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At least 19 recordsLinked to original sources
PubMed1998

Detection of centimeter-sized meteoroid impact events in Saturn's F ring.

Voyager images reveal that three prominent clumps in Saturn's F ring were short-lived, appearing rapidly and then spreading and decaying in brightness over periods of approximately 2 weeks. These features arise from hypervelocity impacts by approximately 10-centimeter meteoroids into F ring bodies. Future ring observations of these impact events could constrain the centimeter-sized component of the meteoroid population, which is otherwise unmeasurable but plays an important role in the evolution of rings and surfaces in the outer solar system. The F ring's numerous other clumps are much longer lived and appear to be unrelated to impacts.

Extraterrestrial Environment↗
PubMed1998

Accretion rate of cosmic spherules measured at the South Pole.

Micrometeorites are terrestrially collected, extraterrestrial particles smaller than about 1 mm, which account for most of the mass being accreted to the Earth. Compared with meteorites, micrometeorites more completely represent the Earth-crossing meteoroid complex and should include fragments of asteroids, comets, Mars and our Moon, as well as pre-solar and interstellar grains. Previous measurements of the flux of micrometeoroids that survive to the Earth's surface have large uncertainties owing to the destruction of particles by weathering, inefficiencies in magnetic collection or separation techniques, low particle counts, poor age constraint or highly variable concentrating processes. Here we describe an attempt to circumvent these problems through the collection of thousands of well preserved and dated micrometeorites from the bottom of the South Pole water well, which supplies drinking water for the Scott-Amundsen station. Using this collection, we have determined precise estimates of the flux and mass distribution for 50-700-microm cosmic spherules (melted micrometeorites). Allowing for the expected abundance of unmelted micrometeorites in the samples, our results indicate that about 90% of the incoming mass of submillimetre particles evaporates during atmospheric entry. Our data indicate the loss of glass-rich and small stony spherules from deep-sea deposits, and they provide constraints for models describing the survival probability of micrometeoroids.

Antarctic Regions↗
PubMed1995

Metal ions in the atmosphere of Neptune.

Microwave propagation experiments performed with Voyager 2 at Neptune revealed sharp layers of electrons in Neptune's lower ionosphere with densities of approximately 10(4) per cubic centimeter. These layers are reminiscent of the sporadic-E layers in the Earth's ionosphere, and when taken together with data from the other giant planets, these data confirm the importance of the magnetic field in layer formation. A photochemical model that incorporates species produced by meteoroid ablation predicts that singly ionized magnesium is the most likely metal to be found in the layers, although laboratory data on the kinetics of metallic atoms and ions in a reducing environment are lacking. The metal chemistry discussed here is directly relevant to the abundant metals observed at the impact site of the G fragment of comet Shoemaker Levy 9 on Jupiter.

Atmosphere↗
PubMed1996

Thin foil preparation of metal particles in brittle ceramic matrices.

Thin foil preparation for metal particles in brittle ceramic matrices is particularly difficult, due to differential thinning of the component materials. This paper considers the preparation of thin foils of stony meteorites. Stony meteorites have a composite nature consisting of small metal particles (< 200 microns) embedded in an inherently brittle silicate matrix. Specimens of this type are difficult to prepare as thin foils because the area of interest is limited to specific regions within the metal particles. The problems of thin foil preparation of metal particles in the stony meteorites were overcome by developing a technique involving electrochemical polishing prior to ion beam thinning. The metal particles were first separated from the silicate matrix and then embedded in Epo-Tek H20E silver epoxy. This specific epoxy was selected for its good electron conductivity which allowed the specimen to be thinned by the electrochemical polishing technique. Specimens prepared by this technique have permitted the direct observation and characterization of the metallic phases in stony meteorites.

Ceramics↗
PubMed1996

Hydrothermal systems on Mars: an assessment of present evidence.

Hydrothermal processes have been suggested to explain a number of observations for Mars, including D/H ratios of water extracted from Martian meteorites, as a means for removing CO2 from the Martian atmosphere and sequestering it in the crust as carbonates, and as a possible origin for iron oxide-rich spectral units on the floors of some rifted basins (chasmata). There are numerous examples of Martian channels formed by discharges of subsurface water near potential magmatic heat sources, and hydrothermal processes have also been proposed as a mechanism for aquifer recharge needed to sustain long term erosion of sapping channels. The following geological settings have been identified as targets for ancient hydrothermal systems on Mars: channels located along the margins of impact crater melt sheets and on the slopes of ancient volcanoes; chaotic and fretted terranes where shallow subsurface heat sources are thought to have interacted with ground ice; and the floors of calderas and rifted basins (e.g. chasmata). On Earth, such geological environments are often a locus for hydrothermal mineralization. But we presently lack the mineralogical information needed for a definitive evaluation of hypotheses. A preferred tool for identifying minerals by remote sensing methods on Earth is high spatial resolution, hyperspectral, near-infrared spectroscopy, a technique that has been extensively developed by mineral explorationists. Future efforts to explore Mars for ancient hydrothermal systems would benefit from the application of methods developed by the mining industry to look for similar deposits on Earth. But Earth-based exploration models must be adapted to account for the large differences in the climatic and geological history of Mars. For example, it is likely that the early surface environment of Mars was cool, perhaps consistently below freezing, with the shallow portions of hydrothermal systems being dominated by magma-cryosphere interactions. Given the smaller gravitational field, declining atmospheric pressure, and widespread, permeable megaregolith on Mars, volatile outgassing and magmatic cooling would have been more effective than on Earth. Thus, hydrothermal systems are likely to have had much lower average surface temperatures than comparable geological settings on Earth. The likely predominance of basaltic crust on Mars suggests that hydrothermal fluids and associated deposits should be enriched in Fe, Mg, Si and Ca, with surficial deposits being dominated by lower temperature, mixed iron oxide and carbonate mineralogies.

Deuterium↗
PubMed1996

The transfer of viable microorganisms between planets.

There is increasing acceptance that catastrophic cosmic impacts have played an important role in shaping the history of terrestrial life. Large asteroid and cometary impacts are also capable of displacing substantial quantities of planetary surface material into space. The discovery of Martian rocks on Earth suggests that viable microorganisms within such ejecta could be exchanged between planets. If this conjecture is correct, it will have profound implications for the origin and evolution of life in the solar system.

Archaea↗
PubMed1998

Life on Mars: chemical arguments and clues from Martian meteorites.

Primitive terrestrial life-defined as a chemical system able to transfer its molecular information via self-replication and to evolve-probably originated from the evolution of reduced organic molecules in liquid water. Several sources have been proposed for the prebiotic organic molecules: terrestrial primitive atmosphere (methane or carbon dioxide), deep-sea hydrothermal systems, and extraterrestrial meteoritic and cometary dust grains. The study of carbonaceous chondrites, which contain up to 5% by weight of organic matter, has allowed close examination of the delivery of extraterrestrial organic material. Eight proteinaceous amino acids have been identified in the Murchison meteorite among more than 70 amino acids. Engel reported that L-alanine was surprisingly more abundant than D-alanine in the Murchison meteorite. Cronin also found excesses of L-enantiomers for nonprotein amino acids. A large collection of micrometeorites has been recently extracted from Antarctic old blue ice. In the 50- to 100-micron size range, carbonaceous micrometeorites represent 80% of the samples and contain 2% of carbon, on average. They might have brought more carbon than that involved in the present surficial biomass. The early histories of Mars and Earth clearly show similarities. Liquid water was once stable on the surface of Mars, attesting the presence of an atmosphere capable of deccelerating C-rich micrometeorites. Therefore, primitive life may have developed on Mars as well and fossilized microorganisms may still be present in the near subsurface. The Viking missions to Mars in 1976 did not find evidence of either contemporary or past life, but the mass spectrometer on the lander aeroshell determined the atmospheric composition, which has allowed a family of meteorites to be identified as Martian. Although these samples are essentially volcanic in origin, it has been recognized that some of them contain carbonate inclusions and even veins that have a carbon isotopic composition indicative of an origin from Martian atmospheric carbon dioxide. The oxygen isotopic composition of these carbonate deposits allows calculation of the temperature regime existing during formation from a fluid that dissolved the carbon dioxide. As the composition of the fluid is unknown, only a temperature range can be estimated, but this falls between 0 degree and 90 degrees C, which would seem entirely appropriate for life processes. It was such carbonate veins that were found to host putative microfossils. Irrespective of the existence of features that could be considered to be fossils, carbonate-rich portions of Martian meteorites tend to have material, at more than 1000 ppm, that combusts at a low temperature; i.e., it is an organic form of carbon. Unfortunately, this organic matter does not have a diagnostic isotopic signature so it cannot be unambiguously said to be indigenous to the samples. However, many circumstantial arguments can be made to the effect that it is cogenetic with the carbonate and hence Martian. If it could be proved that the organic matter was preterrestrial, then the isotopic fractionation between it and the carbon is in the right sense for a biological origin.

Carbonates↗
PubMed1997

Life on Mars: a clue to life on Earth?

A martian meteorite has recently been claimed to show evidence of life and certainly shows the presence of organic matter. What might we learn about how life on Earth developed from studies of Mars?

Earth, Planet↗
PubMed1999

Extraterrestrial handedness: a reply.

Recent investigations of stable isotope ratios of amino acids from the Murchison meteorite have shown them to be of unambiguous extraterrestrial origin, and examinations of their enantiomeric compositions, where terrestrial contamination can be excluded, have found a consistent excess of L-enantiomers. One explanation for this observation has been the asymmetric photolysis of racemic extraterrestrial amino acids by circularly polarized light (CPL) in the synchrotron radiation from orbiting electrons around the pulsar remnants of supernovae. Mason (1997) has attempted to discredit this mechanism on the grounds that circular dichroism (CD) bands for optically active molecules alternate in sign and sum to zero over the entire spectrum, and hence enantioselective photochemical reactions cannot be induced by broad band CPL. We submit arguments disputing this conclusion and present reasons for expecting that broad band CPL synchrotron radiation would be quite capable of inducing asymmetric photolysis, particularly in aliphatic amino acids.

Amino Acids↗
PubMed1999

Flash heating on the early Earth.

It has been suggested that very large impact events (approximately 500 km diameter impactors) sterilized the surface of the young Earth by producing enough rock vapor to boil the oceans. Here, we consider surface heating due to smaller impactors, and demonstrate that surface temperatures conductive to organic synthesis resulted. In particular, we focus on the synthesis of thermal peptides. Previously, laboratory experiments have demonstrated that dry heating a mixture of amino acids containing excess Asp, Glu, or Lys to temperatures approximately 170 degrees C for approximately 2 hours yields polypeptides. It has been argued that such temperature conditions would not have been available on the early Earth. Here we demonstrate, by analogy with the K/T impact, that the requisite temperatures are achieved on sand surfaces during the atmospheric reentry of fine ejecta particles produced by impacts of bolides approximately 10-20 km in diameter, assuming approximately 1-100 PAL CO2. Impactors of this size struck the Earth with a frequency of approximately 1 per 10(4)-10(5) y at 4.2 Ga. Smaller bolides produced negligible global surface heating, whereas bolides > 30 km in diameter yielded solid surface temperatures > 1000 K, high enough to pyrolyze amino acids and other organic compounds. Thus, peptide formation would have occurred globally for a relatively narrow range of bolide sizes.

Amino Acids↗
PubMed1999

Molecular distribution of monocarboxylic acids in Asuka carbonaceous chondrites from Antarctica.

Molecular distribution of low-molecular-weight monocarboxylic acids was studied in three CM2 Asuka carbonaceous chondrites (A-881280, A-881334 and A-881458), which were recovered from Antarctica by the 29th Japanese Antarctic Research Expedition in 1988. GC and GC/MS analyses identified more than 30 monocarboxylic acids in A-881458, including aliphatic and aromatic acids with various structural isomers. Isomeric phenolic compounds were also identified. The aliphatic carboxylic acids have straight-chain structures having 2 to 12 carbon atoms (C2 to C12), and branched-chain structures (C4 to C9). The quantities of straight-chain acids decrease logarithmically with increasing carbon number. At the same carbon number, a straight-chain isomer is always predominant compared to branched-chain isomers. All of the 14 possible C4, C5 and C6 aliphatic monocarboxylic acids (not including optical isomers) have been identified, although all the isomers were not reported in Murchison and Y-791198 meteorites. Of the 17 possible isomeric C7 acids, at least 14 isomers were tentatively identified by mass spectra (EI and CI mode). At C8 or above, peaks of branched-chain isomers become obscure, probably due to the large number of isomers and small concentrations. Branched-chain monocarboxylic acids over C6 have never been reported in Muchison. Although occurrence of aliphatic acids are similar between A-881458 and Murchison at C4, C5 and C6 acids, a major difference is that A-881458 as well as Y-791198 have straight-chain predominance among isomers in contrast to Murchison being branched-chain predominant. In the case of isomeric aromatic compounds such as toluic acids and cresols, m-toluic acid and p-cresol are more abundant among their isomers, respectively. The molecular distribution may not reflect thermodynamic equilibrium but rather a kinetically controlled process for their formation mechanism. The other two CM2 chondrites (A-881280 and A-881334) were depleted in carboxylic acids in spite of similar carbon contents. The depletion is not due to weathering on ice, because the degrees of weathering are small and similar among the three chondrites. Probably those latter two chondrites may have been subjected to aqueous alteration or metamorphism on their meteorite parent bodies.

Antarctic Regions↗
PubMed1998

A search for extraterrestrial amino acids in carbonaceous Antarctic micrometeorites.

Antarctic micrometeorites (AMMs) in the 100-400 microns size range are the dominant mass fraction of extraterrestrial material accreted by the Earth today. A high performance liquid chromatography (HPLC) based technique exploited at the limits of sensitivity has been used to search for the extraterrestrial amino acids alpha-aminoisobutyric acid (AIB) and isovaline in AMMs. Five samples, each containing about 30 to 35 grains, were analyzed. All the samples possess a terrestrial amino acid component, indicated by the excess of the L-enantiomers of common protein amino acids. In only one sample (A91) was AIB found to be present at a level significantly above the background blanks. The concentration of AIB (approximately 280 ppm), and the AIB/isovaline ratio (> or = 10), in this sample are both much higher than in CM chondrites. The apparently large variation in the AIB concentrations of the samples suggests that AIB may be concentrated in rare subset of micrometeorites. Because the AIB/isovaline ratio in sample A91 is much larger than in CM chondrites, the synthesis of amino acids in the micrometeorite parent bodies might have involved a different process requiring an HCN-rich environment, such as that found in comets. If the present day characteristics of the meteorite and micrometeorite fluxes can be extrapolated back in time, then the flux of large carbonaceous micrometeorites could have contributed to the inventory of prebiotic molecules on the early Earth.

Amino Acids↗
PubMed1998

Observation of indigenous polycyclic aromatic hydrocarbons in 'giant' carbonaceous antarctic micrometeorites.

Two-step laser desorption/laser ionization mass spectrometry (microL2 MS) was used to establish the nature and mass distribution of polycyclic aromatic hydrocarbons (PAHs) in fragments of fifteen 'giant' (approximately 200 microns) carbonaceous Antarctic micrometeorites (AMMs). Detectable concentrations of PAHs were observed in all AMMs showing a fine-grained matrix. The range of integrated PAH signal intensities varied between samples by over two orders of magnitude. No evidence of contamination whilst in the Antarctic environment could be found. The dramatic variation of both PAH signal intensities and mass distributions between AMMs along with comprehensive contamination checks demonstrates that particles are not exposed to terrestrial PAHs at or above detection limits, either subsequent, during or prior to collection. Comparison of the observed PAH distributions with those measured in three carbonaceous chondrites [Orgueil (CI1), Murchison (CM2) and Allende (CV3)] under identical conditions demonstrated that marked differences exist in the trace organic composition of these two sources of extraterrestrial matter. In general, AMMs show a far richer distribution of unalkylated 'parent' PAHs with more extended alkylation series (replacement of -H with -(CH2)n-H; n = 1, 2, 3 ...). The degree of alkylation loosely correlates with a metamorphic index that represents the extent of frictional heating incurred during atmospheric entry. A search for possible effects of the chemical composition of the fine-grain matrix of host particles on the observed PAH distributions reveals that high degrees of alkylation are associated with high Na/Si ratios. These results, in addition to other observations by Maurette, indicate that 'giant' micrometeorites survive hypervelocity (> or = 11 km s-1) atmospheric entry unexpectedly well. Because such micrometeorites are believed to represent the dominant mass fraction of extraterrestrial material accreted by the Earth, they may have played a significant role in the prebiotic chemical evolution of the early Earth through the delivery of complex organic matter to the surface of the planet.

Antarctic Regions↗
PubMed1998

Extraterrestrial organic matter: a review.

We review the nature of the widespread organic material present in the Milky Way Galaxy and in the Solar System. Attention is given to the links between these environments and between primitive Solar System objects and the early Earth, indicating the preservation of organic material as an interstellar cloud collapsed to form the Solar System and as the Earth accreted such material from asteroids, comets and interplanetary dust particles. In the interstellar medium of the Milky Way Galaxy more than 100 molecular species, the bulk of them organic, have been securely identified, primarily through spectroscopy at the highest radio frequencies. There is considerable evidence for significantly heavier organic molecules, particularly polycyclic aromatics, although precise identification of individual species has not yet been obtained. The so-called diffuse interstellar bands are probably important in this context. The low temperature kinetics in interstellar clouds leads to very large isotopic fractionation, particularly for hydrogen, and this signature is present in organic components preserved in carbonaceous chondritic meteorites. Outer belt asteroids are the probable parent bodies of the carbonaceous chondrites, which may contain as much as 5% organic material, including a rich variety of amino acids, purines, pyrimidines, and other species of potential prebiotic interest. Richer in volatiles and hence less thermally processed are the comets, whose organic matter is abundant and poorly characterized. Cometary volatiles, observed after sublimation into the coma, include many species also present in the interstellar medium. There is evidence that most of the Earth's volatiles may have been supplied by a 'late' bombardment of comets and carbonaceous meteorites, scattered into the inner Solar System following the formation of the giant planets. How much in the way of intact organic molecules of potential prebiotic interest survived delivery to the Earth has become an increasingly debated topic over the last several years. The principal source for such intact organics was probably accretion of interplanetary dust particles of cometary origin.

Astronomical Phenomena↗
PubMed1998

Carbonaceous micrometeorites and the origin of life.

Giant micrometeorites (sizes ranging from approximately 50 to 500 micrometers), such as those that were first recovered from clean pre-industrial Antarctic ices in December 1987, represent by far the dominant source of extraterrestrial carbonaceous material accreted by the Earth's surface, about 50,000 times the amount delivered by meteorites (sizes > or = a few cm). They correspond to large interplanetary dust particles that survived unexpectedly well their hypervelocity impact with the Earth's atmosphere, contrary to predictions of theoretical models of such impacts. They are related to relatively rare groups of carbonaceous chondrites (approximately 2% of the meteorite falls) and not to the most abundant meteorites (oridinary chondrites and differentiated micrometeorites). About 80% of them appear to be highly unequilibrated fine-grained assemblages of mineral grains, where an abundant carbonaceous component is closely associated on a scale of < or = 0.1 micron to both hydrous and anhydrous minerals, including potential catalysts. These observations suggest that micrometeorites could have functioned as individual microscopic chemical reactors to contribute to the synthesis of prebiotic molecules on the early Earth, about 4 billion years ago. The recent identification of some of their complex organics (amino acids and polycyclic aromatic hydrocarbons), and the observation that they behave as very efficient 'cosmochromatographs', further support this 'early carbonaceous micrometeorite' scenario. Future prospects include identifying the host phases (probably ferrihydrite) of their complex organics, evaluating their catalytic activity, and assessing whether synergetic interactions between micrometeorites and favorable zones of the early Earth (such as submarine hydrothermal vents) accelerated and/or modified such synthesis.

Meteoroids↗
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↗