Supernovae as sources of interstellar diamonds

Small hydrocarbon grains in the vicinity of a supernova could be annealed by the absorption of several far-ultraviolet photons to produce the tiny diamonds found in meteorites. These freshly-synthesized diamond grains would be bombarded by the heavy ions and neutrals in the supernovae outflow and would thereby acquire the distinctive noble-gas isotopic signature by which they were first isolated. Only diamonds formed relatively close to supernovae would acquire such a signature, since grains formed farther out would be subjected to a much diluted and less energetic plasma environment.     

Mercury: Radar images of the equatorial and midlatitude zones

Radar imaging results for Mercury's non-polar regions are presented. The dual-polarization, delay-Doppler images were obtained from several years of observations with the upgraded Arecibo S-band (λ12.6-cm) radar telescope. The images are dominated by radar-bright features associated with fresh impact craters. As was found from earlier Goldstone-VLA and pre-upgrade Arecibo imaging, three of the most prominent crater features are located in the Mariner-unimaged hemisphere. These are: “A,” an 85-km-diameter crater (348° W, 34° S) whose radar ray system may be the most spectacular in the Solar System; “B,” a 95-km-diameter crater (343° W, 58° N) with a very bright halo but less distinct ray system; and “C,” an irregular feature with bright ejecta and rays distributed asymmetrically about a 125-km source crater (246° W, 11° N). Due south of “C” lies a “ghost” feature (242° W, 27° S) that resembles “A” but is much fainter. An even fainter such feature is associated with Bartok Crater. These may be two of the best mercurian examples of large ejecta/ray systems observed in an intermediate state of degradation. Virtually all of the bright rayed craters in the Mariner 10 images show radar rays and/or bright rim rings, with radar rays being less common than optical rays. Radar-bright craters are particularly common in the H-7 quadrangle. Some diffuse radar albedo variations are seen that have no obvious association with impact ejecta. In particular, some smooth plains regions such as the circum-Caloris plains in Tir, Budh, and Sobkou Planitiae and the interiors of Tolstoj and “Skinakas” basins show high depolarized brightness relative to their surroundings, which is the reverse of the mare/highlands contrast seen in lunar radar images. Caloris Basin, on the other hand, appears dark and featureless in the images.     

The five stable isotope compositions of Fig Tree barites: Implications on sulfur cycle in ca. 3.2 Ga oceans

The discovery of ³³S anomalies in Archean sedimentary rocks has established that the early Earth before ∼2.2 Ga (billion years ago) had a very different sulfur cycle than today. The origin of the anomalies and the nature of early sulfur cycle are, however, poorly known and debated. In this study, we analyzed the total sulfur and oxygen isotope compositions, the δ¹⁸O, Δ¹⁷O, δ³⁴S, Δ³³S, and Δ³⁶S, for the >3.2 Ga Fig Tree barite deposits from the Barberton Greenstone Belt, South Africa. The goal is to address two questions: (1) was Archean barite sulfate a mixture of ³³S-anomalous sulfate of photolysis origin and ³³S-normal sulfate of other origins? (2) did the underlying photochemical reactions that generated the observed ³³S anomalies for sulfide and sulfate also generate ¹⁷O anomalies for sulfate?We developed a new method in which pure barite sulfate is extracted for oxygen and sulfur isotope measurements from a mixture of barite sands, cherts, and other oxygen-bearing silicates. The isotope data reveal that (1) there is no distinct ¹⁷O anomaly for Fig Tree barite, with an average Δ¹⁷O value the same as that of the bulk Earth (−0.02 ± 0.07‰, N = 49); and (2) the average δ¹⁸O value is +10.6 ± 1.1‰, close to that of the modern seawater sulfate value (+9.3‰). Evidence from petrography and from the δ¹⁸O of barites and co-existing cherts suggest minimum overprinting of later metamorphism on the sulfate’s oxygen isotope composition. Assuming no other processes (e.g., biological) independently induced oxygen isotope exchange between sulfate and water, the lack of reasonable correlation between the δ¹⁸O and Δ³³S or between the δ³⁴S and Δ³³S suggests two mutually exclusive scenarios: (1) An overwhelming majority of the sulfate in the Archean ocean was of photolysis origin, or (2) The early Archean sulfate was a mixture of ³³S-normal sulfates and a small portion (<5%?) of ³³S-anomalous sulfate of photolysis origin from the atmosphere. Scenario 1 requires that sulfate of photolysis origin must have had only small ³³S or ³⁶S anomalies and no ¹⁷O anomaly. Scenario 2 requires that the photolysis sulfate have had highly negative δ³⁴S and Δ³³S values, recommending future theoretical and experimental work to look into photochemical processes that generate sulfate in Quadrant I and sulfide in Quadrant III in a δ³⁴S (X)-Δ³³S (Y) Cartesian plane. A total sulfur and oxygen isotope analysis has provided constraints on the underlying chemical reactions that produced the observed sulfate isotope signature as well as the accompanying atmospheric, oceanic, and biological conditions.     

A statistical model for foraminiferal paleobathymetry, with application to the middle Miocene of the Hokuroku district, Japan

A paleobathymetric Bayes estimator, based on a uniform prior distribution of foraminiferal species over the depth range from 0 to 5000 m, is developed and calibrated from the occurrence of 35 species of foraminifera from present day sites on the Pacific and Indian Ocean seafloors. The species are grouped into 12 clusters in order to more nearly satisfy the independence assumptions of the model. Objective depth-independent criteria are developed for ruling certain samples inadmissible for estimation. The accuracy of the estimator is assessed by comparing estimated and measured depths for the admissible calibration samples, yielding a mean absolute error of estimation of between 300 and 400 m. The depth estimator is applied to foraminifera-bearing Miocene samples from the Hokuroku district of Japan, where paleobathymetric depths of between 3000 and 4000 m are indicated at the time of massive sulfide ore formation.     

Quasi-periodic structure in solar microwave bursts

High sensitivity, high time resolution recordings of microwave radio bursts show a number of periodic and quasi-periodic bursts which exhibit intervals of the order of 10–20 s. Some of the bursts are accompanied by simultaneous pulsations of the same interval detected in X-rays, type III-m, and extreme ultraviolet emissions. Mechanisms to explain solar radio pulsations are reviewed to see which can explain or be extended to explain these observations.Supported by a company-financed research program of The Aerospace Corporation.     

Lunar soil grain size distribution

A comprehensive review has been made of the currently available data for lunar grain size distributions. It has been concluded that there is little or no statistical difference among the large majority of the soil samples from the Apollo 11, 12, 14, and 15 missions. The grain size distribution for these soils has reached a steady state in which the comminution processes are balanced by the aggregation processes. The median particle size for the steady-state soil is 40 to 130 µm. The predictions of lunar grain size distributions based on the Surveyor television photographs have been found to be quantitatively in error and qualitatively misleading.     

Characterizing the severe turbulence environments associated with commercial aviation accidents. Part 2: Hydrostatic mesoscale numerical simulations of supergradient wind flow and streamwise ageostrophic frontogenesis

Summary Simulation experiments reveal key processes that organize a hydrostatic environment conducive to severe turbulence. The paradigm requires the juxtaposition of the entrance region of a curved jet streak, which is highly subgeostrophic, with the entrance region of a straight jet streak, which is highly supergeostrophic. The wind and mass fields become misphased as the entrance regions converge, resulting in significant spatial variation of inertial forcing, centripetal forcing, as well as along and cross-stream pressure gradient forcing over a meso- scale region. Maxima of these forces are misphased where the two dissimilar jet streaks converge and geostrophic balance is disrupted. Velocity divergence within the subgeostrophic region of largest upstream-directed pressure gradient force and velocity convergence near the region of largest downstream-directed centripetal/inertial-advective forcing act to produce a mesoscale front due to spatially varying confluent flow flanked by zones of increasing velocity divergence. This results in frontogenesis as well as the along-stream divergence of cyclonic and convergence of cyclonic ageostrophic vertical vorticity. The nonuniform centripetally forced mesoscale front becomes the locus of large gradients of ageostrophic vertical vorticity along an overturning isentrope. This region becomes favorable for streamwise vorticity gradient formation enhancing the environment for the organization of horizontal vortex tubes in the presence of buoyant forcing. This is because the mesoscale convergence of vertical vorticity on an overturning isentropic surface creates vertical rotation for the development of horizontal vorticity in regions where isentropic surfaces overturn. Vorticity, shear, and buoyancy are focused in one location by this front thus favoring an environment favorable for microvortex formation leading to turbulence.     

Eros: Shape, Topography, and Slope Processes

Stereogrammetric measurement of the shape of Eros using images obtained by NEAR's Multispectral Imager provides a survey of the major topographic features and slope processes on this asteroid. This curved asteroid has radii ranging from 3.1 to 17.7 km and a volume of 2535±20 km³. The center of figure is within 52 m of the center of mass provided by the Navigation team; this minimal difference suggests that there are only modest variations in density or porosity within the asteroid. Three large depressions 10, 8, and 5.3 km across represent different stages of degradation of large impact craters. Slopes on horizontal scales of ∼300 m are nearly all less than 35°, although locally scarps are much steeper. The area distribution of slopes is similar to those on Ida, Phobos, and Deimos. Regions that have slopes greater than 25° have distinct brighter markings and have fewer large ejecta blocks than do flatter areas. The albedo patterns that suggest downslope transport of regolith have sharper boundaries than those on Phobos, Deimos, and Gaspra. The morphology of the albedo patterns, their lack of discrete sources, and their concentration on steeper slopes suggest transport mechanisms different from those on the previously well-observed small bodies, perhaps due to a reduced relative effectiveness of impact gardening on Eros. Regolith is also transported in talus cones and in connected, sinuous paths extending as much as 2 km, with some evident as relatively darker material. Talus material in at least one area is a discrete superposed unit, a feature not resolved on other small bodies. Flat-floored craters that apparently contain ponded material also suggest discrete units that are not well mixed by impacts.     

Anthropogenic, Climatic, and Hydrologic Trends in the Kosi Basin, Himalaya

A great debate exists concerning theinfluence of land-use and climatic changes onhydrology in the Himalayan region and its adjacentplains. As a representative basin of the Himalayas, westudied basinwide land-use, climatic and hydrologictrends over the Kosi Basin (54,000 km²) in themountainous area of the central Himalayan region. Theassessment of anthropogenic inputs showed that thepopulation of the basin grew at a compound rate ofabout one percent per annum during the past fourdecades. The comparison of land-use data between thesurveys made during the 1960s and 1978–1979 did notreveal noticeable trends in land-use change. Theanalysis of meteorological and hydrological timeseries from 1947 to 1993 showed some increasingtendency of temperature and precipitation. Thestatistical tests of hydrologic trends indicated anoverall decrease in discharge on the Kosi River andits major tributaries. The decreasing trends ofstreamflow were more significant during the low-flowmonths. The statistical analysis of homogeneityshowed that the climatic as well as the hydrologictrends were more localized in nature lacking adistinct basinwide significance.     

Observational Evidence of Recent Change in the Northern High-Latitude Environment

Studies from a variety of disciplines documentrecentchange in the northern high-latitude environment.Prompted by predictions of an amplified response oftheArctic to enhanced greenhouse forcing, we present asynthesis of these observations. Pronounced winter andspring warming over northern continents since about 1970ispartly compensated by cooling over the northern NorthAtlantic. Warming is also evident over the centralArcticOcean. There is a downward tendency in sea ice extent,attended by warming and increased areal extent of theArctic Ocean's Atlantic layer. Negative snow coveranomalies have dominated over both continents sincethelate 1980s and terrestrial precipitation has increasedsince 1900. Small Arctic glaciers have exhibitedgenerally negative mass balances. While permafrost haswarmed in Alaska and Russia, it has cooled in easternCanada. There is evidence of increased plant growth,attended by greater shrub abundance and northwardmigration of the tree line. Evidence also suggeststhatthe tundra has changed from a net sink to a net sourceofatmospheric carbon dioxide.Taken together, these results paint a reasonablycoherent picture of change, but their interpretationassignals of enhanced greenhouse warming is open todebate.Many of the environmental records are either short,areof uncertain quality, or provide limited spatialcoverage. The recent high-latitude warming is also nolarger than the interdecadal temperature range duringthis century. Nevertheless, the general patterns ofchange broadly agree with model predictions. Roughlyhalfof the pronounced recent rise in Northern Hemispherewinter temperatures reflects shifts in atmosphericcirculation. However, such changes are notinconsistentwith anthropogenic forcing and include generallypositive phases of the North Atlantic and ArcticOscillations and extratropical responses to theEl-NiñoSouthern Oscillation. An anthropogenic effect is alsosuggested from interpretation of the paleoclimaterecord,which indicates that the 20th century Arctic is thewarmest of the past 400 years.