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PubMed1998

X-ray emission from clusters and groups of galaxies.

Recent major advances in x-ray imaging and spectroscopy of clusters have allowed the determination of their mass and mass profile out to approximately 1/2 the virial radius. In rich clusters, most of the baryonic mass is in the gas phase, and the ratio of mass in gas/stars varies by a factor of 2-4. The baryonic fractions vary by a factor of approximately 3 from cluster to cluster and almost always exceed 0.09 h50-[3/2] and thus are in fundamental conflict with the assumption of Omega = 1 and the results of big bang nucleosynthesis. The derived Fe abundances are 0.2-0.45 solar, and the abundances of O and Si for low redshift systems are 0.6-1.0 solar. This distribution is consistent with an origin in pure type II supernova. The amount of light and energy produced by these supernovae is very large, indicating their importance in influencing the formation of clusters and galaxies. The lack of evolution of Fe to a redshift of z approximately 0.4 argues for very early enrichment of the cluster gas. Groups show a wide range of abundances, 0.1-0.5 solar. The results of an x-ray survey indicate that the contribution of groups to the mass density of the universe is likely to be larger than 0.1 h50-2. Many of the very poor groups have large x-ray halos and are filled with small galaxies whose velocity dispersion is a good match to the x-ray temperatures.

Astronomical Phenomena↗
PubMed1998

Oxygen and evolutionary patterns in the sea: onshore/offshore trends and recent recruitment of deep-sea faunas.

Over the last 15 years a striking pattern of diversification has been documented in the fossil record of benthic marine invertebrates. Higher taxa (orders) tend to originate onshore, diversify offshore, and retreat into deep-water environments. Previous studies attribute this macroevolutionary pattern to a variety of causes, foremost among them the role of nearshore disturbance in providing opportunities for the evolution of novel forms accorded ordinal rank. Our analysis of the post-Paleozoic record of ordinal first appearances indicates that the onshore preference of ordinal origination occurred only in the Mesozoic prior to the Turonian stage of the Cretaceous, a period characterized by relatively frequent anoxic/dysoxic bottom conditions in deeper marine environments. Later, in the Cretaceous and Cenozoic, ordinal origination of benthic organisms did not occur exclusively, or even preferentially, in onshore environments. This change in environmental pattern of ordinal origination roughly correlates with Late Cretaceous: (i) decline in anoxia/dysoxia in offshore benthic environments; (ii) extinction of faunas associated with dysoxic conditions; (iii) increase in bioturbation with the expansion of deep burrowing forms into offshore environments; and (iv) offshore expansion of bryozoan diversity. We also advance a separate argument that the Cenomanian/Turonian and latest Paleocene global events eliminated much of the deep-water benthos. This requires a more recent origin of modern vent and deep-sea faunas, from shallower water refugia, than the Paleozoic or early Mesozoic origin of these faunas suggested by other workers.

Animals↗
PubMed1999

Is redshift-dependent evolution of galaxies a theoretical artifact?

The physical validity of the hypothesis of (redshift-dependent) luminosity evolution in galaxies is tested by statistical analysis of an intensively studied complete high-redshift sample of normal galaxies. The necessity of the evolution hypothesis in the frame of big-bang cosmology is confirmed at a high level of statistical significance; however, this evolution is quantitatively just as predicted by chronometric cosmology, in which there is no such evolution. Since there is no direct observational means to establish the evolution postulated in big-bang studies of higher-redshift galaxies, and the chronometric predictions involve no adjustable parameters (in contrast to the two in big-bang cosmology), the hypothesized evolution appears from the standpoint of conservative scientific methodology as a possible theoretical artifact.

Astronomical Phenomena↗
PubMed2000

Fullerenes: an extraterrestrial carbon carrier phase for noble gases.

In this work, we report on the discovery of naturally occurring fullerenes (C60 to C400) in the Allende and Murchison meteorites and some sediment samples from the 65 million-year-old Cretaceous/Tertiary boundary layer (KTB). Unlike the other pure forms of carbon (diamond and graphite), fullerenes are extractable in an organic solvent (e.g., toluene or 1,2,4-trichlorobenzene). The recognition of this unique property led to the detection and isolation of the higher fullerenes in the Kratschmer/Huffmann arc evaporated graphite soot and in the carbon material in the meteorite and impact deposits. By further exploiting the unique ability of the fullerene cage structure to encapsulate and retain noble gases, we have determined that both the Allende and Murchison fullerenes and the KTB fullerenes contain trapped noble gases with ratios that can only be described as extraterrestrial in origin.

Carbon↗
PubMedDate not supplied

The early atmosphere: a new picture.

Over the last several years, many of the fundamental ideas concerning the composition and chemical evolution of the Earth's early atmosphere have changed. While many aspects of this subject are clouded--either uncertain or unknown, a new picture is emerging. We are just beginning to understand how astronomical, geochemical, and atmospheric processes each contributed to the development of the gaseous envelope around the third planet from the sun some 4.6 billion years ago and how that envelope chemically evolved over the history of our planet. Simple compounds in that gaseous envelope, energized by atmospheric lightning and/or solar ultraviolet radiation, formed molecules of increasing complexity that eventually evolved into the first living systems on our planet. This process is called "chemical evolution" and immediately preceded biological evolution; once life developed and evolved, it began to alter the chemical composition of the atmosphere that provided the very essence of its creation. Photosynthetic organisms which have the ability to biochemically transform carbon dioxide and water to carbohydrates, which they use for food, produce large amounts of molecular oxygen (O2) as a by-product of the reaction. Atmospheric oxygen photochemically formed ozone, which absorbs ultraviolet radiation from the sun and shields the Earth's surface from this biologically lethal radiation. Once atmospheric ozone levels increased sufficiently, life could leave the safety of the oceans and go ashore for the first time. Throughout the history of our planet, there has been strong interaction between life and the atmosphere. Understanding our cosmic roots is particularly relevant as we embark on a search for life outside the Earth. At this very moment, several radio telescopes around the world are searching for extraterrestrial intelligence (SETI).

Atmosphere↗
PubMed1987

On the evolution and activity of cometary nuclei.

The thermal evolution of a spherical cometary nucleus (initial radius of 2.5 km), composed initially of very cold amorphous ice and moving in comet Halley's orbit, is simulated numerically for 280 revolutions. It is found that the phase transition from amorphous to crystalline ice constitutes a major internal heat source. The transition does not occur continuously, but in five distinct rounds, during the following revolutions: 1, 7, 40-41, 110-112, and 248-252. Due to the (slow) heating of the amorphous ice between crystallization rounds, the phase transition front advances into the nucleus to progressively greater depths: 36 m on the first round, and then 91 m, 193 m, 381 m, and 605 m respectively. Each round of crystallization starts when when the boundary between amorphous and crystalline ice is brought to approximately 15 m below the surface, as the nucleus radius decreases due to sublimation. At the time of crystallization, the temperature of the transformed ice rises to 180 K. According to experimental studies of gas-laden amorphous ice, a large fraction of the gas trapped in the ice at low temperatures is released. Whereas some of the released gas may find its way out through cracks in the crystalline ice layer, the rest is expected to accumulate in gas pockets that may eventually explode, forming "volcanic calderas." The gas-laden amorphous ice thus exposed may be a major source of gas and dust jets into the coma, such as those observed on comet Halley by the Giotto spacecraft. The activity of new comets and, possibly, cometary outbursts and splits may also be explained in terms of explosive gas release following the transition from amorphous to crystalline ice.

Crystallization↗
PubMed1984

Photochemistry of the atmosphere of Titan: comparison between model and observations.

The photochemistry of simple molecules containing carbon, hydrogen, nitrogen, and oxygen atoms in the atmosphere of Titan has been investigated using updated chemical schemes and our own estimates of a number of key rate coefficients. Proper exospheric boundary conditions, vertical transport, and condensation processes at the tropopause have been incorporated into the model. It is argued that he composition, climatology, and evolution of Titan's atmosphere are controlled by five major processes: (a) photolysis and photosensitized dissociation of CH4; (b) conversion of H to H2 and escape of hydrogen; (c) synthesis of higher hydrocarbons; (d) coupling between nitrogen and hydrocarbons; (e) coupling between oxygen and hydrocarbons. Starting with N2, CH4, and H2O, and invoking interactions with ultraviolet sunlight, energetic electrons, and cosmic rays, the model satisfactorily accounts for the concentrations of minor species observed by the Voyager IRIS and UVS instruments. Photochemistry is responsible for converting the simpler atmospheric species into more complex organic compounds, which are subsequently condensed at the tropopause and deposited on the surface. Titan might have lost 5.6 x 10(4), 1.8 x 10(3), and 4.0 g cm-2, or the equivalent of 8, 0.25, and 5 x 10(-4) bars of CH4, N2, and CO, respectively, over geologic time. Implications of abiotic organic synthesis on Titan for the origin of life on Earth are briefly discussed.

Atmosphere↗
PubMed2000

Microbialite morphostratigraphy as a tool for correlating Late Cambrian-Early Ordovician sequences.

Microbialite morphostratigraphy is a new tool for intrabasinal correlation using diverse microbialite structures (morphotypes). The recognition of the succession of morphotypes over constrained temporal intervals and broad areas is a function of the complex interactions that operate to create the structure. Because so many nonlinked variables (e.g., biotic, sedimentological, physicochemical) are involved, similar morphotypes do not reoccur over long temporal intervals. To demonstrate the technique, the upper Cambrian-lowermost Ordovician shelf strata of the Great Basin, United States, were correlated using both morphostratigraphy and standard lithostratigraphy. Six morphozones and one morphosubzone were recognized, as were four main lithologic successions. Because the boundaries between the morphozones and lithologic successions did not coincide, it is inferred that the characteristics of the various microbialite structures are not solely controlled by physical factors. The principles for establishing a morphostratigraphy outlined in this article allow for the potential to correlate along other ancient marine margins in both the same Cambrian and Ordovician interval, as well as any interval in the Phanerozoic in which diverse microbialite structures occur.

Biological Evolution↗
PubMed1989

Paleosols and their relevance to Precambrian atmospheric composition

Various paleosols have been reported from within the Witwatersrand and Ventersdorp Supergroups, South Africa. They were studied in an attempt to constrain the amount of oxygen available in the atmosphere during deposition of the gold- and uranium-bearing Witwatersrand conglomerates. The majority of these horizons do not have any physical characteristics of paleosols, and none of them have a chemistry consistent with weathering, suggesting they have been subjected to modification by later alteration processes. A similar chemistry, indicating overprinting of any original soil chemistry, has been reported from paleosols elsewhere (Elliot Lake region, Canada; Hekpoort basalt, South Africa), but it does not appear that significant cognizance has been taken of this fact when using these paleosols to determine the composition of the Precambrian atmosphere. It is concluded here that characteristics previously attributed to Precambrian weathering in an oxygen-deficient atmosphere are better explained by post-burial, hydrothermal alteration along lithological contacts.

Aerosols↗
PubMed2003

A search for soot from global wildfires in central Pacific Cretaceous-Tertiary boundary and other extinction and impact horizon sediments.

Hypotheses of global wildfires following the Cretaceous-Tertiary (KT) boundary impact are supported by high concentrations of elemental carbon (3.6 mg cm(-2)) and soot (1.8 mg cm(-2)) in DSDP Site 465, which was located several thousand kilometers from potential continental sources at 65 Ma. Soot is not preserved at four other central Pacific KT localities, but this is attributed to loss during oxic diagenesis. We find no evidence for wildfires related to major impacts in the late Eocene or to Ir anomalies and extinctions in the late Cenomanian.

Carbon↗
PubMed2003

Environmental consequences of impact cratering events as a function of ambient conditions on Earth.

The end of the Mesozoic Era is defined by a dramatic floral and faunal turnover that has been linked with the Chicxulub impact event, thus leading to the realization that impact cratering can affect both the geologic and biologic evolution of Earth. However, the environmental consequences of an impact event and any subsequent biological effects rely on several factors, including the ambient environmental conditions and the extant ecosystem structures at the time of impact. Some of the severest environmental perturbations of the Chicxulub impact event would not have been significant in some periods of Earth history. Consequently, the environmental and biological effects of an impact event must be evaluated in the context in which it occurs.

Air↗
PubMed2003

Large aerial bursts: an important class of terrestrial accretionary events.

Large aerial bursts similar to the 1908 Tunguska bolide but much larger in magnitude have surely been responsible for many catastrophic events in the history of the Earth. Because aerial bursts produce shallow (or even negligible) craters, their existence is difficult to document in the geological record. Even aerial bursts as small as Tunguska deposit enough energy to melt approximately 1mm of dry soil. Silica-rich glass formed in such melts has the potential to survive in the soil for many Ma, thus a potential indicator of large aerial bursts is glass that was formed as thick regions within silicate melt sheets. The layered tektites from Southeast Asia and the Libyan desert glass may have formed by a combination of sedimentation and downslope flow of silicate melt heated by radiation from large aerial bursts. The alternative, formation of layered tektites as crater ejecta, cannot account for observations such as uniformly high 10Be contents, the orientation of the magnetic remanence field, and the absence of splash-form (e.g., teardrop or dumbbell) tektites in regions where layered tektites are common. The largest asteroids or comets make craters no matter what their strength. Recent reviews suggest that, for events in the energy range up to 10(19)-10(20) J (about two orders of magnitude larger than the Meteor Crater impact), aerial bursts are more likely than cratering events, and the layered tektites of Southeast Asia imply the existence of aerial bursts one to two orders of magnitude larger still.

Air↗
PubMed2003

Lossless compression of stromatolite images: a biogenicity index?

It has been underappreciated that inorganic processes can produce stromatolites (laminated macroscopic constructions commonly attreibuted to microbiological activity), thus calling into question the long-standing use of stromatolites as de facto evidence for ancient life. Using lossless compression on unmagnified reflectance red-green-blue (RGB) images of matched stromatolite-sediment matrix pairs as a complexity metric, the compressibility index (delta(c), the log ratio of the ratio of the compressibility of the matrix versus the target) of a putative abiotic test stromatolite is significantly less than the delta(c) of a putative biotic test stromatolite. There is a clear separation in delta(c) between the different stromatolites discernible at the outcrop scale. In terms of absolute compressibility, the sediment matrix between the stromatolite columns was low in both cases, the putative abiotic stromatolite was similar to the intracolumnar sediment, and the putative biotic stromatolite was much greater (again discernible at the outcrop scale). We propose tht this metric would be useful for evaluating the biogenicity of images obtained by the camera systems available on every Mars surface probe launched to date including Viking, Pathfinder, Beagle, and the two Mars Exploration Rovers.

Algorithms↗
PubMed2003

Necessary, but not sufficient: Raman identification of disordered carbon as a signature of ancient life.

To identify microscopic particles as actual fossil material, it would be useful to have a means of unambiguously recognizing which carbonaceous deposits found in rocks are residues from once-living organisms (i.e., biogenic material). Those residues consist of many different, mostly aromatic (i.e., benzene ring-containing), C-O-H-dominated molecules, and typically are called kerogens. Raman microprobe spectroscopy can be applied to minute samples of ancient kerogens either isolated from their host rocks or in situ in thin section. The Raman spectra generated by monochromatic blue or green laser excitation (e.g., at 488, 514, 532 nm) typically show only generic spectral features indicative of discontinuous arrays of condensed benzene rings (i.e., structures referred to as "disordered carbonaceous material"). Thus, despite the complex chemistry of kerogens and the expected presence of H, O, and N, the Raman spectra typically do not show any evidence of functional groups, such as CH, CH(2), CH(3), CO, and CN. Moreover, the same kind of Raman spectral signature as is obtained from kerogens also is obtained from many other poorly ordered carbonaceous materials that arise through nonbiological processes, such as in situ heating of organic or inorganic compounds (whether or not they are of biological origin), metamorphic mobilization of preexisting carbon compounds, and high-temperature precipitation from hydrothermal solutions. Thus, neither a Raman spectrum, nor a Raman image derived from such spectra, definitively can identify a sample as "kerogen," but only as "disordered carbonaceous material." Clearly, the fact that small, opaque grains consist of disordered carbonaceous material is necessary, but not sufficient, to prove them to be residues of cellular material and, thus, biogenic.

Carbon↗
PubMed2003

Earth's earliest biosphere-a proposal to develop a collection of curated archean geologic reference materials.

The discovery of evidence indicative of life in a Martian meteorite has led to an increase in interest in astrobiology. As a result of this discovery, and the ensuing controversy, it has become apparent that our knowledge of the early development of life on Earth is limited. Archean stratigraphic successions containing evidence of Earth's early biosphere are well preserved in the Pilbara Craton of Western Australia. The craton includes part of a protocontinent consisting of granitoid complexes that were emplaced into, and overlain by, a 3.51-2.94 Ga volcanigenic carapace - the Pilbara Supergroup. The craton is overlain by younger supracrustal basins that form a time series recording Earth history from approximately 2.8 Ga to approximately 1.9 Ga. It is proposed that a well-documented suite of these ancient rocks be collected as reference material for Archean and astrobiological research. All samples would be collected in a well-defined geological context in order to build a framework to test models for the early evolution of life on Earth and to develop protocols for the search for life on other planets.

Archaea↗
PubMed2003

Spectral behavior of hydrated sulfate salts: implications for Europa mission spectrometer design.

Remote sensing of the surface of Europa with near-infrared instruments has suggested the presence of hydrated materials, including sulfate salts. Attention has been focused on these salts for the information they might yield regarding the evolution of a putative interior ocean, and the evaluation of its astrobiological potential. These materials exhibit distinct infrared absorption features due to bound water. The interactions of this water with the host molecules lead to fine structure that can be used to discriminate among these materials on the basis of their spectral behavior. This fine structure is even more pronounced at the low temperatures prevalent on icy satellites. Examination of hydrated sulfate salt spectra measured under cryogenic temperature conditions provides realistic constraints for future remote-sensing missions to Europa. In particular, it suggests that a spectrometer system capable of 2-5 nm spectral resolution or better, with a spatial resolution approaching 100 m, would be able to differentiate among proposed hydrated surface materials, if present, and constrain their distributions across the surface. Such information would provide valuable insights into the evolutionary history of Europa.

Astronomical Phenomena↗
PubMed2003

Tírez lake as a terrestrial analog of Europa.

Tírez Lake (La Mancha, central Spain) is proposed as a terrestrial analogue of Europa's ocean. The proposal is based on the comparison of the hydrogeochemistry of Tírez Lake with the geochemical features of the alteration mineralogy of meteoritic precursors and with Galileo's Near Infrared Mapping Spectrometer data on Europa's surface. To validate the astrobiological potential of Tírez Lake as an analog of Europa, different hydrogeochemical, mineral, and microbial analyses were performed. Experimental and theoretical modeling helped to understand the crystallization pathways that may occur in Europa's crust. Calculations about the oxidation state of the hypothetical Europan ocean were estimated to support the sulfate-rich neutral liquid model as the origin of Europa's observed hydrated minerals and to facilitate their comparison with Tírez's hydrogeochemistry. Hydrogeochemical and mineralogical analyses showed that Tírez waters corresponded to Mg-Na-SO(4)-Cl brines with epsomite, hexahydrite, and halite as end members. A preliminary microbial ecology characterization identified two different microbial domains: a photosynthetically sustained community represented by planktonic/benthonic forms and microbial mat communities, and a subsurficial anaerobic realm in which chemolithotrophy predominates. Fluorescence in situ hybridization has been used to characterize the prokaryotic diversity of the system. The subsurficial community seemed to be dominated by sulfate-reducing bacteria and methanogens. Frozen Tírez brines were analyzed by Fourier-transform infrared techniques providing spectra similar to those reported previously using pure components and to the Galileo spectral data. Calorimetric measurements of Tírez brines showed pathways and phase metastability for magnesium sulfate and sodium chloride crystallization that may aid in understanding the processes involved in the formation of Europa's icy crust. The use of fluorescence hybridization techniques for microbial detection and characterization in hypersaline environments makes this methodology strongly advisable for future Europa astrobiological missions.

Chemical Phenomena↗