in icy grain mantles in interstellar clouds. Due to the low efficiency in the formation of CO₂ in the gas phase and relatively high abundance of CO in icy grain mantles, it is generally accepted that CO₂ forms on the surface of icy grain mantles in interstellar clouds. CO₂ formation on/in icy grain mantles has been extensively studied experimentally, with the aid of energetic sources such as UV or cosmic-rays, and also without them. In this review, we summarize experimental results on the formation of CO₂ on cold surfaces through energetic and non-energetic processes under the simulative conditions of interstellar molecular clouds.

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I review the circulation style, storage capacity, and current amount of water in the mantle. Possible water subduction and circulation processes in the deep mantle are discussed on the basis of calculated phase relations of simplified hydrous peridotite in the system MgO-SiO₂-H₂O. For water transportation by subduction of lithospheric peridotites, important phase relations are (1) of serpentine at lower pressures less than 10 GPa and (2) of seven different high-pressure hydrous phases including dense hydrous magnesium silicates at higher pressures. Along the cold slab geotherm, large fluid fluxes are predicted at shallow (〜300km depth) and deep (〜700km depth) levels, depending on the slab temperature. The whole mantle may have the storage capacity of water more than ten times as much as the ocean. Nevertheless, the whole mantle seems not to be saturated with water. This would give rise to a mystery of the existence of the ocean at the surface, because the solid Earth should have drained all the water during its accretion period. A solution to this question will contribute to the astrobiology, particularly to the conditions for the habitable planet.

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The history of the theory and experimental evidence that sulfide minerals are linked to abiotic organic chemistry is reviewed. Modern biocatalysts that promote the formation of organic molecules from small components include protein enzymes that contain transition metal sulfide clusters at their active sites. The important roles of metal sulfide clusters in microbial biosynthesis inspired two distinct hypotheses by Wächtershäuser, and Russell. Both of two hypotheses were proposed that sulfide minerals might play significant roles promoting the chemical evolution. During the 1990s and into the twenty-first century, experiments have revealed that sulfide minerals have the capacity to both catalyze and, in some cases, participate in organosynthetic reactions that are similar to modern biosynthetic pathways. Therefore, sulfide minerals could provide the early Earth with valuable organic molecules.

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Since the discovery of Anomalous Isotope Fractionation (AIF) of sulfur, or more commonly termed Mass-Independent Fractionation (MIF) of sulfur, in many sedimentary rocks older than 2.45Ga, and its virtual absence in younger rocks, the AIF record has been cited by many researchers as unequivocal evidence for a dramatic change from an anoxic to oxic atmosphere around 2.45Ga. However, multiple sulfur isotope data from these natural samples have been interpreted based on the assumption that sulfur chemistry in gas phase is only responsible for AIF of sulfur in nature. The objective of this review is to evaluate this assumption. First, results of ab initio calculations for equilibrium isotope exchange between S-bearing species show that deviations from the mass-dependent relationships during equilibrium processes are much smaller than those found in Archean sedimentary rocks. Consequently, AIF requires a property that is a discontinuous function of mass (e.g., nuclear volume, magnetic moment, asymmetry of molecules, isotopic abundance). The number of bound states may be another discontinuous function of mass, which fractionates S isotopes anomalously during chemisorption at a high temperature (e.g., >100℃). A recent experimental study also showed that thermochemical sulfate reduction by simple amino acids produces AIF of sulfur. These results suggest that an alternative process, such as reactions between organic matter in sediments and sulfate-rich hydrothermal solutions, may have caused AIF signatures of sulfur in Archean sedimentary rocks.

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Early to Middle Archean fossil-like carbonaceous and pyritic microstructures from the Pilbara Craton, Western Australia were reviewed, referring to origins and depositional environments of host rocks. While their biogenicity is not always widely accepted but even controversial, it is plausible that Early to Middle Archean microorganisms were significantly diverse (filamentous, lenticular, spheroidal, film-like) and adapted to wide ranges of environments including deep-sea hydrothermal area, shallow sea and even hyper-saline lake. Hence, these records of microfossil and stromatolites reported from contemporaneous strata could not provide direct information about origin of life and its very early evolution. On the other hand, recently accumulated data of large microfossils (>20μm in major dimension), some of which are morphologically similar to cyanbacteria and eukaria, could provide new framework for studies on the early evolution of life.

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Seafloor hydrothermal systems are known to support a variety of biological communities that are sustained by primary production of chemolithoautotrophic microorganisms. The symbiotic and free-living chemolithoautotrophic microorganisms obtain energy from inorganic substances such as H₂S, H₂, and CH₄ derived from hydrothermal vent fluids. Thus, the diversity and abundances of the hydrothermal vent-endemic biological communities are considered to be controlled by chemical compositions of hydrothermal fluids. In order to elucidate biogeochemical relationships between chemolithoautotrophic microbial activities and hydrothermal fluid chemistry in seafloor hydrothermal systems, the amount of metabolic energy available for primary production by chemolithoautotrophic microorganisms is evaluated using geochemical models, and the model results are compared to observed variability in microbial community in seafloor hydrothermal vents. The results of our investigations clearly show that H₂ concentartions in hydrothermal fluids have a significant impact on wide range of not only anaerobic but also aerobic reactions. In addition, the concentrations of CH₄ and Fe²⁺ also affect their oxydation reactions. On the other hand, because almost all hydrothermal vent fluids contain sufficient amounts of H₂S, a variation in H₂S concentration of hydrothermal fluids has essentially no effect on sulfuroxydation reactions, except only under low-temerature conditions less than 25℃. We also present a comparison of potential chemolithoautotrophic microbial activities between modern and possible early Earth's seafloor hydrothermal vents. Assuming that early Earth's seawater had only very low O₂-levels, all aerobic reactions can not be available, and availability of some anaerobic reactions using SO₄ are confined to low-temerature condisions. In striking contrast, methanogenesis utilizing H₂ and CO₂ is essentially unaffected by a variation in seawater O₂-level, suggesting the importance of hydrogen and hydrogenotrophic methanogenesis for life on early Earth as well as other planets and moons.

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