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PubMed1996

Photopolymerization and Mass-Independent Sulfur Isotope Fractionations in Carbon Disulfide

Irradiation of gaseous carbon disulfide [CS2(g)] at 313 nanometers produces a dark brown aerosol of (CS2)x. Its thermal decomposition products include disulfur (S2), carbon monosulfide (CS), and (CS)x. The photopolymerization process is accompanied by a large mass-independent isotopic fractionation of sulfur (a 5 to 10 per mil sulfur-33 excess and a 61 to 84 per mil sulfur-36 deficit). Excess sulfur-33 has been observed in several classes of meteorites. Photochemical production of (CS2)x may be important in the origin and evolution of cosmochemical environments such as the presolar nebula, meteorites, asteroids, and planetary atmospheres.

Journal Article↗
PubMedDate not supplied

Life's origin: the cosmic, planetary and biological processes.

From elements formed in interstellar furnaces to humans peering back at the stars, the evolution of life has been a long, intricate and perhaps inevitable process. Life as we know it requires a planet orbiting a star at just the right distance so that water can exist in liquid form. It needs a rich supply of chemicals and energy sources. On Earth, the combination of chemistry and energy generated molecules that evolved ways of replicating themselves and of passing information from one generation to the next. Thus, the thread of life began. This chart traces the thread, maintained by DNA molecules for much of its history, as it weaves its way through the primitive oceans, gaining strength and diversity along the way. Organisms eventually moved onto the land, where advanced forms, including humans, ultimately arose. Finally, assisted by a technology of its own making, life has reached back out into space to understand its own origins, to expand into new realms, and to seek other living threads in the cosmos.

Animals↗
PubMed2006

H3+ cooling in primordial gas.

Simulations of the thermal and dynamical evolution of primordial gas typically focus on the role played by H2 cooling. H2 is the dominant coolant in low-density primordial gas and it is usually assumed that it remains dominant at high densities. However, H2 is not an effective coolant at high densities, owing to the low critical density at which it reaches local thermodynamic equilibrium and to the large opacities that develop in its emission lines. It is therefore important to quantify the contribution made to the cooling rate by emission from the other molecules and ions present in the gas. A particularly interesting candidate is the H3+ ion, which is known to be an effective coolant at high densities in planetary atmospheres. In this paper, we present results from simulations of the thermal and chemical evolution of gravitationally collapsing primordial gas, which include a detailed treatment of H3+ chemistry and an approximate treatment of H3+ cooling. We show that in most cases, the contribution from H3+ is too small to be important, but if a sufficiently strong ionizing background is present, then H3+ cooling may become significant.

Journal Article↗
PubMed1999

Sublimation from icy jets as a probe of the interstellar volatile content of comets.

Comets are some of the most primitive bodies left over from the Solar System's early history. They may preserve both interstellar material and material from the proto-solar nebula, and so studies of their volatile components can provide clues about the evolution of gases and ices, as a collapsing molecular cloud transforms into a mature planetary system. Previous observations of emission from rotational transitions in molecules have averaged over large areas of the inner coma, and therefore include both molecules that sublimed from the nucleus and those that result from subsequent chemical processes in the coma Here we present high-resolution observations of emission from the molecules HNC, DCN and HDO associated with comet Hale-Bopp. Our data reveal arc-like structures-icy jets-offset from (but close to) the nucleus. The measured abundance ratios on 1-3" scales are substantially different from those on larger scales, and cannot be accounted for by models of chemical processes in the coma; they are, however, similar to the values observed in the cores of dense interstellar clouds and young stellar objects. We therefore propose that sublimation from millimetre-sized icy grains ejected from the nucleus provides access to relatively unaltered volatiles. The D/H ratios inferred from our data suggest that, by mass, Hale-Bopp (and by inference the outer regions of the early solar nebula) consists of > or =15-40% of largely unprocessed interstellar material.

Deuterium↗
PubMed1989

The role of cometary particle coalescence in chemical evolution.

Important prebiotic organic compounds might have been transported to Earth in dust or produced in vapor clouds resulting from atmospheric explosions or impacts of comets. These compounds coalesced in the upper atmosphere with particles ejected from craters formed by impacts of large objects. Coalescence during exposure to UV radiation concentrated organic monomers and enhanced formation of oligomers. Continuing coalescence added material to the growing particles and shielded prebiotic compounds from prolonged UV radiation. These particles settled into the lower atmosphere where they were scavenged by rain. Aqueous chemistry and evaporation of raindrops containing nomomers in high temperature regions near the Earth's surface also promoted continued formation of oligomers. Finally, these oligomers were deposited in the oceans where continued prebiotic evolution led to the most primitive cell. Results of our studies suggest that prebiotic chemical evolution may be an inevitable consequence of impacting comets during the late accretion of planets anywhere in the universe if oceans remained on those planetary surfaces.

Amino Acids↗
PubMed2003

Biogeochemistry of hypersaline microbial mats illustrates the dynamics of modern microbial ecosystems and the early evolution of the biosphere.

Photosynthetic microbial mats are remarkably complete self-sustaining ecosystems at the millimeter scale, yet they have substantially affected environmental processes on a planetary scale. These mats may be direct descendents of the most ancient biological communities in which even oxygenic photosynthesis might have developed. Photosynthetic mats are excellent natural laboratories to help us to learn how microbial populations associate to control dynamic biogeochemical gradients.

Anaerobiosis↗
PubMed2004

"Species" radiations of symbiotic dinoflagellates in the Atlantic and Indo-Pacific since the Miocene-Pliocene transition.

Endosymbiotic dinoflagellates, or "zooxanthellae," are required for the survival of a diverse community of invertebrates that construct and dominate shallow, tropical coral reef ecosystems. Molecular systematics applied to this once understudied symbiont partner, Symbiodinium spp., divide the group into divergent lineages or subgeneric "clades." Within each clade, numerous closely related "types," or species, exhibit distinctive host taxon, geographic, and/or environmental distributions. This diversity is greatest in clade C, which dominates the Indo-Pacific host fauna and shares dominance in the Atlantic-Caribbean with clade B. Two "living" ancestors in this group, C1 and C3, are common to both the Indo-Pacific and Atlantic-Caribbean. With these exceptions, each ocean possesses a diverse clade C assemblage that appears to have independently evolved (adaptively radiated) through host specialization and allopatric differentiation. This phylogeographic evidence suggests that a worldwide selective sweep of C1/C3, or their progenitor, must have occurred before both oceans separated. The probable timing of this event corresponds with the major climactic changes and low CO(2) levels of the late Miocene and/or early Pliocene. Subsequent bursts of diversification have proceeded in each ocean since this transition. An ecoevolutionary expansion to numerous and taxonomically diverse hosts by a select host-generalist symbiont followed by the onset of rapid diversification suggests a radical process through which coral-algal symbioses respond and persist through the vicissitudes of planetary climate change.

Adaptation, Biological↗
PubMed2001

The organic content of the Tagish Lake meteorite.

The Tagish Lake meteorite fell last year on a frozen lake in Canada and may provide the most pristine material of its kind. Analyses have now shown this carbonaceous chondrite to contain a suite of soluble organic compounds (approximately 100 parts per million) that includes mono- and dicarboxylic acids, dicarboximides, pyridine carboxylic acids, a sulfonic acid, and both aliphatic and aromatic hydrocarbons. The insoluble carbon exhibits exclusive aromatic character, deuterium enrichment, and fullerenes containing "planetary" helium and argon. The findings provide insight into an outcome of early solar chemical evolution that differs from any seen so far in meteorites.

Amino Acids↗
PubMed1993

Gaia: goddess or thermostat.

This is a independent look at the concept of Gaia as a tightly coupled system, whose constituents are the biota and the physical environment. Within the system the biota, through normal mechanisms of evolution, unwittingly help regulate the environment within the bounds necessary for their survival. The example of maintaining a broad equilibrium of planetary transpiration is given. Some have given Gaia religious or poetical attributes, others see the concept as an intellectual tool which contributes to understanding of the relationship between animate and inanimate nature.

Animals↗
PubMedDate not supplied

Exobiology, the study of the origin, evolution and distribution of life within the context of cosmic evolution: a review.

The primary goal of exobiological research is to reach a better understanding of the processes leading to the origin, evolution and distribution of life on Earth or elsewhere in the universe. In this endeavour, scientists from a wide variety of disciplines are involved, such as astronomy, planetary research, organic chemistry, palaeontology and the various subdisciplines of biology including microbial ecology and molecular biology. Space technology plays an important part by offering the opportunity for exploring our solar system, for collecting extraterrestrial samples, and for utilizing the peculiar environment of space as a tool. Exobiological activities include comparison of the overall pattern of chemical evolution of potential precursors of life, in the interstellar medium, and on the planets and small bodies of our solar system; tracing the history of life on Earth back to its roots; deciphering the environments of the planets in our solar system and of their satellites, throughout their history, with regard to their habitability; searching for other planetary systems in our Galaxy and for signals of extraterrestrial civilizations; testing the impact of space environment on survivability of resistant life forms. This evolutionary approach towards understanding the phenomenon of life in the context of cosmic evolution may eventually contribute to a better understanding of the processes regulating the interactions of life with its environment on Earth.

Astronomical Phenomena↗
PubMed1986

Review of methodology and technology available for the detection of extrasolar planetary systems.

Anyone undertaking an interstellar voyage might wish to be assured of the existence of a safe planetary harbor at the other end! Aside from the obvious interest of the participants in this Symposium, astronomers and astrophysicists are also eager to detect and study other planetary systems in order to better understand the formation of our own Solar System. Scientists involved in the search for extraterrestrial intelligence argue that planets suitable for the evolution of life may abound elsewhere within our own Milky Way Galaxy. On theoretical grounds, they are probably correct, but they lack any observational support. For in spite of decades of claimed astrometric detections of planetary companions and the recent exciting and tantalizing observations from the IRAS satellite and the IR speckle observations of Van Biesbroeck 8 and other cool stars, there is no unambiguous proof for the existence of another planetary system beyond our own. In this paper we review the various methods for detecting extrasolar planets and briefly describe the Earth and space based technology currently available and discuss the near-term plans to implement these different search techniques. In each case an attempt is made to identify the limiting source of systematic error inherent to the methodology and to assess the potential for technological improvements.

Astronomical Phenomena↗
PubMed1990

Open systems living in a closed biosphere: a new paradox for the Gaia debate.

While energetically open, the biosphere is appreciably closed from the standpoint of matter exchange. Matter cycling and recycling is hence a necessary and emergent property of the global-scale system known as Gaia. But how can an aggregate of open-system life forms have evolved and persisted for billions of years within a planetary system that is largely closed to matter influx and outflow? The puzzling nature of a closed yet persistent biosphere draws our attention to the course of evolution of fundamental metabolic strategies and matter-capture techniques. It suggests a facet of the Gaia hypothesis, framed in terms of persistence. The oceans, atmosphere, soils and biota constitute a complex system which maintains and adjusts matter cycling and recycling within the constraints of planetary closure such that open-system forms of life can persist. This weaker version of the Gaia hypothesis may be useful because it readily lends itself to at least one form of test. What is the solution to the closed biosphere puzzle, and does it indicate that Gaia merits status as a discrete entity? We suggest several disciplines within the field of biology that might provide tools and perspectives toward reaching a solution. These disciplines include artificial closed ecosystems, prokaryote evolution, the nexus of thermodynamics and evolutionary biology, and hierarchy theory in ecosystem modeling and evolution theory.

Biological Evolution↗
PubMed2025

Logan: Planetary-Scale Genome Assembly Surveys Life's Diversity.

The breadth of life's diversity is unfathomable, but public nucleic acid sequencing data offers a window into the dispersion and evolution of genetic diversity across Earth. However the rapid growth and accumulation of sequence data have outpaced efficient analysis capabilities. The largest collection of freely available sequencing data is the Sequence Read Archive (SRA), comprising 27.3 million datasets or 5 × 1016 basepairs. To realize the potential of the SRA, we constructed Logan, a massive sequence assembly transforming short reads into long contigs and compressing the data over 100-fold, enabling highly efficient petabase-scale analysis. We created Logan-Search, a k-mer index of Logan for free planetary-scale sequence search, returning matches in minutes. We used Logan contigs to identify >200 million plastic-degrading enzyme homologs, and validate novel enzymes with catalytic activities exceeding current reference standards. Further, we vastly expand the known diversity of proteins (30-fold over UniRef50), plasmids (22-fold over PLSDB), P4 satellites (4.5-fold), and the recently described Obelisk RNA elements (3.7-fold). Logan also enables ecological and biomedical data mining, such as global tracking of antimicrobial resistance genes and the characterization of viral reactivation across millions of human BioSamples. By transforming the SRA, Logan democratizes access to the world's public genetic data and opens frontiers in biotechnology, molecular ecology, and global health.

Journal Article↗
PubMed2005

The role of phosphorus in chemical evolution.

In this tutorial review we consider the role of phosphorus and its compounds within the context of chemical evolution in galaxies. Following an interdisciplinary approach we first discuss the position of P among the main biogenic elements by considering its relevance in most essential biochemical functions as well as its peculiar chemistry under different physicochemical conditions. Then we review the phosphorus distribution in different cosmic sites, such as terrestrial planets, interplanetary dust particles, cometary dust, planetary atmospheres and the interstellar medium (ISM). In this way we realize that this element is both scarce and ubiquitous in the universe. These features can be related to the complex nucleosynthesis of P nuclide in the cores of massive stars under explosive conditions favouring a wide distribution of this element through the ISM, where it would be ready to react with other available atoms. A general tendency towards more oxidized phosphorus compounds is clearly appreciated as chemical evolution proceeds from circumstellar and ISM materials to protoplanetary and planetary condensed matter phases. To conclude we discuss some possible routes allowing for the incorporation of phosphorus compounds of prebiotic interest during the earlier stages of solar system formation.

Cosmic Dust↗
PubMed1984

Chemical and biological evolution in space.

Astronomical infrared spectra are used to confirm the existence of complex organic molecules produced by ultraviolet photoprocessing of interstellar grain mantles. This material is shown to be the major component of the interstellar grains between the sun and the galactic center and, by inference, constitutes more than 10 million solar masses--or close to one part in a thousand of the entire mass of the milky way galaxy. It may be demonstrated that the primitive chemistry of the earth's surface was dominated by these extraterrestrial molecules after aggregated into comets if the rate of comet impacts with the earth was comparable with that required to account for the extinction of species over the past 300 million years. Ultraviolet irradiation of bacterial spores has been studied for the first time under simulated interstellar conditions. The inactivation time predicted for the less dense regions of space is at most several hundred years. Within molecular clouds it is shown on theoretical and experimental grounds that this time may be extended to tens of million of years which is the estimated time required for their transport from one solar system to another by a molecular cloud. However survival of spores during their initial exposure to the solar ultraviolet presents a problem for panspermia because it requires that in the process of ejection from the earth's surface they must be enclosed within a cocoon (or mantle) of ultraviolet absorbing material of approximately 0.6 micrometer thickness. Thus, although panspermia can not be rejected on the basis of lack of interstellar survival there may remain insurmountable obstacles to its occurring because of the very special protective shield requirements during ejection from its planetary source.

Bacillus subtilis↗
PubMed2000

Daisyworld is Darwinian: constraints on adaptation are important for planetary self-regulation.

The Daisyworld model demonstrates that self-regulation of the global environment can emerge from competition amongst types of life altering their local environment in different ways. Robertson & Robinson (1998. J. theor. Biol.195, 129-134) presented what they describe as a "Darwinian Daisyworld" in which the ability of organisms to adapt their internal physiology in response to environmental change undermines their ability to regulate their environment. They assume that there are no bounds on the environmental conditions that organisms can adapt to and that equal growth rates can potentially be achieved under any conditions. If adaptation could respond sufficiently rapidly to changes in the environment, this would eliminate any need for the environment to be regulated in the first place, because all possible states of the environment would be equally tolerable to life. However, the thermodynamics, chemistry and structure of living organisms set bounds on the range of environmental conditions that can be adapted to. As these bounds are approached, environmental conditions limit growth rate, and adaptations necessary for survival can also cost energy. Here we take account of such constraints and find that environmental regulation is recovered in the Daisyworld model. Hence, we suggest that constraints are an important part of a self-regulating planetary system.

Adaptation, Physiological↗
PubMed2001

Planetary habitability: is Earth commonplace in the Milky Way?

Is there life beyond planet Earth? This is one of the grand enigmas which humankind tries to solve through scientific research. Recent progress in astronomical measurement techniques has confirmed the existence of a multitude of extra-solar planets. On the other hand, enormous efforts are being made to assess the possibility of life on Mars. All these activities have stimulated several investigations about the habitability of cosmic bodies. The habitable zone (HZ) around a given central star is defined as the region within which an Earth-like planet might enjoy the moderate surface temperatures required for advanced life forms. At present, there are several models determining the HZ. One class of models utilises climate constraints for the existence of liquid water on a planetary surface. Another approach is based on an integrated Earth system analysis that relates the boundaries of the HZ to the limits of photosynthetic processes. Within the latter approach, the evolution of the HZ for our solar system over geological time scales is calculated straightforwardly, and a convenient filter can be constructed that picks the candidates for photosynthesis-based life from all the extra-solar planets discovered by novel observational methods. These results can then be used to determine the average number of planets per planetary system that are within the HZ. With the help of a segment of the Drake equation, the number of "Gaias" (i.e. extra-solar terrestrial planets with a globally acting biosphere) is estimated. This leads to the thoroughly educated guess that there should exist half a million Gaias in the Milky Way.

Astronomical Phenomena↗
PubMed1984

NASA's Exobiology Program.

The goal of NASA's Exobiology Program is to understand the origin, evolution, and distribution of life, and life-related molecules, on Earth and throughout the universe. Emphasis is focused on determining how the rate and direction of these processes were affected by the chemical and physical environment of the evolving planet, as well as by planetary, solar, and astrophysical phenomena. This is accomplished by a multi-disciplinary program of research conducted by over 60 principal investigators in both NASA and university laboratories. Major program thrusts are in the following research areas: biogenic elements; chemical evolution; origin of life; organic geochemistry; evolution of higher life forms; solar system exploration; and the search for extraterrestrial intelligence (SETI).

Biological Evolution↗