ALBERT

Description: The formation of a large volcano loads the underlying lithospheric plate and can lead to lithospheric flexure and faulting. In turn, lithospheric deformation affects the stress field beneath and within the volcanic edifice and can influence magma transport. Modeling the interaction of these processes is crucial to an understanding of the history of eruption characteristics and tectonic deformation of large volcanoes. We develop models of time-dependent stress and deformation for the Tharsis volcanoes on Mars. By means of a finite element code, we calculate stresses and displacements due to a volcano-shaped load emplaced on an elastic plate overlying a viscoelastic mantle. Models variously incorporate growth of the volcanic load with time and a detachment between volcano and lithosphere. The models illustrate the manner in which time-dependent stresses induced by lithospheric plate flexure beneath the volcanic load may affect eruption histories, and the derived stress fields can be related to tectonic features on and surrounding Martian volcanoes. As a result of flexure there are three regions where stresses become sufficiently large to cause failure by faulting, according to the Mohr-Coulomb criterion: at the surface of the plate just outward of the volcano, near the base of the elastic lithosphere beneath the center of the volcano, and on the upper flanks of the volcano early in its growth history.

Description: The temperature structure within the northern auroral region of Jupiter is studied by reanalyzing the Voyager 1/infrared interferometer and radiometer spectrometer (IRIS) spectra. The total measured excess infrared auroral zone emission (averaged over the IRIS field of view) in the hydrocarbon bands between 7 and 13 microns is found to be about 208 ergs/cm/s over an area of about 2 x 10(exp 18) sq cm with a resulting power output of 4 x 10(exp 13) W. In comparison, the total energy deposition by magnetospheric charged particles has been estimated on the basis of UV observations to range between 1 x 10(exp 13) and 4 x 10(exp 13) W over a comparable area. The large amount of radiated energy observed in the infrared may imply an additional heat source in the auroral regions (possibly Joule heating). A new set of thermal profiles of Jupiter's high-latitude upper atmosphere has also been derived. These profiles have a large temperature enhancement in the upper stratosphere and are constrained to reproduce the CH4 emission at 7.7 microns. The emission in the other hydrocarbon bands (C2H2 and C2H6) is found to depend on the depth to which the temperature enhancement extends, which further constrains the thermal profiles. This study shows that a large temperature enhancement in the upper stratosphere and lower thermosphere can explain the observed excess hydrocarbon emission bands; thus smaller variations in hydrocarbon abundances (between the high latitudes and the equatorial and middle latitudes) are required than has been assumed in previous models.

Description: Observations of (O I) 6300-A emission near Io have been obtained in 1990, 1991, and 1992 by the National Solar Observatory staff using the solar-stellar spectrography on the McMath-Pierce telescope at Kitt Peak. High-resolution spectra with a resolving power of about 1.2 x 10(exp 5) were obtained with an integration time of 10-15 min each. The viewing aperture for the observations was 5.2 arc sec x 5.2 arc sec centered on Io, with spatial resolution limited within this area by seeing conditions. Observations thus far have been reduced to obtain average brightness values over the aperture which range from approximately 200 to 1000 R for a number of different Io phase angles and Io system III longitudes. The (O I) 6300-A emission brightness exhibits an east-west asymmetry, where the average intensity for Io phase angles in the west (receding ansa) is 1.5 times brighter than in the east (approaching ansa). Similar east-west intensity ratios have also been observed for neutrals near Io and ions in the plasma torus for a number of other optical and ultraviolet emission lines which are excited by electron impact. In addition to the east-west asymmetry, the (O I) 6300-A emission brightness exhibits a strong dependence on the Io system III longitude angle, with a maximum value occurring in the range 200 deg +/- 50 deg. Earlier IUE observations of ultraviolet emission lines of O and S near Io obtained over a number of years have measured the east-west asymmetry, but the long IUE integration times of approximately 7-14 hours masked any detection of system III variability. For the (O I) 6300-A emission, the O(1D) state may be excited by electron impact of atomic oxygen and by electron impact dissociation of SO. The molecule SO may be present at the exobase or may be produced above the exobase as the dominant product of SO2 dissociation by electron impact. Preliminary assessment indicates that production of O(1D) by molecular dissociation may be more important. The (O I) 6300-A emission may therefore provide a remote signature for monitoring (1) the upward transport rates of molecular species in Io's atmosphere, (2) the relative abundance and time-variable dissociation of SO2 and/or SO at the exobase and in the corona of Io, and (3) the spatial distribution of these escaping molecular and atomic species and their ion production rates in the planetary magnetosphere.

Description: Multiangle Advanced Solid State Array Spectroradiometer (ASAS) and Portable Apparatus for Rapid Acquisition of Bidirectional Observations of Land and Atmosphere (PARABOLA) visible and near infrared reflectance data covering the Lunar Lake playa, Nevada, were modeled using specular and volume scattering theory. The volume component used to model the data was based on the Hapke (1986) model and the specular scattering component was based on Fresnel reflection from surface facets with a distribution of tilts. Specular scattering was needed to explain the several-fold increase in reflectance observed at high phase angles in the solar principal plane. Results imply single scattering albedos of approximately 0.93 to 0.95, a real index of refraction value of approximately 1.52 (at a wavelength of 0.66 micron), a small imaginary index of refraction and an exponential facet tilt probability function for the playa. Electron micrographs showed th playa surface to consist of ensembles of smooth, micrometer-scale ellipsoidal particles on gently undulating topography at the millimeter to centimeter scale. The exponential probability function is consistent with this observation. Mars and the icy satellites are discussed as possible places to look for a significant specular scattering component.

Description: We present a comprehensive analysis of data obtained during the 1989 July 3 occultation of 28 Sgr by Titan. The data set includes 23 lightcurves from 15 separate stations, spanning wavelengths from 0.36 to 0.89 micron. A detailed model of the structure of Titan's atmosphere in the altitude range 250 to 450 km is developed, giving the distribution of temperature, pressure, haze optical depth, and zonal wind velocity as a function of altitude and latitude. Haze layers detected in Titan's stratosphere are about one scale height higher than inferred from Voyager data, and show a wavelength dependence indicative of particle sizes on the order of 0.1 micron. A marked north-south dichotomy in haze density is observed with a transition to lower density south of about -20 deg latitude. Zonal wind speeds are inferred from global distortions from spherical symmetry and are of the order of 100 m/s with significant increase toward higher latitudes. Titan's high atmosphere shows substantial axial symmetry; the position angle of the symmetry axis is equal to the position angle of Saturn's spin axis to within about 1 deg.

Description: The atmospheres of Uranus and Neptune are discussed in the light of the Voyager 2 flybys of these planets. A basic overview of their atmospheres is presented, with emphasis on thermal structure, composition, energy and opacity sources, cloud structure, and the horizontal structure of the atmospheres. The nature and implications of the different internal heat flows on the two planets, and the implications of the deuterium and helium abundances for the origin and evolution of these ice giants, as distinct from Jupiter and Saturn, are discussed. Selected chemical and physical processes in the atmospheres of Uranus and Neptune are illustrated.

Description: Our studies of the silicate-bearing inclusions in the IIICD iron meteorites Maltahohe, Carlton, and Dayton suggest that their mineralogy and mineral compositions are related to the composition of the metal in the host meteorites. An inclusion in the low-Ni Maltahohe is similar in mineralogy to those in IAB irons, which contain olivine, pyroxene, plagioclase, graphite, and troilite. With increasing Ni concentration of the metal, silicate inclusions become poorer in graphite, richer in phosphates, and the phosphate and silicate assemblages become more complex. Dayton contains pyroxene, plagioclase, SiO2, brianite, panethite, and whitlockite, without graphite. In addition, mafic silicates become more FeO-rich with increasing Ni concentration of the hosts. In contrast, silicates in IAB irons show no such correlation with host Ni concentration, nor do they have the complex mineral assemblages of Dayton. These trends in inclusion composition and mineralogy in IIICD iron meteorites have been established by reactions between the S-rich metallic magma and the silicates, but the physical setting is uncertain. Of the two processes invoked by other authors to account for groups IAB and IIICD, fractional crystallization of S-rich cores and impact generation of melt pools, we prefer core crystallization. We suggest that the solidification of the IIICD core may have been very complex, involving fractional crystallization, nucleation effects and, possibly, liquid immiscibility.

Description: Escape rates of oxygen atoms from dissociative recombination of O2(+) above the Martian exobase are computed in light of new information from ab initio calculations of the dissociative recombination process and our recently revised understanding of the Martian dayside ionosphere. Only about 60 percent of the dissociative recombinations occur in channels in which the O atoms are released with energies in excess of the escape velocity. Futhermore, we find that the computed escape fluxes for O depend greatly on the nature of the ion loss process that has been found necessary to reproduce the topside ion density profiles measured by Viking. If it is assumed that the ions are not lost from the gravitational field of the planet, as required by an analysis of nitrogen escape, the computed average O escape rate is 3 x 10 exp 6/sq cm/s, much less than half the H escape rates inferred from measurements of the Lyman-alpha dayglow, which are in the range (1-2) x 10 exp 8/sq cm/s. Suggestions for restoring the relative escape rates of H and O to the stoichiometric ratio of water are explored.

Description: Primitive meteorites contain a few parts per million (ppm) of pristine interstellar grains that provide information on nuclear and chemical processes in stars. Their interstellar origin is proven by highly anomalous isotopic ratios, varying more than 1000-fold for elements such as C and N. Most grains isolated thus far are stable only under highly reducing conditions (C/O greater than 1), and apparently are 'stardust' formed in stellar atmospheres. Microdiamonds, of median size about 10 A, are most abundant (about 400-1800 ppm) but least understood. They contain anomalous noble gases including Xe-HL, which shows the signature of the r- and p-processes. Silicon carbide, of grain size 0.2-10 microns and abundance about 6 ppm, shows the signature of the s-process and apparently comes mainly from red giant carbon (AGB) stars of 1-3 solar masses. Some grains appear to be not less than 10 exp 9 a older than the solar system. Graphite spherules of grain size 0.8-7 microns and abundance less than 2 ppm contain highly anomalous C and noble gases, as well as large amounts of fossil Mg-26 from the decay of extinct Al-26. They seem to come from at least three sources, probably AGB stars, novae, and Wolf-Rayet stars.

Description: Lunar soils contain micrometer-sized mineral grains surrounded by thin amorphous rims. Similar features have been produced by exposure of pristine grains to a simulated solar wind, leading to the widespread belief that the amorphous rims result from radiation damage. Electron microscopy studies show, however, that the amorphous rims are compositionally distinct from their hosts and consist largely of vapor-deposited material generated by micrometeorite impacts into the lunar regolith. Vapor deposits slow the lunar erosion rate by solar wind sputtering, influence the optical properties of the lunar regolith, and may account for the presence of sodium and potassium in the lunar atmosphere.