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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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NASA NTRS1990

Preferred orientations in extruded nickel and iron aluminides

A plotting of inverse pole figures is used to characterize the preferred orientations in both powder-extruded and cast-and-extruded NiAl, FeAl, and Ni3Al; the preferred 111 orientation has been noted along the extrusion direction in both powder-extruded and cast-and-extruded NiAl. While powder-extruded FeAl also had a 111 preferred orientation in the as-extruded condition, 110 was observed upon casting and extrusion; this was in turn replaced by a 211 orientation preference upon annealing, although annealing did not change the preferred orientation in either NiAl or in powder-extruded FeAl.

Khadkikar, P. S.↗

NASA NTRS1996

The Lunar Crust: Global Structure and Signature of Major Basins

New lunar gravity and topography data from the Clementine Mission provide a global Bouguer anomaly map corrected for the gravitational attraction of mare fill in mascon basins. Most of the gravity signal remaining after corrections for the attraction of topography and mare fill can be attributed to variations in depth to the lunar Moho and therefore crustal thickness. The large range of global crustal thickness (approx. 20-120 km) is indicative of major spatial variations in melting of the lunar exterior and/or significant impact-related redistribution. The 6l-km average crustal thickness, constrained by a depth-to-Moho measured during the Apollo 12 and 14 missions, is preferentially distributed toward the farside, accounting for much of the offset in center-of-figure from the center-of-mass. While the average farside thickness is 12 km greater than the nearside, the distribution is nonuniform, with dramatic thinning beneath the farside, South Pole-Aitken basin. With the global crustal thickness map as a constraint, regional inversions of gravity and topography resolve the crustal structure of major mascon basins to half wavelengths of 150 km. In order to yield crustal thickness maps with the maximum horizontal resolution permitted by the data, the downward continuation of the Bouguer gravity is stabilized by a three- dimensional, minimum-slope and curvature algorithm. Both mare and non-mare basins are characterized by a central upwarped moho that is surrounded by rings of thickened crust lying mainly within the basin rims. The inferred relief at this density interface suggests a deep structural component to the surficial features of multiring lunar impact basins. For large (greater than 300 km diameter) basins, moho relief appears uncorrelated with diameter, but is negatively correlated with basin age. In several cases, it appears that the multiring structures were out of isostatic equilibrium prior to mare emplacement, suggesting that the lithosphere was strong enough to maintain their state of stress to the present.

Neumann, Gregory A.↗

NASA NTRS1995

NH3, H2S, and the Radio Brightness Temperature Spectra of the Giant Planets

Recent radio interferometer observations of Neptune enable comparisons of the radio brightness temperature (T(sub B)) spectra of all four giant planets. This comparison reveals evidence for fundamental differences in the compositions of Uranus' and Neptune's upper tropospheres, particularly in their ammonia (NH3) and hydrogen sulfide (H2S) mixing ratios, despite those planets' outward similarities. The tropospheric abundances of these constituents yield information about their deep abundances, and ultimately about the formation of the planets from the presolar nebula (Atreya et al.). Figures 1, 2, 3, and 4 show the T(sub B) spectra of Jupiter, Saturn, Uranus, and Neptune, respectively, from 0.1 to tens of cm wavelength. The data shown are collected from many observers. Data for Jupiter, Saturn, and Uranus are those cataloged by de Pater and Massie (1985), plus the Saturn Very Large Array (VLA) data by Grossman et al. Figure 3, Uranus, shows only data acquired since 1973. Before 1973 Uranus' T(sub B) increased steadily as its pole moved into view, causing significant scatter in those data. Neptune data at greater than 1 cm, all taken at the VLA, are collected from de Pater and Richmond, de Pater et al., and Hofstadter. For a variety of reasons, such as susceptibility to source confusion, single-dish data at those wavelengths are much noisier than the more reliable VLA data and have been ignored. Single-dish data by Griffin and Orton shortward of 0.4 cm are shown, along with the Owens Valley Radio Observatory (interferometer) datum at 0.266 cm by Muhleman and Berge. Spectra of Jupiter, Saturn, and Neptune share certain gross characteristics. In each spectrum, T(sub B) at 1.3 cm is approximately 120-140 K, less than approximately 30 K different from that at 0.1 cm. All three spectra show a break in slope at or near 1.3 cm, with T(sub B) increasing fairly rapidly with wavelength longward of 1.3 cm. Visible and IR spectroscopy show that NH3, whose strong inversion spectrum peaks at appropximately 1.3 cm, is an important tropospheric species at Jupiter and Saturn. Its signature on the Jovian radio spectrum is obvious, causing the prominent "hole" at 1.3 cm. At Saturn it is more subdued but is the source of that spectrum's change in slope at 1.3 cm. Radiative transfer models of Jupiter and Saturn with near-solar deep NH3 abundances agree well with the data (e.g., de Pater).

Spilker, Thomas R.↗