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At least 19 recordsLinked to original sources
PubMed1982

Criteria for the emergence and evolution of life in the solar system.

During the past years we have explored most of the bodies of the solar system by means of the Apollo, Venera, Viking, Voyager, and other space missions. We are now in a better position to be able to compare the conditions of other planets and satellites with those of the Earth in order to determine what is unique about our planet which permitted the emergence and evolution of life on it. On the basis of this and other available scientific information we have arrived at the conclusion that there are at least some twentyfive specific conditions or requirements which have to be fulfilled in order for life as we know it to appear and evolve in a planetary system such as ours. Most of these necessary conditions or requirements are mutually interdependent, but in order to discuss their role in depth they have been divided into five major general areas which are discussed in some detail herein. Planetary criteria, which relate to the physical properties of the planet as it is formed and as it becomes a differentiated cosmic body and potential abode of life. The mass, orbital characteristics and energetic relationships with the central star as well as the discrete separation of gas, liquid and solid phases of the planet are of utmost importance. Chemical criteria, which are concerned with the composition, availability of effective energy sources, and chemical constraints (solvent, pH range, redox potential) of the environment(s) where reactions take place for the prebiological formation of biochemical compounds. Protobiological criteria, which relate to the prebiologically synthesized oligomeric and polymeric biomolecules, how they interact cooperatively to form protobiological structures and functions (replication, catalysis, information transfer, etc.) and self-assemble to give rise to a living system. Evolutionary criteria, which are concerned with the processes responsible for the increase in complexity of organisms by genomic multiplication, symbiotic integration and cellular differentiation, as well as with the negentropic ability of organisms to continuously recycle all the volatile biogenic elements. Altogether these processes made possible the development and evolution of life from the simplest prokaryotic cell ancestor to a cognitive and manipulative multicellular organism (man). In order to extend this inquiry to other systems beyond our solar system a fifth set of requirements based on astronomical observations is also discussed, namely, the Stellar criteria, which relate to the elemental composition mass, lifetime, and other features of Main Sequence stars which may be surrounded by planetary systems similar to our own. Finally, a brief review is made on the probability of the existence of extraterrestrial life as well as of civilizations capable of interstellar communication in our Galaxy.

Biological Evolution↗
PubMed2000

Extinct (129)I in halite from a primitive meteorite: evidence for evaporite formation in the early solar system

Halite crystals from the Zag H3-6 chondrite contain essentially pure (monoisotopic) xenon-129 ((129)Xe) produced in the early history of the solar system by the decay of short-lived iodine-129 ((129)I) (half-life = 15.7 million years). Correlated release of (129)Xe and (128)Xe, produced artificially from (127)I by neutron irradiation, corresponds to an initial ((129)I/(127)I) ratio of (1.35 +/- 0.05) x 10(-4), close to the most primitive early solar system value. If the (129)Xe was produced by in situ decay, then the halite formed from an aqueous fluid within 2 million years of the oldest known solar system minerals.

Journal Article↗
PubMed1997

The early solar system.

Life arose on an early Earth which was the product to the conditions present, and processes operating, during formation of the solar system. The formation and early state of the solar system are reviewed in order to better understand the nature of the early Earth, and to constrain the conditions present during the origin and early evolution of life on this planet.

Cold Temperature↗
PubMed1999

The origin of chaos in the outer solar system

Classical analytic theories of the solar system indicate that it is stable, but numerical integrations suggest that it is chaotic. This disagreement is resolved by a new analytic theory. The theory shows that the chaos among the jovian planets results from the overlap of the components of a mean motion resonance among Jupiter, Saturn, and Uranus, and provides rough estimates of the Lyapunov time (10(7) years) and the dynamical lifetime of Uranus (10(18) years). The jovian planets must have entered the resonance after all the gas and most of the planetesimals in the protoplanetary disk were removed.

Journal Article↗
PubMed1996

Hf-W Isotopic Evidence for Rapid Accretion and Differentiation in the Early Solar System

The time scales over which inner solar system objects accreted and differentiated are unclear because the isotopic systems of many meteorites are disturbed. 182Hf decays to 182W with a half-life of 9 million years and is a particularly useful chronometer because both elements are highly refractory and immobile. Tungsten isotopic data for IIA, IIIA, IVA, and anomalous iron meteorites and H, L, and LL chondrites indicate that their parent bodies accreted rapidly and segregated metal within just a few million years.

Journal Article↗
PubMed1999

Mass-independent isotope effects in planetary atmospheres and the early solar system.

A class of isotope effects that alters isotope ratios on a mass-independent basis provides a tool for studying a wide range of processes in atmospheres of Earth and other planets as well as early processes in the solar nebula. The mechanism for the effect remains uncertain. Mass-independent isotopic compositions have been observed in O3, CO2, N2O, and CO in Earth's atmosphere and in carbonate from a martian meteorite, which suggests a role for mass-independent processes in the atmosphere of Mars. Observed mass-independent meteoritic oxygen and sulfur isotopic compositions may derive from chemical processes in the presolar nebula, and their distributions could provide insight into early solar system evolution.

Atmosphere↗
PubMed1999

The formation of Uranus and Neptune in the Jupiter-Saturn region of the Solar System.

Planets are believed to have formed through the accumulation of a large number of small bodies. In the case of the gas-giant planets Jupiter and Saturn, they accreted a significant amount of gas directly from the protosolar nebula after accumulating solid cores of about 5-15 Earth masses. Such models, however, have been unable to produce the smaller ice giants Uranus and Neptune at their present locations, because in that region of the Solar System the small planetary bodies will have been more widely spaced, and less tightly bound gravitationally to the Sun. When applied to the current Jupiter-Saturn zone, a recent theory predicts that, in addition to the solid cores of Jupiter and Saturn, two or three other solid bodies of comparable mass are likely to have formed. Here we report the results of model calculations that demonstrate that such cores will have been gravitationally scattered outwards as Jupiter, and perhaps Saturn, accreted nebular gas. The orbits of these cores then evolve into orbits that resemble those of Uranus and Neptune, as a result of gravitational interactions with the small bodies in the outer disk of the protosolar nebula.

Evolution, Planetary↗
PubMed1984

Exobiology and the solar system: the Cassini mission to Titan.

The recent Voyager mission and the simulation experiments in the laboratory suggest that a complex nitrogen-organic chemistry is occuring at the periphery of Titan. Thus, this satellite of Saturn appears as a privileged place in the solar system for the study of extraterrestrial organic chemistry which can be considered as part of Exobiology. Projects of space mission relating to Titan are already under investigation, in particular with the "CASSINI" proposal. The CASSINI project is a combination of a Saturn orbiter and a Titan probe mission. Such a mission would allow the first study "in situ" of a complex extraterrestrial organic chemistry in atmospheric phase.

Animals↗
PubMed2000

The ascent of parity-violation: exochirality in the solar system and beyond

We review recent developments stemming from Stephen Mason's work on the origin of homochirality, focusing in particular on the parity-violating energy difference (PVED) between enantiomers. We summarize results of calculations of the PVED--both our own and those of other groups--which show that the natural enantiomers are indeed favoured by the weak force in most cases. The PVED has become important not only to explain the selection of the L-amino acids in the origin of life, but also as a "molecular footprint" of fundamental physics, leading to proposals to derive values of the Weinberg angle from future spectroscopic measurements of the PVED. The new field of exochirality--chirality outside the Earth--is now taking off, with reports of excesses of L-amino acids in meteorites, and proposals to look for homochirality as a signature of life on other planets and even in other solar systems. If it was indeed the PVED that determined life's handedness, we would expect to find L-amino acids rather than D everywhere in the universe.

Journal Article↗
PubMed1999

Interiors of giant planets inside and outside the solar system.

An understanding of the structure and composition of the giant planets is rapidly evolving because of (i) high-pressure experiments with the ability to study metallic hydrogen and define the properties of its equation of state and (ii) spectroscopic and in situ measurements made by telescopes and satellites that allow an accurate determination of the chemical composition of the deep atmospheres of the giant planets. However, the total amount of heavy elements that Jupiter, Saturn, Uranus, and Neptune contain remains poorly constrained. The discovery of extrasolar giant planets with masses ranging from that of Saturn to a few times the mass of Jupiter opens up new possibilities for understanding planet composition and formation. Evolutionary models predict that gaseous extrasolar giant planets should have a variety of atmospheric temperatures and chemical compositions, but the radii are estimated to be close to that of Jupiter (between 0.9 and 1.7 Jupiter radii), provided that they contain mostly hydrogen and helium.

Astronomical Phenomena↗
PubMed1997

Analogs of the early solar system.

Within the last few decades, the existence of protoplanetary disks has been inferred on the basis of emission from T Tauri stars that does not arise from a stellar photosphere. More recently, high-resolution interferometric techniques have resolved the dust continuum emission, and millimeter arrays have imaged circumstellar molecular gas. These measurements corroborate the disk interpretation; many T Tauri stars are surrounded by centrifugally supported circumstellar disks with radial sizes of order 100 AU. Further proof issues from Hubble Space Telescope images of disks that are illuminated externally. The morphology of circumstellar dust is revealed in striking detail and affirms the prevalence and dimensions of disks imaged at longer wavelengths. The fate of circumstellar material around young stars must be understood in order to discern the degree to which these disks are proto-planetary. Observational studies of circumstellar disks which are in the beginning of a dispersal phase are challenging and place great demands on astronomical techniques. Nevertheless, the connection between disks and the formation of extra-solar planets is supported by increasing circumstantial evidence. Optically thin dust continuum emission persists in T Tauri stars and is detected around some young main sequence stars. Since the dust is subject to rapid dispersal by radiation pressure and Poynting-Robertson drag, some mechanism of replenishment is required. Disks around nearby young main sequence stars show evidence for inner voids and disk asymmetries that should also disappear on short timescales. The presence of large orbiting bodies which collide and interact with the resulting debris can explain both the persistence of optically thin dust and the maintenance of otherwise-ephemeral dynamical features. Together with recent detections of extra-solar planets, these observations lend some support to the hypothesis that circumstellar disks commonly give birth to planetary systems.

Astronomy↗