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At least 829 records · Page 46
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NASA NTRS1988

How well do we know the Martian abundances of highly volatile elements

The table is given that summarizes four models based on observations and gives some reasonable inferences of each for Martian volatiles. The table columns give: (1) the volatile assumptions of the model; (2) the inferred Martian volatile concentrations relative to earth; (3) the N/36 Ar and C/36 Ar ratios; (4) the percentage of Martian degassing inferred by the model, and 5 and 6) the equivalent global column heights of liquid water and solid CaCO3 over the entire surface of Mars that would be formed from the model quantities of degassed volatiles.

Bogard, Donald↗
NASA NTRS1988

Deccan volcanism at the Cretaceous-Tertiary boundary

The accuracy with which one can claim that Deccan trap volcanism occurred at the Cretaceous-Tertiary boundary (KTB) over a very short time interval is of key importance in deciding whether a volcanic origin of the KTB events should be taken seriously. In the two years since paleomagnetic, paleontological and geodynamic evidence was published, further data have become available and the case now appears to be well constrained. The Ar-40/Ar-39 results from six labs have yielded some 24 reliable plateau ages that narrow the age range to 65 to 69 Ma. Moreover, it appears that a significant part of this range results from inter-lab spread and possible minor alteration. Paleontology demonstrates that volcanism started in the Maestrichtian, more precisely in the A. mayaroensis zone. Paleomagnetism shows that volcanism spanned only 3 chrons and only one correlation remains possible, that of the main central reversed chron with 29R. Therefore, whereas Ar-40/Ar-39 is able only to restrict the duration of volcanism to some 4 Ma, paleomagnetism restricts it to 0.5 Ma. Using some geochemical indicators such as C-13 as proxy, it is suggested that volcanism actually consists of a few shorter events of unequal magnitude. Extrusion rates may be as high as 100 cu km/yr and fissure lengths as long as several 100 km. Such a scenario appears to be at least as successful as others in accounting for most anomalies observed at the KTB. Particularly important are Iridium and other platinum group elements (PGE) profiles, Sr-87/Sr-86, C-13, 0-18, other exotic geochemical signatures, spherules, soot, shocked minerals, selective and stepwise extinctions. The environmental impact of CO2 possibly released during explosive phases of volcanism, and SO2 released during effusive phases, and the ability of volcanism to ensure worldwide distribution of KTB products are now all addressed. In conclusion, the case for a causal link between internal hotspot activity, birth of the Reunion hotspot itself as the Deccan and KTB events appears to rest on an increasingly stronger basis.

Courtillot, V.↗
NASA NTRS1988

Isotopic investigations in the area of the Tunguska catastrophe in 1908 year

The hypotheses of the annihilation and thermonuclear character of the Tunguska explosion have been tested by measuring inductive Ar-39 radioactivity from K and Ca in rocks and soil under the explosion epicentrum. Ar-39 was not detected though its estimated radioactivity was expected to be 100 times higher than the radio-metrical plant sensitivity. These results testify against the nuclear nature of the Tunguska explosion. The contents of 11 elements in the ultrasmall quantity of matter of the silicate microspherules isolated from catastrophe peat layer at the explosion site were measured by method of neutron activation analysis. It was demonstrated that the enrichment of microspherules by light and volatile elements (Al, Na, Zn, Cs) and the impoverishment by more heavy and hard volatile ones (Fe, Co, Sc). It was shown that the microspherules were not the product of differentiation of the terrestrial soil or of an ordinary meteorite material. It was demonstrated that Pb isotopic content in catastrophe peat layer had more Pb-204, Pb-207, Pb-208 than Pb-206 as compared with Pb isotopic content of other peat layers and common Pb in this area. In the peat column taken at the Ostraya hill area in three nearcatastrophe layers, small increases of isotopic C-13 was determined. Isotopic C effect in the peat layer is confirmed also for the North peatbog. Observed isotopic changes are not accounted for by climatic changes or other physico-chemical reasons. They seem to be related to preservation in the peat of matter resembling carbonaceous chrondrites of the Cl type or more probably of cometary matter enriched much more in volatile elements.

Kolesnikov, E. M.↗
NASA NTRS1988

Some volcanologic aspects of Columbia River basalt volcanism relevant to the extinction controversy

The Columbia River Basalt Group is the youngest and most thoroughly studied flood-basalt province known; information about it should be relevant to questions about the possible relation of flood-basalt volcanism to mass extinctions. The group has a total volume of about 174,000 cu km and covers an area of about 164,000 sq km. It was erupted between 17.5 and 6 Ma, as measured by K-Ar and Ar-40/Ar-39 dates. Early eruptions formed the Imnaha Basalt. More than 85 percent of the group was produced during a 1.5 my period between 17 and 15.5 Ma, forming the Grande Ronde and greatly subordinate Picture Gorge Basalts. Later flows formed the Wanapum Basalt, which includes the well-known Roza Member, and the Saddle Mountains Basalt. Linear vent systems for many of the flows are known and are located only in the eastern third of the Columbia Plateau. No systematic migration of vents occurred throughout the 11.5 my period of activity; this and other considerations make it unlikely that the province is related to a hot spot. Model calculations based on observations that little cooling occurred during flow of hundreds of kilometers suggest eruption and emplacement durations of a few days. Some voluminous flows occur in all formations, but most such flows apparently were erupted during Grande Ronde time. The eruption and emplacement of more than 1,000 cu km of 1100 C basaltic lava on the surface within several days doubtless had at least local meteorologic effects. Whether the effects were broader can at present only be hypothesized. Grande Ronde Basalt and Picture Gorge Basalts contain moderately common but thin sedimentary interbeds between flows, whereas earlier and later formations contain numerous, locally thick sediment accumulations. Volcaniclastic debris derived from extra-plateau sources commonly occurs in the testbeds.

Swanson, Donald A.↗
NASA NTRS1988

Apollo 14 regolith breccias - Different glass populations and their potential for charting space time variations

Apollo 14 regolith breccias (14313, 14307, 14301, 14049, 14047) have been found to have different populations of nonagglutinitic, mare-derived glasses. These variations appear to not only reflect different source regoliths but also different closure ages for these breccias. Based upon these different glass populations, 14301 is inferred to have a closure age sometime during the epoch of mare volcanism. All of the other four breccias were formed after the termination of mare volcanism with a possible age sequence from old to young of the following: 14307, 14313, 14049, 14047. Due to the relative simplicity of acquiring high-quality chemical data on large numbers of glasses by electron microprobe, mare glass populations allow: (1) classification of regolith breccias with respect to provenance and (2) estimation of their relative and absolute closure ages. The determination of (Ar-40)-(Ar-39) ages on individual glass spherules within breccias using the laser probe should in the future prove to be a promising extension of the present study.

Delano, John W.↗
NASA NTRS1989

Titan atmospheric composition by hypervelocity shock layer analysis

The Cassini Mission, a NASA/ESA cooperative project which includes a deployment of probe into the atmosphere of Titan, is described, with particular attention given to the shock radiometer experiment planned for the Titan probe for the analysis of Titan's atmosphere. Results from a shock layer analysis are presented, demonstrating that the mole fractions of the major species (N2, CH4, and, possibly Ar) in the Titan atmosphere can be successfully determined by the Titan-probe radiometer, by measuring the intensity of the CN(violet) radiation emitted in the shock layer during the high velocity portion of the probe entry between 200 and 400 km altitude. It is shown that the sensitivity of the CN(violet) radiation makes it possible to determine the mole fractions of N2, CH4, and Ar to about 0.015, 0.003, and 0.01, respectively, i.e., much better than the present uncertainties in the composition of Titan atmosphere.

Nelson, H. F.↗
NASA NTRS1989

Spectral shifts of methane lines in collisions with hydrogen, helium, nitrogen, and argon

Pressure-induced shifts of rotational-vibrational lines in CH4 have been measured at very high spectral resolution in the R-branch of the 3nu(3) overtone. The colliding gases were H2, He, N2, and Ar. Results are presented as functions of rotational and tetrahedral quantum numbers. Mean values (in 0.001/cm per torr) for the R0-R6 multiplets are -0.01109 (CH4-H2), +0.00547 (opposite sense) (CH4-He), -0.01990 (CH4-N2), and -0.02126 (CH4-Ar).

Fox, Kenneth↗
NASA NTRS1990

Mass fractionation of noble gases in diffusion-limited hydrodynamic hydrogen escape

The theory of mass fractionation by hydrogen is presently extended to atmospheres in which hydrogen is not the major constituent. This theoretical framework is applied to three different cases. In the first, it is shown that the fractionation of terrestrial atmospheric neon with respect to mantle neon is explainable as a consequence of diffusion-limited hydrogen escape from a steam atmosphere toward the end of the accretion process. In the second, the anomalously high Ar-38/Ar-36 ratio of Mars is shown to be due to hydrodynamic fractionation by a vigorously escaping and very pure hydrogen wind. In the last case, it is speculated that the currently high Martian D/H ratio emerged during the hydrodynamic escape phase which fractionated Ar.

Zahnle, Kevin↗
NASA NTRS1990

Transformation of vector magnetograms and the problems associated with the effects of perspective and the azimuthal ambiguity

Off-center vector magnetograms which use all three components of the measured field provide the maximum information content from the photospheric field and can provide the most consistent potential field independent of the viewing angle by defining the normal component of the field. The required transformations of the magnetic field vector and the geometric mapping of the observed field in the image plane into the heliographic plane have been described. Here we discuss the total transformation of specific vector magnetograms to detail the problems and procedures that one should be aware of in analyzing observational magnetograms. The effect of the 180-deg ambiguity of the observed transverse field is considered as well as the effect of curvature of the photosphere. Specific results for active regions AR 2684 (September 23, 1980) and AR 4474 (April 26, 1984) from the Marshall Space Flight Center Vector magnetograph are described which point to the need for the heliographic projection in determining the field structure of an active region.

Gary, G. Allen↗
NASA NTRS1990

Nonpotential magnetic fields at sites of gamma-ray flares

The relation between the degree of nonpotentiality of photospheric magnetic fields and the occurrence of gama-ray flares is examined to determine whether there are special signatures of the stressed fields for this type of flare. Observations of the flares in the active region of April 1984 (AR 4474) are analyzed, showing that the big flare initiated at the location on the magnetic neutral line where the field deviated the most from a potential field. The nonpotential signatures of AR 4474 are compared with those of four other regions. The results suggest that gamma-ray flares are associated with strongly nonpotential fields that extend over relatively larger lengths of the magnetic neutral line that the fields associated with flares that do not produce gamma-ray events.

Hagyard, M. J.↗
NASA NTRS1990

Solar coronal Ar/Fe and Ne/Mg abundance ratios derived from ultraviolet forbidden line spectra

An Ar/Fe abundance ratio of 0.031 + or - 35 percent and an upper limit of 0.4 to the Ne/Mg abundance ratio are determined for solar coronal active regions using solar spectra. Forbidden lines occur at 1324.45 A for Mg V, 1349.38 A for Fe XII, 1375.98 A for Ca XV, and 1392.12 A for Ar XI. The unobserved Ne V line is predicted to fall at 1574.68 A. These abundance ratios are consistent with lower abundances of Ne and Ar in the corona relative to low first ionization potential elements.

Doschek, G. A.↗
NASA NTRS1990

A model for the production of cosmogenic nuclides in chondrites

A model is presented for calculating the production rates of cosmic-ray-produced He, Ne, and Ar as well as Be-10, Al-26, and Mn-53 in chondrites of variable size and shape. The predictions of this model are compared with published data for the meteorites ALH78084, St. Severin, and Keyes. The agreement is found to be about 5 percent for the concentrations of Be-10, Ne-21, Ne-22, Ar-38, and Mn-53, and to be better than 1 percent for the Ne-22/Ne-21 ratios. The correlation between P(Be-10)/P(Ne-21) and Ne-22/Ne-21 ratios is experimentally verified over a wide range of irradiation conditions.

Graf, TH.↗
NASA NTRS1991

Probable age of Autolycus and calibration of lunar stratigraphy

Ar-39 - Ar-40 analyses of three petrographically distinct, shocked Apollo 15 KREEP (i.e., high K, rare earth element, P, and other trace element contents) basalt samples demonstrate that a major impact event affected all three samples at about 2.1 Ga. The Copernican System craters Aristillus and Autolycus are to the north. Autolycus, the older of the two, is in a particularly appropriate terrain and is the most likely source of the 2.1 Ga heating and delivery event. With this calibration point, and if Autolycus really is a Copernican crater, the Copernican System lasted twice as long as has previously been suggested. Furthermore, the moon was not subjected to a constant cratering rate over the past 3 billion years; the average rate in the preceding Eratosthenian must have been twice that in the Copernican.

Ryder, G.↗
NASA NTRS1991

Searching for Crisium Basin ejecta - Chemistry and ages of Luna 20 impact melts

Chemical (INAA) and chronological (Ar-40 - Ar-39) analyses of six Luna 20 impact melts are performed, and these are compared to the results of Podosek et al. (1973), commonly taken to be representative of the Crisium Basin impact. At least two chemical groups of impact melts are identified. One is interpreted as melts derived from the local crust by craters over the last 3.9 Ga. Another group, which includes one of the samples dated by Podosek et al. (1973), is chemically and chronologically (3.85 + or - 0.02 Ga) indistinguishable from Apollo 17 samples interpreted as Serenitatis impact melts, and hence could be melt formed by that event and deposited at the Luna 20 site. One sample (22023,3,F) has a well-defined age of 3.895 + or - 0.017 Ga, and is chemically similar to, but distinct from, impact melts from other basins. This sample, and its chemistry and age are tentatively identified with the Crisium Basin impact. Whether the age of the Crisium impact is given by 22023,3,F, the samples analyzed by Podosek et al. (1973), or neither, it is clear that Crisium impact melt is not abundant in the Luna 20 collection.

Swindle, T. D.↗
NASA NTRS1991

A reevaluation of the solar-wind sputtering rate on the lunar surface

Knowledge of the retention efficiency for solar-wind-implanted Ar-36 in lunar surface material and measurement of the Ar-36 concentration in a sputtered surface make it possible to calculate an upper limit to the effective sputtering rate at which that surface has been eroded. On the basis of data for 71501 plagioclase and the glass of rock 12054, a value is derived for the sputter-erosion rate equal to less than 0.0024 + or - 0.001 A/yr, in good agreement with the limit of less than 0.0035 A/yr previously calculated by Zinner et al. (1977).

Kerridge, J. F.↗
NASA NTRS1991

Argon adsorption and the lunar atmosphere

The results of Ar adsorption experiments on a terrestrial labradorite and lunar rock 15415 crushed in vacuo are reported. The experiments were designed to test lunar atmosphere simulation models for the behavior of Ar on the lunar surface, as determined from the Apollo 17 mass spectrometer results. These models (Hodges, 1980, 1982) used a single adsorption potential to characterize the surfaces of lunar soil grains, with the result that high (6-7 kcal/mol) heats of adsorption were inferred. The present experimental results show that very high adsorption potentials are indeed associated with fresh mineral surfaces, but that these energetic surfaces occupy only small fractions of the total surface area. Nonetheless, these small fractions of surface, if they can be maintained in the lunar regolith in steady-state condition, could be sufficient to account for the Apollo 17 mass spectrometer observations.

Bernatowicz, T. J.↗
NASA NTRS1991

Modeling the Orion nebula as an axisymmetric blister

The ionized gas in the Orion nebula is examined by means of axisymmetric modeling that is based on observational data from the ionized, neutral, and molecular regions. Nonsymmetrical features are omitted, radial dependence from the Trapezium is assumed, and azimuthal symmetry in the plane of the sky is used. Stellar properties and abundances of certain elements are described, and these data are used to compare the present axisymmetric-blister model to a previous spherical model. Strong singly-ionized emission that are visible near the Trapezium are found to originate in the ionization-bounded region in the dense Trapezium zone. The model can be more tightly constrained by adding near-IR data on noncentral zones for (Ar II), (AR III), (Ne II), and (S IV). The quadrant with the 'bar' creates an nonsymmetry that influences the observational data, and the model can therefore be improved with the additional data.

Rubin, R. H.↗
NASA NTRS1991

Revived interest in the lunar atmosphere

The recent discoveries of Na and K in the lunar atmosphere are emphasized in a review of lunar atmospheric observations which identifies possible directions of research. Observations are briefly reviewed through the Apollo era which identified the atmosphere as a surface-boundary exosphere, and identified atmospheric constituents included He and Ar which rose and fell by a factor of 20 during the lunar diurnal cycle. Ar-36 and -40 were also identified as were the sources of ionospheric source and sink processes including the solar EUV flux. Na and K are discussed which were identified in a study of the spectroscopic ring effect, and the effects of elemental interaction with the lunar surface can be approximated in laboratory experiments. Observations and measurements of the lunar surface are important for the continued investigation of lunar and other planetary mechanisms.

Morgan, Thomas H.↗