Prebiotic Organic Molecules Research Articles

Oxidation of iron by giant impact and its implication on the formation of reduced atmosphere in the early Earth

Giant impact–driven redox processes in the atmosphere and magma ocean played crucial roles in the evolution of Earth. However, because of the absence of rock records from that time, understanding these processes has proven challenging. Here, we present experimental results that simulate the giant impact–driven reactions between iron and volatiles (H 2 O and CO 2 ) using x-ray free electron laser (XFEL) as fast heat pump and structural probe. Under XFEL pump, iron is oxidized to wüstite (FeO), while volatiles are reduced to H 2 and CO. Furthermore, iron oxidation proceeds into formation of hydrides (γ-FeH x ) and siderite (FeCO 3 ), implying redox boundary near 300-km depth. Through quantitative analysis on reaction products, we estimate the volatile and FeO budgets in bulk silicate Earth, supporting the Theia hypothesis. Our findings shed light on the fast and short-lived process that led to reduced atmosphere, required for the emergence of prebiotic organic molecules in the early Earth.

Advances in superconductor quantum and thermal detectors for analytical instruments

Analytical instruments or scientific instruments are indispensable for scientific research and industry. The analytical instruments require a detector that converts physical quantities to be measured (measurands) to electric signals. This Tutorial describes the basics of quantum and thermal detectors, the operation principles of superconductor detectors, and the ultimate performance of state-of-art analytical instruments with superconductivity. We still face fundamental issues, such as the classical Fano factor, the relation between energy gap and mean carrier creation energy, quasiparticle dynamics, and the intermediate state in the middle of superconducting transition; and engineering issues, such as the small sensitive area and the spatially nonuniform response. Nevertheless, enormous efforts have matured superconductor detectors, which enables us to solve the inherent problems of conventional analytical instruments. As an example of the analytical results, we describe x-ray spectroscopy and mass spectrometry at our institute by using three detector types: superconductor tunnel junction, transition edge sensor, and superconductor strip. Microwave kinetic inductance and metallic magnetic calorimetric types are also described. The analytical results may contribute to a wide range of fields, such as dentistry, molecular biology, energy-saving society, planetary science, and prebiotic organic molecules in space.

Cosmology, astrobiology and the RNA world: just add quintessential water

AbstractLaboratory generation of water nanoclusters from amorphous ice and strong terahertz (THz) radiation from water nanoclusters ejected from water vapour into a vacuum suggest the possibility of water nanoclusters ejected into interstellar space from abundant amorphous ice-coated cosmic dust produced by supernovae explosions. Water nanoclusters (section ‘Water nanoclusters’) offer a hypothetical scenario connecting major mysteries of our Universe: dark matter (section ‘Baryonic dark matter’), dark energy (section ‘Dark energy’), cosmology (section ‘Cosmology’), astrobiology (section ‘Astrobiology’) and the RNA world (section ‘The RNA world’) as the origin of life on Earth and habitable exoplanets. Despite their expected low density in space compared to hydrogen, their quantum-entangled diffuse Rydberg electronic states make cosmic water nanoclusters a candidate for baryonic dark matter that can also absorb, via the microscopic dynamical Casimir effect, the virtual photons of zero-point-energy vacuum fluctuations above the nanocluster cut-off vibrational frequencies, leaving only vacuum fluctuations below these frequencies to be gravitationally active, thus leading to a possible common origin of dark matter and dark energy. This picture includes novel explanations of the small cosmological constant, the coincidence of energy and matter densities, possible contributions of the red-shifted THz radiation from cosmic water nanoclusters at redshift z ≅ 10 to the cosmic microwave background (CMB) spectrum, the Hubble constant crisis, the role of water as a known coolant for rapid early star formation and ultimately, how life may have originated from RNA protocells on Earth and exoplanets and moons in the habitable zones of developed solar systems. Together, they lead to a cyclic universe cosmology – based on the proposed equivalence of cosmic water nanoclusters to a quintessence scalar field – instead of a multiverse based on cosmic inflation theory. Recent CMB birefringence measurements may support quintessence. Finally, from the quantum chemistry of water nanoclusters interacting with prebiotic organic molecules, amino acids and RNA protocells on early Earth and habitable exoplanets, this scenario is consistent with the anthropic principle that our Universe must have those properties which allow life, as we know it – based on water, to develop at the present stage of its history.

Extraterrestrial ribose and other sugars in primitive meteorites

Sugars are essential molecules for all terrestrial biota working in many biological processes. Ribose is particularly essential as a building block of RNA, which could have both stored information and catalyzed reactions in primitive life on Earth. Meteorites contain a number of organic compounds including key building blocks of life, i.e., amino acids, nucleobases, and phosphate. An amino acid has also been identified in a cometary sample. However, the presence of extraterrestrial bioimportant sugars remains unclear. We analyzed sugars in 3 carbonaceous chondrites and show evidence of extraterrestrial ribose and other bioessential sugars in primitive meteorites. The 13C-enriched stable carbon isotope compositions (δ13C vs.VPDB) of the detected sugars show that the sugars are of extraterrestrial origin. We also conducted a laboratory simulation experiment of a potential sugar formation reaction in space. The compositions of pentoses in meteorites and the composition of the products of the laboratory simulation suggest that meteoritic sugars were formed by formose-like processes. The mineral compositions of these meteorites further suggest the formation of these sugars both before and after the accretion of their parent asteroids. Meteorites were carriers of prebiotic organic molecules to the early Earth; thus, the detection of extraterrestrial sugars in meteorites establishes the existence of natural geological routes to make and preserve them as well as raising the possibility that extraterrestrial sugars contributed to forming functional biopolymers like RNA on the early Earth or other primitive worlds.

Organic compounds in fluid inclusions of Archean quartz-Analogues of prebiotic chemistry on early Earth.

The origin of life is still an unsolved mystery in science. Hypothetically, prebiotic chemistry and the formation of protocells may have evolved in the hydrothermal environment of tectonic fault zones in the upper continental crust, an environment where sensitive molecules are protected against degradation induced e.g. by UV radiation. The composition of fluid inclusions in minerals such as quartz crystals which have grown in this environment during the Archean period might provide important information about the first organic molecules formed by hydrothermal synthesis. Here we present evidence for organic compounds which were preserved in fluid inclusions of Archean quartz minerals from Western Australia. We found a variety of organic compounds such as alkanes, halocarbons, alcohols and aldehydes which unambiguously show that simple and even more complex prebiotic organic molecules have been formed by hydrothermal processes. Stable-isotope analysis confirms that the methane found in the inclusions has most likely been formed from abiotic sources by hydrothermal chemistry. Obviously, the liquid phase in the continental Archean crust provided an interesting choice of functional organic molecules. We conclude that organic substances such as these could have made an important contribution to prebiotic chemistry which might eventually have led to the formation of living cells.

Mackinawite and greigite in ancient alkaline hydrothermal chimneys: Identifying potential key catalysts for emergent life

One model for the emergence of life posits that ancient, low temperature, submarine alkaline hydrothermal vents, partly composed of iron-sulfides, were capable of catalyzing the synthesis of prebiotic organic molecules from CO2, H2 and CH4. Specifically, hydrothermal mackinawite (FeIIS) and greigite (FeIIFeIII2S4) have been highlighted in previous studies as analogs of the active centers of hydrogenase, ferredoxin, acetyl coenzyme-A synthase and carbon monoxide dehydrogenase featured in the biochemistry of certain autotrophic prokaryotes that occupy the base of the evolutionary tree. Despite the proposed importance of iron sulfide minerals and clusters in the synthesis of abiotic organic molecules, the mechanisms for the formation of these sulfides from solution and their preservation under the anoxic and low temperature (below 100 °C) conditions expected in off-axis submarine alkaline vent systems is not well understood (Bourdoiseau et al., 2011; Rickard and Luther, 2007). To rectify this, single hydrothermal chimneys were precipitated using a unique apparatus to simulate growth at hydrothermal vents of moderate temperature under supposed Hadean ocean-bottom conditions. Iron sulfide phases were observed through Raman spectroscopy at growth temperatures ranging from 40° to 80 °C. Fe(III)-containing mackinawite is confirmed to be present with mackinawite and greigite, supporting an FeIII-mackinawite intermediate mechanism for the transformation of mackinawite to greigite below 100 °C. Raman spectroscopy of the chimneys revealed a maximum yield of greigite at 75 °C. These results suggest abiotic production of catalytically active mackinawite and greigite are possible under early Earth hydrothermal conditions as well as on other wet, rocky worlds geochemically similar to the Earth.

Atmospheric Prebiotic Chemistry and Organic Hazes.

Earth’s atmospheric composition at the time of the origin of life is not known, but it has often been suggested that chemical transformation of reactive species in the atmosphere was a significant source of prebiotic organic molecules. Experimental and theoretical studies over the past half century have shown that atmospheric synthesis can yield molecules such as amino acids and nucleobases, but these processes are very sensitive to gas composition and energy source. Abiotic synthesis of organic molecules is more productive in reduced atmospheres, yet the primitive Earth may not have been as reducing as earlier workers assumed, and recent research has reflected this shift in thinking. This work provides a survey of the range of chemical products that can be produced given a set of atmospheric conditions, with a particular focus on recent reports. Intertwined with the discussion of atmospheric synthesis is the consideration of an organic haze layer, which has been suggested as a possible ultraviolet shield on the anoxic early Earth. Since such a haze layer – if formed – would serve as a reservoir for organic molecules, the chemical composition of the aerosol should be closely examined. The results highlighted here show that a variety of products can be formed in mildly reducing or even neutral atmospheres, demonstrating that contributions of atmospheric synthesis to the organic inventory on early Earth should not be discounted. This review intends to bridge current knowledge of the range of possible atmospheric conditions in the prebiotic environment and pathways for synthesis under such conditions by examining the possible products of organic chemistry in the early atmosphere.

Could Life Originate between Mica Sheets?: Mechanochemical Biomolecular Synthesis and the Origins of Life

AbstractThe materials properties of mica have surprising similarities to those of living systems. The mica hypothesis is that life could have originated between mica sheets, which provide stable compartments, mechanical energy for bond formation, and the isolation needed for Darwinian evolution. Mechanical energy is produced by the movement of mica sheets, in response to forces such as ocean currents or temperature changes. The energy of a carbon-carbon bond at room temperature is comparable to a mechanical force of 6 nanoNewtons (nN) moving a distance of 100 picometers. Mica's movements may have facilitated mechanochemistry, resulting in the synthesis of prebiotic organic molecules. Furthermore, mica's movements may have facilitated the earliest cell divisions, at a later stage of life's origins. Mica's movements, pressing on lipid vesicles containing proto-cellular macromolecules, might have facilitated the blebbing off of ‘daughter’ protocells. This blebbing-off process has been observed recently in wall-less L-form bacteria and is proposed to be a remnant of the earliest cell divisions (Leaver, et al. Nature457, 849 (2009).

The birth and death of organic molecules in protoplanetary disks

AbstractThe most intriguing question related to the chemical evolution of protoplanetary disks is the genesis of pre-biotic organic molecules in the planet-forming zone. In this contribution we briefly review current observational knowledge of physical structure and chemical composition of disks and discuss whether organic molecules can be present in large amounts at the verge of planet formation. We predict that some molecules, including CO-bearing species such as H2CO, can be underabundant in inner regions of accreting protoplanetary disks around low-mass stars due to the high-energy stellar radiation and chemical processing on dust grain surfaces. These theoretical predictions are further compared with high-resolution observational data and the limitations of current models are discussed.

Complete Course in Astrobiology

1 Astrobiology: From the Origin of Life on Earth to Life in the Universe (Andre Brack). 1.1 General Aspects of Astrobiology. 1.2 Reconstructing Life in a Test Tube. 1.3 The Search for Traces of Primitive Life. 1.4 The Search for Life in the Solar System. 1.5 The Search for Life Beyond the Solar System. 1.6 Conclusions. 1.7 Further Reading. 2 From the Big Bang to the Molecules of Life (Harry J. Lehto). 2.1 Building Blocks of Life. 2.2 Big Bang: Formation of H and He. 2.3 First Stars: Formation of Small Amounts of C, O, N, S and P and Other Heavy Elements. 2.4 Normal Modern Stars, Bulk Formation of C, O, N, S, P and Other Heavy Elements. 2.5 The First Molecules (CO and H2O). 2.6 Interstellar Matter. 2.7 Generation of Stars: Formation of the Sun and Planets. 2.8 Further Reading. 2.9 Questions for Students. 3 Basic Prebiotic Chemistry (Herve). 3.1 Key Molecules of Life. 3.2 Historical Milestones. 3.3 Sources of Prebiotic Organic Molecules. 3.4 From Simple to Slightly More Complex Compounds. 3.5 Conclusions. 3.6 Further Reading. 3.7 Questions for Students. 4 From Molecular Evolution to Cellular Life (Kirsi Lehto). 4.1 History of Life at Its Beginnings. 4.2 Life as It Is Known. 4.3 Last Universal Common Ancestor (LUCA). 4.4 "Life" in the RNA-Protein World: Issues and Possible Solutions. 4.5 "Life" Before the Appearance of the Progenote. 4.6 The RNA World. 4.7 Beginning of Life. 4.8 Further Reading. 4.9 Questions for Students. 5 Extremophiles, the Physicochemical Limits of Life (Growth and Survival) (Helga Stan-Lotter). 5.1 A Brief History of Life on Earth. 5.2 Extremophiles and Extreme Environments. 5.3 Microbial Survival of Extreme Conditions. 5.4 Conclusions. 5.5 Further Reading. 5.6 Questions for Students. 6 Habitability (Charles S. Cockell). 6.1 A Brief History of the Assessment of Habitability. 6.2 What Determines Habitability? 6.3 Uninhabited Habitable Worlds. 6.4 Factors Determining Habitability. 6.5 A Postulate for Habitability. 6.6 Some Test Cases for Habitability. 6.7 Conclusions. 6.8 Further Reading. 6.9 Questions for Students. 7 Astrodynamics and Technological Aspects of Astrobiology Missions in Our Solar System (Stefanos Fasoulas and Tino Schmiel). 7.1 Introduction. 7.2 The Rocket Equation. 7.3 Orbital Mechanics and Astrodynamics. 7.4 Orbital Maneuvers. 7.5 Example: Missions to Mars. 7.6 Further Reading. 7.7 Questions for Students. 8 Astrobiology of the Terrestrial Planets, with Emphasis on Mars (Monica M. Grady). 8.1 The Solar System. 8.2 Terrestrial Planets. 8.3 Further Reading. 8.4 Questions for Students. 9 Astrobiology of Saturn's Moon Titan (Francois Raulin). 9.1 Extraterrestrial Bodies of Astrobiological Interest. 9.2 Some Historical Milestones in the Exploration of Titan. 9.3 General Properties, Formation and Internal Structure of Titan. 9.4 Atmosphere and Surface of Titan. 9.5 Astrobiological Aspects of Titan. 9.6 Outlook: Astrobiology and Future Exploration of Titan. 9.7 Further Reading. 9.8 Questions for Students. 10 Jupiter's Moon Europa: Geology and Habitability (Christophe Sotin and Daniel Prieur). 10.1 A Short Survey of the Past Exploration of Europa. 10.2 Geology of the Moon Europa. 10.3 Internal Structure of the Moon Europa. 10.4 Models of Evolution of the Moon Europa. 10.5 Astrobiological Considerations about Possibilities for Life on the Moon Europa. 10.6 Summary and Conclusions. 10.7 Outlook and Plans for Future Missions. 10.8 Further Reading. 10.9 Questions for Students. 11 Astrobiology Experiments in Low Earth Orbit: Facilities, Instrumentation, and Results (Pietro Baglioni, Massimo Sabbatini, and Gerda Horneck). 11.1 Low Earth Orbit Environment, a Test Bed for Astrobiology. 11.2 Astrobiology Questions Tackled by Experiments in Earth Orbit. 11.3 Exposure Facilities for Astrobiology Experiments. 11.4 Results from Astrobiology Experiments in Earth Orbit. 11.5 Future Development and Applications of Exposure Experiments. 11.6 Further Reading. 11.7 Questions for Students. 12 Putting Together an Exobiology Mission: The ExoMars Example (Jorge L. Vago and Gerhard Kminek). 12.1 Background of the ExoMars Mission. 12.2 ExoMars Science Objectives. 12.3 ExoMars Science Strategy. 12.4 ExoMars Mission Description. 12.5 Outlook and Conclusions. 12.6 Further Reading. 12.7 Questions for Students. 13 Astrobiology Exploratory Missions and Planetary Protection Requirements (Gerda Horneck, Andre Debus, Peter Mani, and J. Andrew Spry). 13.1 Rationale and History of Planetary Protection. 13.2 Current Planetary Protection Guidelines. 13.3 Implementation of Planetary Protection Guidelines. 13.4 Astrobiology Exploratory Missions of Concern to Planetary Protection. 13.5 Outlook: Future Tasks of Planetary Protection. 13.6 Further Reading. 13.7 Questions for students. Index.

Presidential Address to the Mineralogical Society of America, Salt Lake City, October 18, 2005: Mineral surfaces and the prebiotic selection and organization of biomolecules

One of the most enigmatic steps in Earth.s ancient transition from a lifeless planet to a living world was the process or processes by which prebiotic organic molecules were selected, concentrated, and organized into the essential macromolecules of life. More than a half-century of theory and experiment points to the critical roles of mineral surfaces in the assembly of proteins, lipid bilayers, and genetic polymers. This review considers three aspects of this problem: (1) the self-assembly of lipids, which may be enhanced in the presence of minerals; (2) the role of minerals in polymerization of amino acids and nucleic acids; and (3) the selective adsorption of organic species, including chiral molecules, onto mineral surfaces.

Prebiotic materials from on and off the early Earth

One of the greatest puzzles of all time is how did life arise? It has been universally presumed that life arose in a soup rich in carbon compounds, but from where did these organic molecules come? In this article, I will review proposed terrestrial sources of prebiotic organic molecules, such as Miller-Urey synthesis (including how they would depend on the oxidation state of the atmosphere) and hydrothermal vents and also input from space. While the former is perhaps better known and more commonly taught in school, we now know that comet and asteroid dust deliver tons of organics to the Earth every day, therefore this flux of reduced carbon from space probably also played a role in making the Earth habitable. We will compare and contrast the types and abundances of organics from on and off the Earth given standard assumptions. Perhaps each process provided specific compounds (amino acids, sugars, amphiphiles) that were directly related to the origin or early evolution of life. In any case, whether planetary, nebular or interstellar, we will consider how one might attempt to distinguish between abiotic organic molecules from actual signs of life as part of a robotic search for life in the Solar System.

Observations of Molecules in Comets

Comets are among the most primitive bodies of the solar system, and their chemical composition is rich in information on the protosolar nebula and its possible connection with interstellar cloud chemistry. Comets are also a source of light atoms and, possibly, prebiotic organic molecules for the early Earth. We know better and better the abundances of cometary volatiles through spectroscopy, mainly at infrared and radio wavelengths. Another crucial component of cometary matter - organic refractories - is still poorly characterized, however.We summarize here the ˜30 abundances and ˜20 upper limits obtained on cometary volatiles and highlight a few species and problems: ethylene glycol, NS, HNC/HCN, ^{14}N/^{15}N, the origin of CN, CS_{2}, PAHs, and H_{2}O. Comet-to-comet variations and comet internal heliocentric variations can now be studied, and cometary comas can be mapped with a variety of techniques. We list a number of temperature indicators, for they can help understand the relation between IS and cometary matters, which present both a global similarity but also marked differences, such as the high ethylene glycol content of comets.We conclude by outlining a few key problems to be addressed by future ground-based and space instruments or by cometary sample analysis. For many species, we stress that laboratory data are missing on spectroscopy, photodissociation and collisions with H_{2}O.

Life in the solar system

Life, defined as a chemical system capable of transferring its molecular information via self-replication and also capable of evolving, must develop within a liquid to take advantage of the diffusion of complex molecules. On Earth, life probably originated from the evolution of reduced organic molecules in liquid water. Organic matter might have been formed in the primitive Earth's atmosphere or near hydrothermal vents. A large fraction of prebiotic organic molecules might have been brought by extraterrestrial-meteoritic and cometary dust grains decelerated by the atmosphere. Any celestial body harboring permanent liquid water may therefore accumulate the ingredients that generated life on the primitive Earth. The possibility that life might have evolved on early Mars when water existed on the surface marks it as a prime candidate in a search for bacterial life beyond the Earth. Europa has an icy carapace. However, cryovolcanic flows at the surface point to a possible water subsurface region which might harbor a basic life form. The atmosphere and surface components of Titan are also of interest to exobiology for insight into a hydrocarbon-rich chemically evolving world. One-handed complex molecules and preferential isotopic fractionation of carbon, common to all terrestrial life forms, can be used as basis indicators when searching for life beyond the Earth.

Mutual catalysis in sets of prebiotic organic molecules: Evolution through computer simulated chemical kinetics

A thorough outlook on the origin of life needs to delineate a chemically rigorous, self-consistent path from highly heterogeneous, random ensembles of relatively simple organic molecules, to an entity that has rudimentary life-like characteristics. Such entity should be endowed with a capacity to express variation, undergo mutation-like changes and manifest a simple evolutionary process. For simulating such system we developed the Graded Autocatalysis Replication Domain (GARD) model for explicit kinetic analysis of mutual catalysis in sets of random oligomers derived from energized precursor monomers. The kinetic properties of the GARD model are based on vesicle enclosure and expansion. With the additional assumption of spontaneous vesicle splitting, a GARD evolution scenario is envisaged as a consequence of pure chemical kinetics. Here we show how the GARD model can serve as a platform for investigating the dynamics of self-organization mechanisms in molecular evolutionary processes.

Endogenous synthesis of prebiotic organic molecules

The necessary condition for the synthesis of organic compounds on the primitive earth is the presence of reducing conditions. This means an atmosphere of CH4, CO, or CO2 + H2. The atmospheric nitrogen can be N2 with a trace of NH3, but NH4(+) is needed in the ocean at least for amino acid synthesis. Many attempts have been made to use CO2 + H2O atmospheres for prebiotic synthesis, but these give at best extremely low yields of organic compounds, except in the presence of H2. Even strong reducing agents such as FeS + H2S or the mineral assemblages of the submarine vents fail to give significant yields of organic compounds with CO2. There appears to be a high kinetic barrier to the non-biological reduction of CO2 at low temperatures using geological reducing agents. The most abundant source of energy for prebiotic synthesis is ultraviolet light followed by electric discharges, with electric discharges being more efficient, although it is not clear which was the important energy source. Photochemical process would also make significant contributions. In an atmosphere Of CO2, N2, and H2O with no H2, the production rates of HCN and H2CO would be very low, 0.001 or less than that of a relatively reducing atmosphere. The concentration of organic compounds under these non-reducing conditions would be so low that there is doubt whether the concentration mechanism would be adequate for further steps toward the origin of life. A number of workers have calculated the influx of comets and meteorites on the primitive earth as a source of organic compounds. We conclude that while some organic material was added to the earth from comets and meteorites the amount available from these sources at a given time was at best only a few percent of that from earth bases syntheses under reducing conditions.

Were micrometeorites a source of prebiotic molecules on the early Earth?

“Interplanetary Dust Particles” with sizes ≈ 10 μm collected in the stratosphere (IDPs), as well as much larger “giant” micrometeorites retrieved from Antarctic ice melt water (AMMs), are mostly composed of unequilibrated assemblages of minerals, thus being related to primitive unequilibrated meteorites. Two independent evaluations of the mass flux of micrometeorites measuring ≈ 50 μm to ≈ 200 μm, recovered from either the Greenland or the Antarctic ice sheets have been reported (≈ 20,000 tons/a). A comparison with recent evaluation of the flux of meteorites reaching the Earth's surface (up to masses of 10,000 tons), indicates that micrometeorites represent about 99.5% of the extraterrestrial material falling on the Earth's surface each year. As they show carbon concentrations exceeding that of the most C-rich meteorite (Orgueil), they are the major contributors of extraterrestrial C-rich matter accreting to the Earth today. Moreover they are complex microstructured aggregates of grains. They contain not only a variety of C-rich matter, such as a new “dirty” magnetite phase enriched in P, S, and minor elements, but also a diversity of potential catalysts (hydrous silicates, oxides, sulfides and metal grains of Fe/Ni composition, etc.). They could have individually functioned on the early Earth, as “micro-chondritic-reactors” for the processing of prebiotic organic molecules in liquid water. Future progress requires the challenging development of meaningful laboratory simulation experiments, and a better understanding of the partial reprocessing of micrometeorites in the atmosphere.

Cometary delivery of organic molecules to the early Earth.

It has long been speculated that Earth accreted prebiotic organic molecules important for the origins of life from impacts of carbonaceous asteroids and comets during the period of heavy bombardment 4.5 x 10(9) to 3.8 x 10(9) years ago. A comprehensive treatment of comet-asteroid interaction with the atmosphere, surface impact, and resulting organic pyrolysis demonstrates that organics will not survive impacts at velocities greater than about 10 kilometers per second and that even comets and asteroids as small as 100 meters in radius cannot be aerobraked to below this velocity in 1-bar atmospheres. However, for plausible dense (10-bar carbon dioxide) early atmospheres, we find that 4.5 x 10(9) years ago Earth was accreting intact cometary organics at a rate of at least approximately 10(6) to 10(7) kilograms per year, a flux that thereafter declined with a half-life of approximately 10(8) years. These results may be put in context by comparison with terrestrial oceanic and total biomasses, approximately 3 x 10(12) kilograms and approximately 6 x 10(14) kilograms, respectively.

Correction: An optical yield that increases with temperature in a photochemically induced enantiomeric isomerization

TRANSFER of chirality and multiplication of chiral molecules are key processes in the chemical and biochemical transformations of natural and synthetic compounds. For entropic reasons, structural immobilization in enzymatic and catalytic processes and the use of low temperatures should generally enhance the optical yield (preference for one enantiomer) in chiral reactions. Studies of the temperature dependence of optical yield in chemical and biochemical processes have, however, been sporadic1–10. Here we report a systematic study of the temperature dependence of the photosensitized enantiodifferentiating isomerization of a simple alkene. We find that above a critical temperature, characteristic of the photosensitizer used, the optical yield increases with increasing temperature, apparently conflicting with the widely accepted view that lower temperatures favour a higher optical yield. Thus both enantiomers may be produced with high efficiency using a single chiral source. This implies that transfer and multiplication of chirality in natural systems, an important aspect of the development of prebiotic organic molecules, may be effected rather more simply than has hitherto been supposed.

An optical yield that increases with temperature in a photochemically induced enantiomeric isomerization

TRANSFER of chirality and multiplication of chiral molecules are key processes in the chemical and biochemical transformations of natural and synthetic compounds. For entropic reasons, structural immobilization in enzymatic and catalytic processes and the use of low temperatures should generally enhance the optical yield (preference for one enantiomer) in chiral reactions. Studies of the temperature dependence of optical yield in chemical and biochemical processes have, however, been sporadic1–10. Here we report a systematic study of the temperature dependence of the photosensitized enantiodifferentiating isomerization of a simple alkene. We find that above a critical temperature, characteristic of the photosensitizer used, the optical yield increases with increasing temperature, apparently conflicting with the widely accepted view that lower temperatures favour a higher optical yield. Thus both enantiomers may be produced with high efficiency using a single chiral source. This implies that transfer and multiplication of chirality in natural systems, an important aspect of the development of prebiotic organic molecules, may be effected rather more simply than has hitherto been supposed.