REPEATED SHOCK AND THERMAL METAMORPHISM OF THE ABERNATHY METEORITE

Based on the example of Abernathy (L6 chondrite), this study shows how petrographic investigation can be used to unravel the nature, chronology and conditions of superposed metamorphic events in chondrites. Features considered include the texture of the rock, optical characteristics of olivine, pyroxene and plagioclase, refractive index of plagioclase, metallographical characteristics and microhardness of Fe-Ni alloys. From these data it is deduced that Abernathy has been involved in at least six metamorphic events since the formation of the chondrite. Four distinct shock events and two separate reheating events have been identified. The chronology of these events is established. The conditions for the last four events are reasonably well constrained. These include successively, i) severe reheating (T > 1200°C), ii) severe shock causing complete melting of plagioclase and local melting of the rock (90 < P < 110 GPa, 1250°GC < T < 1350°C), iii) mild shock (10 < P < 25 GPa, T<500°C), and iv) reheating below 800°C.     

Coronal emission line polarization

A discussion of a program for the computation of coronal emission line polarization is presented. The starting point is a general formulation of the scattering function for magnetic dipole transitions between any two total angular momentum levels, J J, J ± 1. Illustration of the behavior of the scattering function for different transitions is given. The integration of the scattering function over the solar disk and along the line of sight accounting for arbitrary distribution of magnetic fields as well as an inhomogeneous temperature and density structure of the corona is considered next.Sample results are presented for the numerical computation of the angle of maximum polarization and the degree of maximum polarization to be expected from idealized magnetic field configurations such as radial and dipole. A computation is included for a realistic field configuration predicted to exist at the time of the 1966 eclipse. The magnetic field input to the scattering calculation is based upon the potential field extension of photospheric magnetic fields. It is the purpose of the sample calculations to demonstrate how the measurement of emission polarization measurements can be interpreted in terms of the direction of coronal magnetic fields. Factors which lend ambiguity to such interpreations are clearly illustrated from the examples. These include the Hanle-effect depolarization and the depolarization at the Van Vleck angle.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Optimal estimation of in-situ ground stresses from displacement measurements

This paper presents a study on the optimal estimation of initial in-situ stresses, in which the in-situ stresses may be constant or linearly changing along a co-ordinate axis, from a set of relative measurements between adjacent measuring points utilized for monitoring the stability of tunnels. Displacements occurring before installation of measuring devices can be taken into account. The approach may be used for a variety of models used to simulate the ground media through which the tunnels are being excavated and has the advantage of being easy to implement on the computer and having both good precision and calculation stability. Finally, an analysis is carried out on the stability of the algorithm and computational time for different numerical examples.     

Small SiC grains and a nitride grain of circumstellar origin from the Murchison meteorite: implications for stellar evolution and nucleosynthesis

We report the results of SIMS isotopic analyses of carbon, nitrogen, oxygen, and silicon made on 849 small (approximately 1 micrometer) individual silicon carbide grains from the Murchison meteorite. The isotopic compositions of the major elements carbon and silicon of most grains (mainstream) are similar to those observed in larger grain studies suggesting an AGB star origin of these grains. In contrast, the trace element nitrogen shows a clear dependency on grain size. 14N/15N ratios increase with decreasing grain size, suggesting different stellar sources for grains of different size. Typically observed 14N/15N ratios in the small grains of this study are approximately 2700, clearly larger than the values expected from model calculations of AGB stars. In addition to the three dredge-up episodes characteristic for the evolution of AGB stars, extra-mixing of CNO-processed matter in low mass AGB stars appears to be a promising possibility in order to explain the high 14N/15N ratios of the small circumstellar SiC grains. A small fraction of grains shows a silicon isotopic signature not observed in larger circumstellar SiC grains from Murchison. Their stellar origin is still uncertain. The minor type A, B, Y, and X grains were found to be present at a level of a percent, which is similar to their abundance in the larger-grain SiC separates from Murchison. Oxygen isotopic compositions are normal within the experimental uncertainties of several 10%, indicating that oxygen of stellar origin is rare or even absent in the SiC grains. We conclude that most of the oxygen is a contaminant which was introduced into the SiC grains after their formation, e.g., during sample processing in the laboratory. We identified a nitride grain, most likely Si3N4 with little carbon, with highly anomalous isotopic compositions (12C/13C = 157 +/- 33, 14N/15N = 18 +/- 1, delta 29 Si = -43 +/- 56%, delta 30 Si = -271 +/- 50%). The isotopic patterns of carbon, nitrogen, and silicon resemble those of the rare SiC X grains suggesting that these two rare constituents of circumstellar matter formed in the same type of stellar source, namely, Type II supernovae.     

Baroclinic Wave Transitions in the Martian Atmosphere

Surface pressure data from the Viking Lander mission and from GCM simulations of the martian atmosphere have been analyzed using singular systems analysis. Very regular oscillations are found with frequencies that are distributed bimodally with peaks corresponding to periods of approximately 2–4 days and 5–7 days, respectively. Reconstructions of the amplitudes of the two oscillations are often negatively correlated; i.e., when the amplitude of one oscillation is large, that of the other is small. The GCM simulations show that the negative correlation in the amplitudes of the two oscillations can be explained as a flipping between two different wavenumber modes. In the absence of diurnal forcing in the model, transition from an unrealistically regular high frequency mode to a similarly unrealistic regular low frequency mode occurs at most once during the northern winter season. The diurnal cycle in the model, however, acts in a non-linear sense to stimulate the transitions between the two wavenumbers and thus increases the frequency of mode flipping events. The corresponding simulations bear a closer resemblance to the observations.     

The dammed Hikurangi Trough: a channel-fed trench blocked by subducting seamounts and their wake avalanches (New Zealand–France GeodyNZ Project)

The Hikurangi Trough, off eastern New Zealand, is at the southern end of the Tonga–Kermadec–Hikurangi subduction system, which merges into a zone of intracontinental transform. The trough is mainly a turbidite-filled structural trench but includes an oblique-collision, foredeep basin. Its northern end has a sharp boundary with the deep, sediment-starved, Kermadec Trench. Swath-mapping, sampling and seismic surveys show modern sediment input is mainly via Kaikoura Canyon, which intercepts littoral drift at the southern, intracontinental apex of the trough, with minor input from seep gullies. Glacial age input was via many canyons and about an order of magnitude greater. Beyond a narrow, gravelly, intracontinental foredeep, the southern trench-basin is characterized by a channel meandering around the seaward edge of mainly Plio-Pleistocene, overbank deposits that reach 5 km in thickness. The aggrading channel has sandy turbidites, but low-backscatter, and long-wavelength bedforms indicating thick flows. Levées on both sides are capped by tangentially aligned mudwaves on the outsides of bends, indicating centrifugal overflow from heads of dense, fast-moving, autosuspension flows. The higher, left-bank levée also has levée-parallel mudwaves, indicating Coriolis and/or boundary currents effects on dilute flows or tail plumes. In the northern trough, basin-fill is generally less than 2 km thick and includes widespread overbank turbidites, a massive, blocky, avalanche deposit and an extensive, buried, debris flow deposit. A line of low seamounts on the subducting plate acts as a dam preventing modern turbidity currents from reaching the Kermadec Trench. Major margin collapse probably occurred in the wake of a large subducting seamount; this seamount and its wake debris flow probably dammed the trench from 2 Ma to 0.5 Ma. Before this, similar dams may have re-routed turbidity currents across the plateau.     

Optical characteristics of the whipple observatory TeV gamma-ray imaging telescope

In TeV -ray astronomy, large mirrors are used to collect erenkov light from electromagnetic cascades in the atmosphere in order to obtain low energy thresholds. The flux sensitivity of TeV -ray detectors is limited by background due to erenkov light bursts from isotropic, cosmic-ray showers which are much more numerous than -ray showers. It has recently been established that most of this background can be eliminated on the basis of the shapes of erenkov light images on the focal plane of a telescope. In order for this technique to work, the light collector must have adequate resolution over a relatively wide field of view. In this paper, the optical characteristics of the 10 m reflector used in the imaging detection of the Crab Nebula are examined and contrasted with those of a standard parabolic design. This 10 m reflector has a unique (Davies-Cotton) design with small spherical facet mirrors placed on spherical support structure with radius equal to exactly 1/2 the curvature radius of the facet mirrors. The off-axis focusing properties of this type of telescope have not been examined previously.     

Atmospheric and cloud structures of the Jovian planets

Wet adiabatic atmospheric models are computed for the four Jovian planets. Possible cloud-forming condensates considered are H₂O ice, aqueous NH₃ solution, NH₄SH, and solid NH₃, CH₄, and Ar. Some techniques for computer calculation of condensation in a multiphase system of variable composition are discussed. Nominal models are computed for all four-planets, and the effects of variations in composition and different pressure-temperature regimes are investigated in detail for Jupiter. The meaning of the calculated cloud densities and their relation to the actual densities are discussed. The computed atmospheric profiles represent average conditions, subject to considerable local variations.