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PubMed2000

Supernova Sources and the 92Nb-92Zr p-Process Chronometer.

We report new Zr isotope evidence for live (92)Nb (mean life: tau&d1;92Nb=52 Myr) within the early solar system resulting in &parl0;92Nb&solm0;93Nb&parr0;initial approximately 10-3. The meteoritic minerals rutile and zircon have, respectively, very high and very low Nb/Zr ratios and are ideal for exploring the (92)Nb-(92)Zr chronometer. Rutiles exhibit high positive straightepsilon92Zr ( approximately 14-36) while a zircon has a negative straightepsilon92Zr ( approximately -4), as would be expected if (92)Nb was live in the early solar system. The meteoritic rutiles appear to be young, with apparent times of formation of approximately 80-220 Myr subsequent to the origin of the solar system. The initial (92)Nb/(92)Mo for the solar system is broadly compatible with a model of uniform production if the (92)Nb/(92)Mo production ratio for Type II supernova (SNII) sources with neutrino-driven winds is used. Data for all the now extinct p-process nuclides ((92)Nb, (97)Tc, and (146)Sm) are consistent with these isotopes being derived by uniform production from SNII sources and a free decay interval of approximately 10 Myr. Consideration of a range of models indicates that the average p-process production ratio of (92)Nb/(92)Mo needs to be at least in the range of 0.06-0.25.

Journal Article↗
PubMed2001

Planetary exploration in the time of astrobiology: protecting against biological contamination.

These are intriguing times in the exploration of other solar-system bodies. Continuing discoveries about life on Earth and the return of data suggesting the presence of liquid water environments on or under the surfaces of other planets and moons have combined to suggest the significant possibility that extraterrestrial life may exist in this solar system. Similarly, not since the Viking missions of the mid-1970s has there been as great an appreciation for the potential for Earth life to contaminate other worlds. Current plans for the exploration of the solar system include constraints intended to prevent biological contamination from being spread by solar-system exploration missions.

Exobiology↗
PubMed1997

Physics and chemistry of the solar nebula.

The solar system is thought to have begun in a flattened disk of gas and dust referred to traditionally as the solar nebula. Such a construct seems to be a natural product of the collapse of dense parts of giant molecular clouds, the vast star-forming regions that pepper the Milky Way and other galaxies. Gravitational, magnetic and thermal forces within the solar nebula forced a gradual evolution of mass toward the center (where the sun formed) and angular momentum (borne by a small fraction of the mass) toward the outer more distant regions of the disk. This evolution was accompanied by heating and a strong temperature contrast from the hot, inner regions to the cold, more remote parts of the disk. The resulting chemistry in the disk determined the initial distribution of organic matter in the planets; most of the reduced carbon species, in condensed form, were located beyond the asteroid belt (the 'outer' solar system). The Earth could have received much of its inventory of pre-biological material from comets and other icy fragments of the process of planetary formation in the outer solar system.

Astronomy↗
PubMed2002

A short timescale for terrestrial planet formation from Hf-W chronometry of meteorites.

Determining the chronology for the assembly of planetary bodies in the early Solar System is essential for a complete understanding of star- and planet-formation processes. Various radionuclide chronometers (applied to meteorites) have been used to determine that basaltic lava flows on the surface of the asteroid Vesta formed within 3 million years (3 Myr) of the origin of the Solar System. Such rapid formation is broadly consistent with astronomical observations of young stellar objects, which suggest that formation of planetary systems occurs within a few million years after star formation. Some hafnium-tungsten isotope data, however, require that Vesta formed later (approximately 16 Myr after the formation of the Solar System) and that the formation of the terrestrial planets took a much longer time (62(-14)(+4504) Myr). Here we report measurements of tungsten isotope compositions and hafnium-tungsten ratios of several meteorites. Our measurements indicate that, contrary to previous results, the bulk of metal-silicate separation in the Solar System was completed within <30 Myr. These results are completely consistent with other evidence for rapid planetary formation, and are also in agreement with dynamic accretion models that predict a relatively short time (approximately 10 Myr) for the main growth stage of terrestrial planet formation.

Journal Article↗
PubMed2002

Detection of carbonates in dust shells around evolved stars.

Carbonates on large Solar System bodies like Earth and Mars (the latter represented by the meteorite ALH84001) form through the weathering of silicates in a watery (CO3)2- solution. The presence of carbonates in interplanetary dust particles and asteroids (again, represented by meteorites) is not completely understood, but has been attributed to aqueous alteration on a large parent body, which was subsequently shattered into smaller pieces. Despite efforts, the presence of carbonates outside the Solar System has hitherto not been established. Here we report the discovery of the carbonates calcite and dolomite in the dust shells of evolved stars, where the conditions are too primitive for the formation of large parent bodies with liquid water. These carbonates, therefore, are not formed by aqueous alteration, but perhaps through processes on the surfaces of dust or ice grains or gas phase condensation. The presence of carbonates which did not form by aqueous alteration suggests that some of the carbonates found in Solar System bodies no longer provide direct evidence that liquid water was present on large parent bodies early in the history of the Solar System.

Astronomical Phenomena↗
PubMed1989

Use of Martian resources in a Controlled Ecological Life Support System (CELSS).

The exploration of Mars has long been considered as a major goal in the exploration of the Solar system. The Space Station Freedom will make such missions feasible because it will provide a site for the assembly and launch of the large vehicles required. Interest in manned visits to Mars often focus on the possibility of collecting information about the origin of that planet, & hence of the solar system, including the Earth. Interest also involves the history of the planet, its past record of geological and fluvial activity, atmospheric and thermal history and surface chemical activity. The latter is of particular interest to exobiologists who would like to seek evidence of pre-biological physical and chemical activity involving organic molecules. Finally, there is interest in the possibility of planetary ecosynthesis, i.e. specific intervention in the evolution of Mars that could result in the development of a second habitable planet in the solar system. The scenarios for visits and the establishment of bases on Mars are being developed now. The intent of this paper is to consider various possibilities for crew life support on Mars and particularly to explore the use of Martian resources as life support materials.

Carbon Dioxide↗
PubMed2004

The universe: a cryogenic habitat for microbial life.

Panspermia, an ancient idea, posits that microbial life is ubiquitous in the Universe. After several decades of almost irrational rejection, panspermia is at last coming to be regarded as a serious contender for the beginnings of life on our planet. Astronomical data is shown to be consistent with the widespread distribution of complex organic molecules and dust particles that may have a biological provenance. A minuscule (10(-21)) survival rate of freeze-dried bacteria in space is all that is needed to ensure the continual re-cycling of cosmic microbial life in the galaxy. Evidence that terrestrial life may have come from elsewhere in the solar system has accumulated over the past decade. Mars is seen by some as a possible source of terrestrial life, but some hundreds of billions of comets that enveloped the entire solar system, are a far more likely primordial reservoir of life. Comets would then have seeded Earth, Mars, and indeed all other habitable planetary bodies in the inner regions of the solar system. The implications of this point of view, which was developed in conjunction with the late Sir Fred Hoyle since the 1970s, are now becoming amenable to direct empirical test by studies of pristine organic material in the stratosphere. The ancient theory of panspermia may be on the verge of vindication, in which case the entire universe would be a grand crucible of cryomicrobiology.

Astronomical Phenomena↗
PubMedDate not supplied

Non-light based ecosystems and bioastronomy.

The possibility of the existence of life beyond planet Earth has always fascinated humans. However, due to certain circumstances such as the failure of the Viking expeditions to detect any sign of biotic activity on Mars, and the understanding that the presence of life would lead to drastic alterations in the atmosphere of the host planet (alterations that have never been detected on other planets or planetoids of the solar system), the belief that our planet is the only planet to sustain life inside the solar system originated. During the last three decades a series of new complex biological communities have been discovered, in the deep sea, inside caves isolated from the external biosphere, and deep inside the crust of our planet, and found to depend on geothermal energy instead of solar energy for their survival. These discoveries give us new evidence and hope that life might exist not only on other planets, but perhaps even in other planetoids of our solar system. Life may exist in regions other than the surface of a planet, and these areas would be extremely difficult to identify.

Ecosystem↗
PubMed1987

Europa, tidally heated oceans, and habitable zones around giant planets.

Tidal dissipation in the satellites of a giant planet may provide sufficient heating to maintain an environment favorable to life on the satellite surface or just below a thin ice layer. In our own solar system, Europa, one of the Galilean satellites of Jupiter, could have a liquid ocean which may occasionally receive sunlight through cracks in the overlying ice shell. In such case, sufficient solar energy could reach liquid water that organisms similar to those found under Antarctic ice could grow. In other solar systems, larger satellites with more significant heat flow could represent environments that are stable over an order of Aeons and in which life could perhaps evolve. We define a zone around a giant planet in which such satellites could exist as a tidally-heated habitable zone. This zone can be compared to the habitable zone which results from heating due to the radiation of a central star. In our solar system, this radiatively-heated habitable zone contains the Earth.

Biomass↗
PubMed1991

Urey Prize Lecture: Planetary Evolution and the Origin of Life.

The origin of life appears to be closely tied to the formation and early evolution of the solar system. Key questions deal with the source of abiotic organic material on the early Earth, the nature of interstellar organic material and its relationship to the observed organic compounds in the outer solar system, and the possible origin of life on Mars early in its history. From the perspective of planetary environments, liquid water is the essential requirement for life and serves as a surrogate indicator for life. New models and analyses in conjunction with data returned from upcoming missions promise to significantly advance our knowledge of how life originated in our solar system.

Awards and Prizes↗
PubMed1994

Provenance of the terrestrial planets.

Earlier work on the simultaneous accumulation of the asteroid belt and the terrestrial planets is extended to investigate the relative contribution to the final planets made by material from different heliocentric distances. As before, stochastic variations intrinsic to the accumulation processes lead to a variety of final planetary configurations, but include systems having a number of features similar to our solar system. Fifty-nine new simulations are presented, from which thirteen are selected as more similar to our solar system than the others. It is found that the concept of "local feeding zones" for each final terrestrial planet has no validity for this model. Instead, the final terrestrial planets receive major contributions from bodies ranging from 0.5 to at least 2.5 AU, and often to greater distances. Nevertheless, there is a correlation between the final heliocentric distance of a planet and its average provenance. Together with the effect of stochastic fluctuations, this permits variation in the composition of the terrestrial planets, such as the difference in the decompressed density of Earth and Mars. Biologically important light elements, derived from the asteroidal region, are likely to have been significant constituents of the Earth during its formation.

Astronomical Phenomena↗
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↗
PubMed2006

Isotopic enhancements of 17O and 18O from solar wind particles in the lunar regolith.

Differences in isotopic abundances between meteorites and rocks on Earth leave unclear the true composition of the gas out of which the Solar System formed. The Sun should have preserved in its outer layers the original composition, and recent work has indicated that the solar wind is enriched in 16O, relative to Earth, Mars and bulk meteorites. This suggests that self-shielding of CO due to photo-dissociation, which is a well understood process in molecular clouds, also led to evolution in the isotopic abundances in the early Solar System. Here we report measurements of oxygen isotopic abundances in lunar grains that were recently exposed to the solar wind. We find that 16O is underabundant, opposite to an earlier finding based on studies of ancient metal grains. Our result, however, is more difficult to understand within the context of current models, because there is no clear way to make 16O more abundant in Solar System rocks than in the Sun.

Journal Article↗
PubMed2006

Carbon molecules in space: from astrochemistry to astrobiology.

How complex carbonaceous molecules in space are, what their abundance is and on what timescales they form are crucial questions within cosmochemistry. Despite the large heterogeneity of galactic and interstellar regions the organic chemistry in the universe seems to follow common pathways. The largest fraction of carbon in the universe is incorporated into aromatic molecules (gaseous polycyclic aromatic hydrocarbon as well as solid macromolecular aromatic structures). Macromolecular carbon constitutes more than half of the interstellar carbon, approximately 80% of the carbon in meteorites, and is likely to be present in comets. Molecules of high astrobiological relevance such as N-heterocycles, amino acids and pre-sugars have all been identified in trace quantities (ppb) in extracts of carbonaceous meteorites. Their presence in inter- and circumstellar regions is either unknown or contentious. In any event such fragile species are easily destroyed by UV radiation, shocks and thermal processing and are unlikely to survive incorporation into Solar System material without some degradation. The more refractory material, in particular macromolecular carbon may retain an interstellar heritage more faithfully. We present laboratory measurements on the photostability of organic compounds and discuss their survival in regions with elevated UV radiation. We also show recent observations of diffuse interstellar bands indicating the presence of fullerenes. We investigate the link between the carbon chemistry in interstellar space and in the Solar System by analyzing the carbonaceous fraction of meteorites and by reviewing stable isotopic data. It also seems evident that both volatile and refractory material from carbonaceous meteoritic has been substantially altered owing to thermal and aqueous processing within the Solar System.

Carbon↗
PubMed2002

Diverse supernova sources of pre-solar material inferred from molybdenum isotopes in meteorites.

Variations in the isotopic composition of some components in primitive meteorites demonstrate that the pre-solar material was not completely homogenized, nor was it processed at sufficiently high temperatures to erase the signatures of the diverse stellar sources. This is in accord with the observation that accretion disks of young stellar objects are at relatively low temperatures. Carbonaceous chondrites are considered to represent the 'average' Solar System composition; the rare pre-solar grains in the matrixes of carbonaceous chondrites have been used to identify some sources of the pre-solar material. Here we report that the molybdenum isotopic composition of bulk carbonaceous chondrites is distinctly different from the accepted average solar value. We show that the Mo data require the presence of material produced in at least two different r-processes, and that the contribution from the p-process material is decoupled from the r-process, all occurring in supernova explosions. This is consistent with the emerging picture of diverse sources inferred from short-lived isotopes in the early Solar System and elemental analyses of metal-poor stars.

Journal Article↗
PubMed2005

Extreme collisions between planetesimals as the origin of warm dust around a Sun-like star.

The slow but persistent collisions between asteroids in our Solar System generate a tenuous cloud of dust known as the zodiacal light (because of the light the dust reflects). In the young Solar System, such collisions were more common and the dust production rate should have been many times larger. Yet copious dust in the zodiacal region around stars much younger than the Sun has rarely been found. Dust is known to orbit around several hundred main-sequence stars, but this dust is cold and comes from a Kuiper-belt analogous region out beyond the orbit of Neptune. Despite many searches, only a few main-sequence stars reveal warm (> 120 K) dust analogous to zodiacal dust near the Earth. Signs of planet formation (in the form of collisions between bodies) in the regions of stars corresponding to the orbits of the terrestrial planets in our Solar System have therefore been elusive. Here we report an exceptionally large amount of warm, small, silicate dust particles around the solar-type star BD+20,307 (HIP 8920, SAO 75016). The composition and quantity of dust could be explained by recent frequent or huge collisions between asteroids or other 'planetesimals' whose orbits are being perturbed by a nearby planet.

Journal Article↗
PubMed2002

The FUBI system for solar rating nonprescription eyewear.

BACKGROUND: In an effort to develop a comprehensive method of rating nonprescription eyewear for its ability to protect the eye against solar damage, the FUBI (Fashion, Ultraviolet, Blue, and Infrared) System is presented. The system presents a numeric value, from Oto 100, for each of the three known harmful portions of the solar spectrum: ultraviolet UV), blue/violet (B), and infrared (IR). A fourth value was determined for the fashion (F) of the eyewear as it relates to protection of the eye against reflected or scattered radiation that is not transmitted through the eyewear. METHODS: The numeric value of the system for UV, B, and IR was derived by taking the average transmittance of radiation through each tested lens and weighting it by multiplying that value by a relative toxicity factor (RTF) for each waveband of solar radiation tested. The RTF was derived by multiplying the approximate level of radiation reaching a specified anatomic part of the eye at sea level for each wavelength tested (Elambda) by the inverse of the value of its action spectrum (sensitivity, Slambda) on that part of the eye. This weighted average transmitted percentage of radiation was then deducted from 100 to derive the FUBI value for the UV, B, and IR range. The numeric value for F was derived by measuring the scattered or reflected light from five known sources of luminance at a fixed distance around opacified lenses on each tested frame. RESULTS: The FUBI values for six known commercial products of nonprescription eyewear are presented for comparison. CONCLUSIONS: The FUBI system presents a comprehensive, scientifically valid means of rating nonprescription eyewear for solar protective value. The system will provide consumers of nonprescription eyewear with more useful, comparative information about the protective qualities of eyewear against solar radiation.

Eye Protective Devices↗
PubMed2006

NASA and the search for life in the universe.

Almost from its beginnings in 1958, the National Aeronautics and Space Administration (NASA) set up a life-science program. Because one of the priorities of the organization is to search for life beyond Earth, NASA began designing spacecraft to unravel the mysteries of Mars. The effort to search for life on Mars culminated in the landing of two Viking spacecraft on the surface of the planet in 1976. Although the biology experiments conducted as part of these missions provided some evidence for the possibility of life, the scientific consensus was that they drew a blank. In 1996, however, the 'Mars rock' rekindled interest in life in our solar system. The discovery of an ocean on the Jovian moon Europa, of organic molecules on the Saturnian moon Titan and persuasive evidence that water once flowed on Mars suggests that the solar system is still of considerable exobiological interest. In addition, since 1995 approximately 175 planets have been found beyond our solar system. Although these discoveries are gas giants, NASA spacecraft might soon detect Earth-sized planets. The search for life in the universe continues.

Exobiology↗