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PubMed1990

Xenon fractionation in porous planetesimals.

The distinctively fractionated Xe on Mars and Earth may have its root in a common source from which both planets accreted. We begin with Ozima and Nakazawa's hypothesis that terrestrial Xe fractionation was caused by gravitational separation of adsorbed solar nebular gases inside large porous planetesimals. We point out that Xe would have been trapped as the planetesimal grew and pores were squeezed shut by lithostatic pressure. We show that enough fractionated Xe to supply the Earth could have been trapped this way. The degree of fractionation is controlled by the lithostatic pressure at the pore-closing front and so would have been roughly the same for all large planetesimals. The predicted degree of fractionation agrees well with that preserved in terrestrial and martian Xe. Relative to Xe, this source is strongly depleted in other noble gases. In contrast to the original Ozima and Nakazawa hypothesis, our hypothesis predicts the observed fractionation, and it allows planetary accretion to occur after the dissipation of the solar nebula. The required planetesimals are large, representing a class of object now extinct in the solar system.

Atmosphere↗
PubMed1991

Isotopic analyses of amino acids from the Murchison meteorite.

Previous isotopic analyses of the total amino acids of the Murchison meteorite showed these compounds to be substantially enriched in 2H, 13C, and 15N relative to terrestrial organic matter. These analyses have been repeated (2H, 13C) with inclusion of an ultrafiltration step to exclude the possibility that a fine particulate contaminant carried the isotopic excesses observed in the previous work. In addition, the meteorite amino acids were chromatographically separated to rule out the possibility that the isotopic enrichment of the meteorite extract could reside in basic compounds other than amino acids. The results indicate that the Murchison amino acids are truly isotopically unusual, that the isotopic excesses reside in at least several different amino acids, and that the isotopic contents of some of these amino acids reach values of about +40% (delta 13C) and +2500% (delta D). If it is assumed that the high deuterium content of the meteorite alpha-amino acids is a result of the synthesis of their molecular precursors by low temperature ion-molecule reactions in an interstellar cloud, their formation by aqueous phase Strecker reactions in the parent body is consistent with their general characteristics and with known parent body processes.

Amino Acids↗
PubMed1992

Isotopic and molecular analyses of hydrocarbons and monocarboxylic acids of the Murchison meteorite.

The monocarboxylic acids and hydrocarbons of the Murchison meteorite (CM2) were isolated for isotopic analysis. The nonvolatile hydrocarbons were analyzed as crude methanol and benzene-methanol extracts and also after separation by silica gel chromatography into predominantly aliphatic, aromatic, and polar hydrocarbon fractions. The volatile hydrocarbons were obtained after progressive decomposition of the meteorite matrix by freeze-thaw, hot water, and acid treatment. Molecular analyses of the aromatic hydrocarbons showed them to comprise a complex suite of compounds in which pyrene, fluoranthene, phenanthrene, and acenaphthene were the most abundant components, a result similar to earlier analyses. The polar hydrocarbons also comprise a very complex mixture in which aromatic ketones, nitrogen, and sulfur heterocycles were identified. Both delta 13C and delta D values were obtained for all preparations. The monocarboxylic acids, aliphatic, aromatic, and polar hydrocarbons, and the indigenous volatile hydrocarbons were found to be D-rich with delta D values ranging from about +100 to +1000. The delta 13C values ranged overall from -13 to +17. The deuterium enrichment observed in these compounds is suggestive of a relationship to interstellar organic compounds. In two separate analyses, the delta D values of the nonvolatile hydrocarbons were observed to increase in the following order: aliphatic < aromatic < polar. This finding is consistent with an early solar system or parent body conversion of aromatic to aliphatic compounds as well as the earlier suggestion of pyrolytic formation of aromatic from aliphatic compounds.

Benzene↗
PubMed1992

Noble metal abundances in an Early Archean impact deposit.

We report detailed analyses on the concentrations of the noble metals Pd, Os, Ir, Pt, and Au in an early Archean spherule bed (S4) of probable impact origin from the lower Fig Tree Group, Barberton Greenstone Belt, South Africa. Compared to other sedimentary deposits of known or suspected impact origin, some noble metals are present in exceptionally high concentrations. Noble metal abundances are fractionated relative to abundances in chondrites with ratios of Os/Ir, Pt/Ir, Pd/Ir, and Au/Ir at only 80, 80, 41, and 2% of these values in CI chondrites. Although an extraterrestrial source is favored for the noble metal enrichment, the most plausible cause of the fractionation is by regional hydrothermal/metasomatic alteration.

Earth, Planet↗
PubMed1993

Carbon abundance and silicate mineralogy of anhydrous interplanetary dust particles.

We have studied nineteen anhydrous chondritic interplanetary dust particles (IDPs) using analytical electron microscopy. We have determined a method for quantitative light element EDX analysis of small particles and have applied these techniques to a group of IDPs. Our results show that some IDPs have significantly higher bulk carbon abundances than do carbonaceous chondrites. We have also identified a relationship between carbon abundance and silicate mineralogy in our set of anhydrous IDPs. In general, these particles are dominated by pyroxene, olivine, or a subequal mixture of olivine and pyroxene. The pyroxene-dominated IDPs have a higher carbon abundance than those dominated by olivines. Members of the mixed mineralogy IDPs can be grouped with either the pyroxene- or olivine-dominated particles based on their carbon abundance. The high carbon, pyroxene-dominated particles have primitive mineralogies and bulk compositions which show strong similarities to cometary dust particles. We believe that the lower carbon, olivine-dominated IDPs are probably derived from asteroids. Based on carbon abundances, the mixed-mineralogy group represents particles derived from either comets or asteroids. We believe that the high carbon, pyroxene-rich anhydrous IDPs are the best candidates for cometary dust.

Astronomy↗
PubMed1993

A search for C60 in carbonaceous chondrites.

Analysis of interior samples of the Murchison meteorite by two routes yielded an upper limit of 2 ppb for its C60 content, as compared to parts per million levels for individual polycyclic aromatic hydrocarbons (PAHs). Provided the samples contain an interstellar component, which is probable since Murchison hydrocarbons contain excess deuterium, this result argues against the ubiquitous presence of C60 in the interstellar medium. A possible explanation for the absence of C60 was found in experiments showing how PAHs replace fullerenes as stable end products when hydrogen is present during carbon condensation. As a secondary result we found high molecular weight PAHs in the Murchison and Allende meteorites. Coronene and its methyl derivatives are especially interesting since features in the coronene spectrum have been shown to match some of the unidentified interstellar infrared emission bands.

Carbon↗
PubMed1993

Simulations of the Viking Gas Exchange Experiment using palagonite and Fe-rich montmorillonite as terrestrial analogs: implications for the surface composition of Mars.

Simulations of the Gas Exchange Experiment (GEX), one of the Viking Lander Biology Experiments, were run using palagonite and Fe-rich montmorillonite as terrestrial analogs of the Martian soil. These terrestrial analogs were exposed to a nutrient solution of the same composition as that of the Viking Landers under humid (no contact with nutrient) and wet (intimate contact) conditions. The headspace gases in the GEX sample cell were sampled and then analyzed by gas chromatography under both humid and wet conditions. Five gases were monitored: CO2, N2, O2, Ar, and Kr. It was determined that in order to simulate the CO2 gas changes of the Viking GEX experiment, the mixture of soil analog mineral plus nutrient medium must be slightly (pH = 7.4) to moderately basic (pH = 8.7). This conclusion suggests constraints upon the composition of terrestrial analogs to the Mars soil; acidic components may be present, but the overall mixture must be basic in order to simulate the Viking GEX results.

Aluminum Silicates↗
PubMed1993

Cosmogenic and nucleogenic isotopic changes in Mars: their rates and implications to the evolutionary history of Martian surface.

We present calculations of rates of production of several nuclides in the Martian atmosphere and in the regolith due to nuclear interactions of cosmic ray and radiogenic particles and consider their implications to the evolutionary history of Mars. Nuclides selected are those which, considering their chemical properties, may be useful as tracers for delineating the past histories of the Martian atmosphere and regolith. Calculations are presented for different assumed atmospheric pressures. The regolith production rates for the present thin Martian atmosphere (approximately 20 g cm-2) are expected to be fairly robust because they are based primarily on observed cosmogenic effects in the Moon, for which semiempirical estimates of nuclide production rates have been provided earlier by Reedy (1981). Uncertainties which arise in the calculations of nuclide production rates for an earlier hypothetical Martian atmosphere of approximately 300-500 g cm-2 thickness are discussed. Compared to cosmic ray production rates, the nucleogenic production rates are smaller by several orders of magnitude. However, the nucleogenic production extends to much deeper levels, whereas the cosmogenic production is essentially confined to the top 750-1000 g cm-2 depth. Important examples of nucleogenic production are discussed. Isotopes of neon and argon appear to be very promising for delineating relative magnitudes of a number of planetary processes related to the temporal changes in the thickness of the atmosphere, as well as their release from the regolith. However, quantification of the processes would require higher-precision isotopic data for the atmosphere and also direct measurements of isotopic ratios in the Martian regolith, along with supplementary information on changes in the isotopic compositions of hydrogen, carbon, and nitrogen, which are affected by a variety of mechanisms of escape of gases from the atmosphere. Cosmogenic effects are minimal in these cases. We show that although we can at present draw but limited inferences, the planet Mars presents a unique opportunity to use cosmogenic nuclides as tools to delineate the evolutionary history of the planet as a whole, as well as its regolith and the atmosphere. This arises because of two factors: minimal degassing of the planet, and a fairly intense chemical weathering history of the upper surface. Consequently, an appreciable fraction of some of the isotopes of volatile elements is contributed by nuclear reactions.

Argon↗
PubMed1995

Interstellar water in meteorites?

D/H ratios of two meteorites (Renazzo CR and Semarkona LL3), which are known to exhibit the largest departures from the terrestrial hydrogen isotopic ratios, have been determined with the CRPG Nancy ion microprobe. Correlations between the D/H ratios and the chemical compositions (H2O, K, Si, C/H) of plausible hydrogen carriers were observed. From these correlations, it is possible to show that, contrary to previous interpretations, phyllosilicates are the carriers of the deuterium-rich hydrogen in Semarkona and Renazzo: 870 x 10(-6) > or = D/H > or = 670 x 10(-6) (+4600 > or = deltaD > or = 3300%) and > or = 320 x 10(-6) (deltaD > or = 1050%), respectively. Hydrogen is also present in the chondrules of these two deuterium-rich meteorites. The large differences in D/H ratios between matrix (up to 700 x 10(-6), deltaD up to +3500%) and chondrules (from 120 x 10(-6) (deltaD = -230%) to 230 x 10(-6) (deltaD = +475%)) show that hydrogen in chondrules cannot originate from the matrix by simple contamination or diffusion processes. The high D/H ratios measured in water-bearing minerals could not have been produced thermally within a dense solar nebula. Chemical reactions (i.e., involving ions or radicals), taking place in interstellar space or in the outer regions of the nebula at 110-140K are presently the only conceivable mechanisms capable of yielding such isotopic enrichments. Water in these meteorites should no longer be considered as a simple product of nebular condensation under equilibrium thermodynamic conditions at T > or = 160K.

Carbon↗
PubMed1996

A reexamination of amino acids in lunar soils: implications for the survival of exogenous organic material during impact delivery.

Using a sensitive high performance liquid chromatography technique, we have analyzed both the hot water extract and the acid hydrolyzed hot water extract of lunar soil collected during the Apollo 17 mission. Both free amino acids and those derived from acid labile precursors are present at a level of roughly 15 ppb. Based on the D/L amino acid ratios, the free alanine and aspartic acid observed in the hot water extract can be entirely attributed to terrestrial biogenic contamination. However, in the acid labile fraction, precursors which yield amino acids are apparently present in the lunar soil. The amino acid distribution suggests that the precursor is probably solar wind implanted HCN. We have evaluated our results with regard to the meteoritic input of intact organic compounds to the moon based on an upper limit of < or = 0.3 ppb for alpha-aminoisobutyric acid, a non-protein amino acid which does not generally occur in terrestrial organisms and which is not a major amino acid produced from HCN, but which is a predominant amino acid in many carbonaceous chondrites. We find that the survival of exogenous organic compounds during lunar impact is < or = 0.8%. This result represents an example of minimum organic impact survivability. This is an important first step toward a better understanding of similar processes on Earth and on Mars, and their possible contribution to the budget of prebiotic organic compounds on the primitive Earth.

Amino Acids↗
PubMed1985

The Jovian stratosphere in the ultraviolet.

The center-of-disk reflectivity of Jupiter in the wavelength range from 1450 to 3150 angstroms has been computed from 30 low-dispersion IUE spectra taken during solar maximum in 1978-1980. A vertically inhomogeneous radiative transfer program is used to compute model reflectivities of various stratospheric compositions for comparison. Ammonia and acetylene are well determined because they show narrow absorption bands in the ultraviolet. Above 1800 angstroms, these two gases provide a good fit to the data, but not below. At shorter wavelengths the fit would be much improved by a small amount (0.5-1.5 ppb) of propadiene/allene (C3H4). Voyager IRIS spectra show that the IR bands of allene are not strong enough to be detected in such a small amount. Additional absorption around 1600 angstroms can be reproduced best with the presence of cyclopropane (C3H6, <15 ppb), although other absorbers (e.g., hydrocarbon molecules with more than three carbon atoms, oxygen- or nitrogen-containing molecules, or a high-altitude haze) could also explain the spectrum in this region. The data are too noisy to detect possible CO Cameron band absorption near 2000 angstroms.

Acetylene↗
PubMed1985

Spectra of simulated lightning on Venus, Jupiter, and Titan.

Laser-induced plasmas in various gas mixtures were used to simulate lightning in other planetary atmospheres. This method of simulation has the advantage of producing short-duration, high-temperature plasmas free from electrode contamination. The laser-induced plasma discharges in air are shown to accurately simulate terrestrial lightning and can be expected to simulate lightning spectra in other planetary atmospheres. Spectra from 240 to 880 nm are presented for simulated lightning in the atmospheres of Venus, Earth, Jupiter, and Titan. The spectra of lightning on the other giant planets are expected to be similar to that of Jupiter because the atmospheres of these planets are composed mainly of hydrogen and helium. The spectra of Venus and Titan show substantial amounts of radiation due to the presence of carbon atoms and ions and show CN Violet radiation. Although small amounts of CH4 and NH3 are present in the Jovian atmosphere, only emission from hydrogen and helium is observed. Most differences in the spectra can be understood in terms of the elemental ratios of the gas mixtures. Consequently, observations of the spectra of lightning on other planets should provide in situ estimates of the atmospheric and aerosol composition in the cloud layers in which lightning is occurring. In particular, the detection of inert gases such as helium should be possible and the relative abundance of these gases compared to major constituents might be determined.

Ammonia↗
PubMed1986

Amino acids derived from Titan tholins.

An organic heteropolymer (Titan tholin) was produced by continuous dc discharge through a 0.9 N2/0.1 CH4 gas mixture at 0.2 mbar pressure, roughly simulating the cloudtop atmosphere of Titan. Treatment of this tholin with 6N HCl yielded 16 amino acids by gas chromatography after derivatization of N-trifluroacetyl isopropyl esters on two different capillary columns. Identifications were confirmed by GC/MS. Glycine, aspartic acid, and alpha- and beta-alanine were produced in greatest abundance; the total yield of amino acids was approximately 10(-2), approximately equal to the yield of urea. The presence of "nonbiological" amino acids, the absence of serine, and the fact that the amino acids are racemic within experimental error together indicate that these molecules are not due to microbial or other contamination, but are derived from the tholin. In addition to the HCN, HC2CN, and (CN)2 found by Voyager, nitriles and aminonitriles should be sought in the Titanian atmosphere and, eventually, amino acids on the surface. These results suggest that episodes of liquid water in the past or future of Titan might lead to major further steps in prebiological organic chemistry on that body.

Amino Acids↗
PubMed1986

Methods for computing comet core temperatures.

General analytic expressions are derived that relate the surface temperature to the temperature deep within the nucleus of a spherically symmetric layered comet in thermal equilibrium. The relation between the average surface temperature and the mean temperature at great depths depends entirely on the temperature dependence of the thermal conductivity. The core temperature is given by the inverse of the anti-derivative of the thermal conductivity, with respect to temperature, operating on the average value of the anti-derivative of the thermal conductivity evaluated at the surface temperature. Using these expressions detailed numerical models of the surface temperature of comets can be used to directly estimate the core temperature. For the special, albeit unphysical, case of an isothermal, low-conductivity comet nucleus, without sublimation, the core temperature can be determined analytically. To illustrate the dependence of core temperature on eccentricity this simple case is solved assuming that the temperatures dependence of the thermal conductivity is given by that of crystalline ice. For an eccentricity of approximately 0.5, the core temperature obtained is 3% colder than the corresponding value obtained assuming constant thermal conductivity an is 11% colder than the result of Klinger's (1981) formula. This method is also applied to a detailed numerical model with a complicated nonintegrable thermal conductivity.

Astronomy↗
PubMed1986

Ultranarrowband searches for extraterrestrial intelligence with dedicated signal-processing hardware.

Multichannel spectroscopy with millihertz resolution constitutes an attractive strategy for a microwave search for extraterrestrial intelligence (SETI), assuming the transmission of a narrow-band radiofrequency beacon. Such resolution matches the properties of the interstellar medium, and the necessary receiver Doppler corrections provide a high degree of interference rejection. We have constructed a frequency-agile swept receiver with an 8,388,608-channel spectrum analyzer, on-line signal recognition, and multithreshold archiving. A search of 250 Sun-like stars at 1.4 and 2.8 GHz has been carried out with the Arecibo 305-m antenna, and a meridian transit search of the northern sky is in progress at the Harvard-Smithsonian 26-m antenna. Successive spectra of 400 kHz at 0.05 Hz resolution are searched for features characteristic of an intentional narrowband beacon transmission. These spectra are centered on guessable ("magic") frequencies (such as the 21-cm hydrogen hyperfine line), referenced successively to the local standard of rest, the galactic barycenter, and the cosmic blackbody rest frame.

Astronomy↗
PubMed1987

Electric discharge synthesis of HCN in simulated Jovian atmospheres.

HCN has been detected in the Jovian atmosphere at a column density of about 2.2 x 10(-7) moles cm-2. While photochemical synthesis from methylamine and aziridine, upwelling, and lightning have been proposed as possible sources of this HCN, corona discharge has not been previously considered. HCN energy yields (moles J-1) were measured using corona discharge for gas mixtures containing H2, CH4, NH3, with H2/CH4 ratios from 4.4 to 1585. The yields are approximately proportional to the mole fraction of methane in the gas mixture. Assuming that the 3/1 ratio of corona discharge to lightning energy on the Earth applies to Jupiter, HCN column densities from corona discharge could account for approximately 10% of the observed HCN. These estimates are very dependent on the values used for the energy available as lightning on Jupiter and the eddy diffusion coefficients in the region of synthesis.

Ammonia↗
PubMed1987

The case for a wet, warm climate on early Mars.

Theoretical arguments are presented in support of the idea that Mars possessed a dense CO2 atmosphere and a wet, warm climate early in its history. Calculations with a one-dimensional radiative-convective climate model indicate that CO2 pressures between 1 and 5 bars would have been required to keep the surface temperature above the freezing point of water early in the planet's history. The higher value corresponds to globally and orbitally averaged conditions and a 30% reduction in solar luminosity; the lower value corresponds to conditions at the equator during perihelion at times of high orbital eccentricity and the same reduced solar luminosity. The plausibility of such a CO2 greenhouse is tested by formulating a simple model of the CO2 geochemical cycle on early Mars. By appropriately scaling the rate of silicate weathering on present Earth, we estimate a weathering time constant of the order of several times 10(7) years for early Mars. Thus, a dense atmosphere could have persisted for a geologically significant time period (approximately 10(9) years) only if atmospheric CO2 was being continuously resupplied. The most likely mechanism by which this might have been accomplished is the thermal decomposition of carbonate rocks induced directly and indirectly (through burial) by intense, global-scale volcanism. For plausible values of the early heat flux, the recycling time constant is also of the order of several times 10(7) years. The amount of CO2 dissolved in standing bodies of water was probably small; thus, the total surficial CO2 inventory required to maintain these conditions was approximately 2 to 10 bars. The amount of CO2 in Mars' atmosphere would eventually have dwindled, and the climate cooled, as the planet's internal heat engine ran down. A test for this theory will be provided by spectroscopic searches for carbonates in Mars' crust.

Atmosphere↗
PubMed1987

An update of nitrile photochemistry on Titan.

New chemical schemes leading to the formation of cyanogen (C2N2) and dicyanoacetylene (C4N2) in the upper atmosphere of Titan are proposed and examined in light of recent laboratory kinetics experiments and Voyager observations.

Atmosphere↗