Publications of the exobiology program for 1987: A special bibliography
Presented is a list of 1987 publications resulting from research pursued under the auspices of the NASA Exobiology Program.
Yung, Yuk Ling↗
Engineering topics
Epstein, Samuel
Publications and source records attributed to Epstein, Samuel.
S-process krypton of variable isotopic composition in the Murchison meteorite
Data are reported which, for the first time, permit the derivation of the full isotopic spectrum of s-process krypton with reasonable precision. It is shown that this s-Kr in a residue from the Murchison meteorite did not originate in one single s-process but rather is a mixture of contributions from stellar environments where the density of free neutrons was not the same. The astrophysical conditions under which this krypton was produced were distinct from those that have been invoked to explain the solar system s-process abundance. Similar to the C-13-rich carbon component in an aliquot of the same residue, the s-process Kr from different astrophysical sites has retained its identity during the accumulation and subsequent history of the meteorite.
Ott, Urlich↗
Impact-induced devolatilization and hydrogen isotopic fractionation of serpentine: Implications for planetary accretion
Impact-induced devolatilization of porous serpentine was investigated using two independent experimental methods, the gas recovery and the solid recovery method, each yielding nearly identical results. For shock pressures near incipient devolatilization, the hydrogen isotopic composition of the evolved H2O is very close to that of the starting material. For shock pressures at which up to 12 percent impact-induced devolatilization occurs, the bulk evolved gas is significantly lower in deuterium than the starting material. There is also significant reduction of H2O to H2 in gases recovered at these higher shock pressures, probably caused by reaction of evolved H2O with the metal gas recovery fixture. Gaseous H2O-H2 isotopic fractionation suggests high temperature isotopic equilibrium between the gaseous species, indicating initiation of devolatilization at sites of greater than average energy deposition. Bulk gas-residual solid isotopic fractionations indicate nonequilibrium, kinetic control of gas-solid isotopic ratios. Impact-induced hydrogen isotopic fractionation of hydrous silicates during accretion can strongly affect the long-term planetary isotopic ratios of planetary bodies, leaving the interiors enriched in deuterium. Depending on the model used for extrapolation of the isotopic fractionation to devolatilization fractions greater than those investigated experimentally can result from this process.
Tyburczy, James A.↗
Heavy carbon in individual oxide grains from the Murchison meteorite
The carbon isotopic composition of individual oxide grains from Murchison HF-HCl acid residue CFOc has been measured in the ion microprobe. Many grains contain carbon with large C-13 excesses ranging to 7000 per mil. In most cases the carbon is present as micron-sized subgrains. The association of silicon with the anomalous carbon points towards SiC as carrier. If this tentative identification is correct, then the SIC grains most likely originated in the circumstellar atmospheres of red giants. Oxide grains in which the O-18/O-16 ratio has also been measured show variable O-16 excesses which are not correlated with the C-13 excesses. This indicates that the sources of the anomalous carbon and oxygen isotopic compositions of the oxide grains are unrelated.
Zinner, Ernst↗
The effect of the isotopic composition of oxygen on the non-mass-dependent isotopic fractionation in the formation of ozone by discharge of O2
Isotopic fractionation processes in the formation of O3 by 20-kV 60-kHz discharge of O2 in a fused silica container at 77 K are investigated experimentally, with a focus on the effects of changes in the isotopic composition of O2, the O2 pressure, the degree of O3 formation, and the discharge geometry on the isotope abundances in the postdischarge O2 and O3. The results are presented in tables and graphs and compared with the predictions of theoretical models involving vibrational anharmonicity and symmetry effects on O3 predissociation. In experiments using O2 enriched with O-17 and O-18, the heavy isotope is enriched in the residual O2 and not in the O3, whereas the opposite is true when atmospheric isotope abundances are used. It is inferred that the predissociation rate depends on the symmetry and mass of the O3 molecules and not on self-shielding or symmetry effects in the reactant O2. The potential value of the present findings for studies of anomalous isotope abundances in meteorites is indicated.
Yang, Jongmann↗
The effect of pressure and excitation energy on the isotopic fractionation in the formation of ozone by discharge of O2
Isotopic fractionation processes in the formation of O3 by discharge (at 20 kV and 3 MHz) of O2 in a fused silica container at 77 K are investigated experimentally, with a focus on the effects of changes in the isotopic abundances in the predischarge O2, the O2 pressure, and the excitation energy on the isotope abundances in the postdischarge O2 and O3. The results are presented in extensive tables and graphs and characterized in detail, and good general agreement is found with data obtained using 60-kHz excitation (Yang and Epstein, 1987). It is shown that O-17 and O-18 enrichment of the residual O2 (when the predischarge gas is enriched) or of the O3 (when atmospheric isotope abundances are used) can occur independently of one another. It is inferred that the rate of anharmonic predissociation of vibrationally hot O3 depends on the symmetry and mass of the O3 molecules. The significance of the present findings for studies of anomalous oxygen isotope abundances in meteorites is indicated.
Yang, Jongmann↗