Two Different Sources of Water for the Early Solar Nebula

Water is essential for life. This is a trivial fact but has profound implications since the forming of life on the early Earth required water. The sources of water and the related amount of delivery depend not only on the conditions on the early Earth itself but also on the evolutionary history of the solar system. Thus we ask where and when water formed in the solar nebula-the precursor of the solar system. In this paper we explore the chemical mechanics for water formation and its expected abundance. This is achieved by studying the parental cloud core of the solar nebula and its gravitational collapse. We have identified two different sources of water for the region of Earth's accretion. The first being the sublimation of the icy mantles of dust grains formed in the parental cloud. The second source is located in the inner region of the collapsing cloud core - the so-called hot corino with a temperature of several hundred Kelvin. There, water is produced efficiently in the gas phase by reactions between neutral molecules. Additionally, we analyse the dependence of the production of water on the initial abundance ratio between carbon and oxygen.     

Microfossils

Defining biosignatures, i.e. features that are indicative of past or present life, has been one of the major strategies developed over the last few years for the search of life on the early Earth and in the solar system. Current knowledge about microscopic remnants of fossil organisms, namely microfossils are reviewed, focusing on: (i) studies of recent environments used as analogues for the early Earth or extraterrestrial environments; (ii) examination of Precambrian rocks; and (iii) laboratory experiments simulating biotic and abiotic processes and resulting in the formation of genuine or pseudomicrofossils. Fossils’ preservation depends on environment and chemical composition of the primary structure, although they might undergo taphonomic processes that alter their morphology and/or composition. Altogether, these examples illustrate what can be potentially preserved during the very first stages of fossilization and what can be left in the geological record after diagenesis and metamorphism. Finally, this provides a rationale to tentatively define diagnosis criteria for microfossils or ways to look for life on Earth or in extraterrestrial environments.     

Galactic Factors,the Young Sun,the Earth,and the Biophysics of Living Systems

The effects of radiation from the young Sun and galactic cosmic rays on the physical conditions on the early Earth are significantly underestimated in studies of the problems related to the origin and evolution of the biosphere. This review considers the dynamics of solar and galactic processes over the 4.56 billion years of the existence of the Solar System. These factors substantially affected the development of adaptive technologies in ancient and modern living systems. The features of biosphere development are considered for the early Earth under the young Sun, which was fainter, but more flare active. The radiation spectrum of the young Sun is discussed together with the paradoxical mismatch between the solar radiation spectrum and the chlorophyll adsorption spectrum. Ways of solving the paradox are proposed. The role of solar radiation is important when studying models of the early biosphere of the Earth and hypothetical biospheres of giant planet satellites and exoplanets.

     

The cometary connection with prebiotic chemistry

Amino-acids, purines and pyrimidines may have been formed in space and brought down to Earth by comets during the final stage of the Earth's cold accretion from dust. Present-day comets seem to be the remnants of the building blocks of the early solar system, put for 4.6 billion years in parking orbits in the deep cold of space by the same process of orbital diffusion that brought a veneer of cometary material to the primitive Earth. The second part of the present review examines the chemical nature of comets as can be reconstructed from (so far) very incomplete evidence.     

THE SEARCH FOR LIFE ON MARS

Mars appears to have no life on its surface today. However, the presence of fluvial features provides evidence that liquid water was once present on the martian surface. By analogy with Earth, life may have originated on Mars early in its history, possibly during the end of the late heavy bombardment. Analysis of the one meteorite from Mars which dates to this early time appears to contain evidence of this early environment and possibly life. As the climate cooled and liquid water became unavailable, life would have eventually died out. The cold deserts of Antarctica provide a glimpse of what martian ecosystems might have been like as conditions worsened. The search for fossil evidence of past life on Mars may provide the first direct indication of life beyond Earth.     

The significance of the study about the biological effects of solar ultraviolet radiation using the Exposed Facility on the International Space Station.

It is believed that ultraviolet (UV) radiation from the sun participated in events related to the chemical evolution and birth of life on the primitive Earth. Although UV radiation would be also a driving force for the biological evolution of life on Earth, life space of the primitive living organisms would be limited in the UV-shielded place such as in the water at an early stage of the evolution of life. After the formation of stratospheric ozone layer through the production of oxygen by photoautotroph, living organisms were able to expand their domain from water to land. As a result, now, many kinds of living organisms containing human beings are flourishing on the ground. In the near future, increased transmission of harmful solar UV radiation may reach the Earth's surface due to stratospheric ozone layer depletion. In order to learn more about the biological effects of solar UV radiation with or without interruption by the ozone layer, the utilization of an Exposed Facility on the International Space Station is required. Experiments proposed for this facility would provide a tool for the scientific investigation of processes involved in the birth and evolution of life on Earth, and could also demonstrate the importance of protecting the Earth's future environment from future ozone layer depletion.     

Early Earth closely surrounded by young stars

Knowledge of the physical and chemical conditions on the primeval Earth is important for the study of the origin of the biosphere. This paper discusses the latest modification of the theory of the origin of the Earth and other planets. Possible consequences of the formation of the Sun in the area of the star formation closely surrounded by neighboring young stars are considered. The classical problem of the rate of accretion of Earth and other planets is generalized with new estimates allowing the correlation of the results from long-lived (U-Pb) and short-lived (Hf-W) space-chronometers. A model of the early evolution of the Earth, based on both dynamic estimates and the latest geochemical data (earliest Australian zircons, relict xenon pleiad) is discussed. The problems of the theory of early Earth’s evolution, which so far cannot be adequately solved, are discussed.     

Life in space

The physical conditions of Space are most inhospitable and the higher forms of life probably could exist extraterrestrially only on Venus, Jupiter, and Saturn in our Solar System, and the chances there are poor in light of present knowledge. Thus intelligent life probably exists only on the Earth.Although indigenous intelligent extraterrestrial life seems to be improbable it is by no means clear that man cannot learn to live reasonably comfortably on most of our planets and planetoids such as our moon, and it seems certain that he will be able to travel great distances in the solar system.Lower forms of life may well occur extraterrestrially.     

Porphyrin-like compounds genesis under simulated abiotic conditions

Experiments with gas mixtures intended to simulate the primaeval atmosphere of the Earth yielded many biologically important chemicals. Investigations into the synthesis of porphyrinlike compounds from methane, ammonia and water vapour were carried out by using high frequency discharges. Microanalyses of porphyrins showed that porphyrin-like pigments were formed in this way. The presence of divalent cations in the reaction system increased the yield of porphyrin-like pigments also involving the direct synthesis of their metal complexes.The ready formation of these compounds in abiotic conditions is significant, suggesting the possibility of their appearance during the early stage of chemical evolution.     

Boron Enrichment in Martian Clay

We have detected a concentration of boron in martian clay far in excess of that in any previously reported extra-terrestrial object. This enrichment indicates that the chemistry necessary for the formation of ribose, a key component of RNA, could have existed on Mars since the formation of early clay deposits, contemporary to the emergence of life on Earth. Given the greater similarity of Earth and Mars early in their geological history, and the extensive disruption of Earth''s earliest mineralogy by plate tectonics, we suggest that the conditions for prebiotic ribose synthesis may be better understood by further Mars exploration.     

The evolution of the solar ‘constant’

The ultimate source of the energy utilized by life on Earth is the Sun, and the behavior of the Sun determines to a large extent the conditions under which life originated and continues to thrive. What can we say about the history of the Sun? Has the solar ‘constant’, the rate at which energy is received by the Earth from the Sun per unit area per unit time, been constant at its present level since Archean times? Three mechanisms by which it has been suggested that the solar energy output can vary with time will be discussed, characterized by long (～10⁹ yr), intermediate (～10⁸ yr), and short (～ years to decades) time scales.     

Stardust component in tree rings

Tree-ring series collected from different parts of Arctic (Fennoscandia, Kola Peninsula and Northern Siberia) are investigated by means of the multi-taped method (MTM) of spectral analysis. Results of spectral analysis allow us to select the main periods of solar variability (22-, 30–33- and 80–90-year solar cycles) in Kola and Fennoscandia tree-ring chronologies. Besides it was found that only periodicities of around 20 years are present in Siberian and Stockholm series, respectively. With respect to 11-year periodicity, which is the most prominent one in sunspot number spectrum (Schwabe cycle) it may be said that it hardly appeared in Arctic tree-ring series. Although the 22-year cycles in climatic records are perceivable (it is also evident from our and other results), any physical mechanisms by which a reversal in the solar magnetic field could influence climate are still missing. To our mind, a potential cause of this phenomenon seems to be a variation of stardust flux inside the solar system. The most recent observations in frame of the DUST experiment on board the Ulysses spacecraft have shown that stardust level inside of the solar system was trebled during the recent solar maximum (Landgraf et al., 2003. Penetration of the heliosphere by the interstellar dust stream during solar maximum. Journal Geophysical Research 108, 8030). It is possible that the periodic increase of stardust in the solar system will influence the amount of extraterrestrial material that rains down to the Earth and consequently down to the Earth's atmosphere and may affect climate through alteration of atmospheric transparency and albedo.     

Catalytic Role of Manganese Oxides in Prebiotic Nucleobases Synthesis from Formamide

Origin of life processes might have begun with the formation of important biomonomers, such as amino acids and nucleotides, from simple molecules present in the prebiotic environment and their subsequent condensation to biopolymers. While studying the prebiotic synthesis of naturally occurring purine and pyrimidine derivatives from formamide, the manganese oxides demonstrated not only good binding for formamide but demonstrated novel catalytic activity. A novel one pot manganese oxide catalyzed synthesis of pyrimidine nucleobases like thymine is reported along with the formation of other nucleobases like purine, 9-(hydroxyacetyl) purine, cytosine, 4(3 H)-pyrimidinone and adenine in acceptable amounts. The work reported is significant in the sense that the synthesis of thymine has exhibited difficulties especially under one pot conditions and also such has been reported only under the catalytic activity of TiO₂. The lower oxides of manganese were reported to show higher potential as catalysts and their existence were favored by the reducing atmospheric conditions prevalent on early Earth; thereby confirming the hypothesis that mineral having metals in reduced form might have been more active during the course of chemical evolution. Our results further confirm the role of formamide as a probable precursor for the formation of purine and pyrimidine bases during the course of chemical evolution and origin of life.     

Signal transduction in cells of the immune system in microgravity

Life on Earth developed in the presence and under the constant influence of gravity. Gravity has been present during the entire evolution, from the first organic molecule to mammals and humans. Modern research revealed clearly that gravity is important, probably indispensable for the function of living systems, from unicellular organisms to men. Thus, gravity research is no more or less a fundamental question about the conditions of life on Earth. Since the first space missions and supported thereafter by a multitude of space and ground-based experiments, it is well known that immune cell function is severely suppressed in microgravity, which renders the cells of the immune system an ideal model organism to investigate the influence of gravity on the cellular and molecular level. Here we review the current knowledge about the question, if and how cellular signal transduction depends on the existence of gravity, with special focus on cells of the immune system. Since immune cell function is fundamental to keep the organism under imnological surveillance during the defence against pathogens, to investigate the effects and possible molecular mechanisms of altered gravity is indispensable for long-term space flights to Earth Moon or Mars. Thus, understanding the impact of gravity on cellular functions on Earth will provide not only important informations about the development of life on Earth, but also for therapeutic and preventive strategies to cope successfully with medical problems during space exploration.     

Living nanovesicles--chemical and physical survival strategies of primordial biosystems

Life on Earth and Mars could have started with self-assembled nanovesicles similar to the present nanobacteria (NB). To resist extreme environmental stress situations and periods of nutritional deprivation, nanovesicles would have had a chemical composition protected by a closed mineralized compartment, facilitating their development in a primordial soup, or other early wet environment. Their survivability would have been enhanced if they had mechanisms for metabolic communication, and an ability to collect primordially available energy forms. Here, we establish an irreducible model system for life formation starting with NB.     

The chemistry that preceded life's origin: a study guide from meteorites

Carbonaceous meteorites are rare fragments of asteroids that contain organic carbon of diverse composition, various complexity, and whose lineage can in several instances be traced back to pre-solar environments. Their analyses offer a unique glimpse into the chemistry of the solar system that preceded life and may have been available to its emergence on the early Earth. While the heterogeneity of the organic materials of meteorites is indicative of random synthetic processes for their formation, some of their components have identical counterparts in the biosphere, and a group of meteoritic amino acids were found to display chiral asymmetry, a property known since the time of Pasteur to be inextricably linked to life's processes. The ability of these amino acids to act as asymmetric catalysts, as well as indications that molecular asymmetry in meteorites may not be limited to these compounds, encourage the suggestion of possible involvement of meteoritic material in the induction of selective traits in molecular evolution.     

The origin of the biologically coded amino acids

Biology uses essentially 20 amino acids for its coded protein enzymes, representing a very small subset of the structurally possible set. Most models of the origin of life suggest organisms developed from environmentally available organic compounds. A variety of amino acids are easily produced under conditions which were believed to have existed on the primitive Earth or in the early solar nebula. The types of amino acids produced depend on the conditions which prevailed at the time of synthesis, which remain controversial. The selection of the biological set is likely due to chemical and early biological evolution acting on the environmentally available compounds based on their chemical properties. Once life arose, selection would have proceeded based on the functional utility of amino acids coupled with their accessibility by primitive metabolism and their compatibility with other biochemical processes. Some possible mechanisms by which the modern set of 20 amino acids was selected starting from prebiotic chemistry are discussed.     

Enhanced Photocatalytic Performance of ZnS for Reversible Amination of α-oxo Acids by Hydrothermal Treatment

To understand how life could have originated on early Earth, it is essential to know what biomolecules and metabolic pathways are shared by extant organisms and what organic compounds and their chemical reaction channels were likely to have been primordially available during the initial phase of the formation of prebiotic metabolism. In a previous study, we demonstrated for the first time the reversible amination of α-oxo acids on the surface of photo-illuminated ZnS. The sulfide mineral is a typical component at the periphery of submarine hydrothermal vents which has been frequently argued as a very attractive venue for the origin of life. In this work, in order to simulate more closely the precipitation environments of ZnS in the vent systems, we treated newly-precipitated ZnS with hydrothermal conditions and found that its photocatalytic power was significantly enhanced because the relative crystallinity of the treated sample was markedly increased with increasing temperature. Since the reported experimental conditions are believed to have been prevalent in shallow-water hydrothermal vents of early Earth and the reversible amination of α-oxo acids is a key metabolic pathway in all extant life forms, the results of this work provide a prototypical model of the prebiotic amino acid redox metabolism. The amino acid dehydrogenase-like chemistry on photo-irradiated ZnS surfaces may advance our understanding of the establishment of archaic non-enzymatic metabolic systems.     

Formation of Carbon-Carbon Bonds in the Photochemical Alkylation of Polycyclic Aromatic Hydrocarbons

The reaction of polycyclic aromatic hydrocarbons (PAHs) withalkanes was examined in the presence of ultraviolet (UV) lightunder model prebiotic Earth and interstellar medium (ISM)conditions. We observed the formation of alkylated PAHs from avariety of PAHs and alkanes, which was caused by the absorptionof UV light by the PAH molecule. Photoalkylation occurred inPAHs and alkanes in solution, in thin films in contact withsimulated ocean water, and in matrices simulating ISMconditions. Photoalkylation occurred readily, with significantproduct yields within 5 h of irradiation. Because alkanes andPAHs are presumed to be part of the organic inventory in the ISMand on the early Earth, we propose that this photoalkylation reactionis a plausible pathway for the formation of carbon-carbon bondsin both these environments.     

The energetics of organic synthesis inside and outside the cell

Thermodynamic modelling of organic synthesis has largely been focused on deep-sea hydrothermal systems. When seawater mixes with hydrothermal fluids, redox gradients are established that serve as potential energy sources for the formation of organic compounds and biomolecules from inorganic starting materials. This energetic drive, which varies substantially depending on the type of host rock, is present and available both for abiotic (outside the cell) and biotic (inside the cell) processes. Here, we review and interpret a library of theoretical studies that target organic synthesis energetics. The biogeochemical scenarios evaluated include those in present-day hydrothermal systems and in putative early Earth environments. It is consistently and repeatedly shown in these studies that the formation of relatively simple organic compounds and biomolecules can be energy-yielding (exergonic) at conditions that occur in hydrothermal systems. Expanding on our ability to calculate biomass synthesis energetics, we also present here a new approach for estimating the energetics of polymerization reactions, specifically those associated with polypeptide formation from the requisite amino acids.