Chemical aspects of iron oxide coagulation in water: Laboratory studies and implications for natural systems

Initial coagulation rates of colloidal hematite (-Fe₂O₃) particles (diameter less than 0.1 µm) were measured experimentally in well-defined laboratory systems at constant temperature. The relative stability ratio,W, was obtained at various ionic strengths in NaCl medium and at pH values in the range from 3 to 12. ExperimentalW values ranged from 1 to 10⁴ in various systems. The results delineate the roles ofspecific andgeneralized coagulation mechanisms for iron oxides. Among the specifically-interacting species (G _ads ⁰ >G _coul ⁰ ) studied were phosphate, monomeric organic acids of various structures, and polymeric organic acids. The critical coagulation-restabilization concentrations of specifically-interacting anions (from 10^–7 to 10^–4 molar) can be compared with the general effects of non-specific electrolyte coagulants (10^–3 to 10^–1 molar). The laboratory results are interpreted with the help of a Surface Complex Formation/Diffuse Layer Model (SCF/DLM) which describes variations of interfacial charge and potential resulting from variations of coagulating species in solution. Comparison of these laboratory experiments with observations on iron behavior in estuarine and lake waters aids in understanding iron removal mechanisms and coagulation time scales in natural systems.     

Direct imaging of extra-solar planets with stationary occultations viewed by a space telescope

The feasibility of detecting planets outside the solar system through imaging at optical wavelengths by a telescope in space is considered. The “black” limb of the moon can be used as an occulting edge to greatly reduce the background light from the planet's star. With this technique and if certain other technical requirements can be realized, a hypothetical Jupiter-Sun system could be detected at a distance of 10 pc. For this system, a signal-to-noise ratio of 9 could be achieved in less than 20 min with a 2.4-m telescope in space. For diffraction limited optics, the required integration time is inversely proportional to the fifth power of the telescope aperture. An orbit for the telescope is described that could achieve a stationary lunar occultation of any star that would last nearly two hours, providing six times more integration time than required by the hypothetical Jupiter-Sun example.     

Properties of the clouds of Venus,as inferred from airborne observations of its near-infrared reflectivity spectrum

We have measured the shape and absolute value of Venus' reflectivity spectrum in the 1.2-to 4.0-μm spectral region with a circular variable filter wheel spectrometer having a spectral resolution of 1.5%. The instrument package was mounted on the 91-cm telescope of NASA Ames Kuiper Airborne Observatory, and the measurements were obtained at an altitude of about 41,000 feet, when Venus had a phase angle of 86°. Comparing these spectra with synthetic spectra generated with a multiple-scattering computer code, we infer a number of properties of the Venus clouds. We obtain strong confirmatory evidence that the clouds are made of a water solution of sulfuric acid in their top unit optical depth and find that the clouds are made of this material down to an optical depth of at least 25. In addition, we determine that the acid concentration is 84 ± 2% H_2SO4 by weight in the top unit optical depth, that the total optical depth of the clouds is 37.5 ± 12.5, and that the cross-sectional weighted mean particle radius lies between 0.5 and 1.4 μm in the top unit optical depth of the clouds. These results have been combined with a recent determination of the location of the clouds' bottom boundary [Marov et al., Cosmic Res.14, 637–642 (1976)] to infer additional properties about Venus' atmosphere. We find that the average volume mixing ratio of H₂SO₄ and H₂O contained in the cloud material both equal approximately 2× 10^?6. Employing vapor pressure arguments, we show that the acid concentration equals 84 ± 6% at the cloud bottom and that the water vapor mixing ratio beneath the clouds lies between 6 × 10^?4 and 10^?2.     

Microcraters formed in hot glass by hypervelocity projectiles

Microcraters were formed in heated soda-lime glass by the normal incidence of spheres of plastic or fused silica with diameters between 0.8 and 4.5m and velocities between 2.5 and 10 km s^–1. The morphology of the craters in targets at temperatures up to 800°C is little different from those formed in unheated glass. Spallation still occurs to the same extent and above the same velocity threshold, but the spalls sag and sharp edges become dull in a few seconds at temperatures above the softening point. There is a small increase in the flow of glass from the central pit into a narrow lip at the higher temperatures, but this lip is often removed by spallation, especially at the higher velocities of impact. There is no evidence of a splashed lip with strings of melt overlying the spalled area. The results in conjunction with other evidence suggest that most lunar craters of micrometer size with a smooth central pit, splashed lip, and a spallation zone are the result of primary impacts.     

The Process of Competitive Discussion

Every active scientific discipline has arrived at its own peculiar definition of acceptable problems and acceptable ways of attacking them through a social process of mutual consent.     

The formation of saturn's satellites and rings,as influenced by saturn's contraction history

We have used Pollack et al.'s 1976 calculations of the quasi-equilibrium contraction of Saturn to study the influence of the planet's early high luminosity on the formation of its satellites and rings. Assuming that the condensation of ices ceased at the same time within Jupiter's and Saturn's primordial nebulae, and using limits for the time of cessation derived for Jupiter's system by Pollack and Reynolds (1974) and Cameron and Pollack (1975), we arrive at the following tentative conclusions. Titan is the innermost satellite at whose position a methane-containing ice could condense, a result consistent with the presence of methane in this satellite's atmosphere. Water ice may have been able to condense at the position of all the satellites, a result consistent with the occurrence of low-density satellites close to Saturn. The systematic decrease in the mass of Saturn's regular satellites with decreasing distance from Saturn may have been caused partially by the larger time intervals for the closer satellites between the start of contraction and the first condensation of ices at their positions and between the start of contraction and the time at which Saturn's radius became less than a satellite's orbital radius. Ammonia ices, principally NH₄SH, were able to condense at the positions of all but the innermost satellites.Water ice may bave been able to condense in the region of the rings close to the end of the condensation period. We speculate that the rings are unique to Saturn because on the one hand, temperatures within Jupiter's Roche limit never became cool enough for ice particles to form before the end of the condensation period and on the other hand, ice particles formed only very early within Uranus' and Neptune's Roche limits, and were eliminated by gas drag effects that caused them to spiral into the planet before the gas of these planets' nebula was eliminated. Gas drag would also have eliminated any rocky particles initially present inside the Roche limit.We also derive an independent estimate of several million years for the time between the start of the quasi-equilibrium contraction of Saturn and the cessation of condensation. This estimate is based on the density and mass characteristics of Saturn's satellites. Using this value rather than the one found for Jupiter's satellites, we find that the above conclusions about the rings and the condensation of methane-and ammonia-containing ices remain valid.     

The spectral reflectivity of select areas on Europa

The July 12, 1973, occultation of Europa by Io was observed in 30 wavelength channels in the spectral region λλ3200-11, 000 Å with the 200-inch Hale telescope and a multichannel spectrometer. The data are presented in absolute units above the Earth's atmosphere. The data are analyzed to obtain the spectral reflectivity of seven localized areas on the disk of Europa centered on 324°W longitude. The equatorial material is confirmed to be darker than the eastward-skewed bright north polar cap and a hint is evident that the darker material as well may be somewhat redder than the cap material.     

Low temperature volatile production at the Lost City Hydrothermal Field, evidence from a hydrogen stable isotope geothermometer

Although commonly utilized in continental geothermal work, the water-hydrogen and methane-hydrogen isotope geothermometers have been neglected in hydrothermal studies. Here we report δD-CH₄ and δD-H₂ values from high-temperature, black smoker-type hydrothermal vents and low-temperature carbonate-hosted samples from the recently discovered Lost City Hydrothermal Field. Methane deuterium content is uniform across the dataset at − 120 ± 12‰. Hydrogen δD values vary from − 420‰ to − 330‰ at high-temperature vents to − 700‰ to − 600‰ at Lost City. The application of several geothermometer equations to a suite of hydrothermal vent volatile samples reveals that predicted temperatures are similar to measured vent temperatures at high-temperature vents, and 20-60 °C higher than those measured at the Lost City vents. We conclude that the overestimation of temperature at Lost City reflects 1) that methane and hydrogen are produced by serpentinization at > 110 °C, and 2) that isotopic equilibrium at temperatures < 70 °C is mediated by microbial sulfate reduction. The successful application of hydrogen isotope geothermometers to low-temperature Lost City hydrothermal samples encourages its employment with low-temperature diffuse hydrothermal fluids.     

Mass-dependent fractionation of quadruple stable sulfur isotope system as a new tracer of sulfur biogeochemical cycles

Sulfur isotope studies of post-Archean terrestrial materials have focused on the ratio ³⁴S/³²S because additional isotopes, ³³S and ³⁶S, were thought to carry little information beyond the well-known mass-dependent relationship among multiple-isotope ratios. We report high-precision analyses of Δ³³S and Δ³⁶S values, defined as deviations of ³³S and ³⁶S from ideal mass-dependent relationships, for international reference materials and sedimentary sulfides of Phanerozoic age by using a fluorination technique with a dual-inlet isotope ratio mass spectrometer. Measured variations in Δ³³S and Δ³⁶S are explained as resulting from processes involve branching reactions (two or more reservoirs formed) or mixing. Irreversible processes in closed systems (Rayleigh distillation) amplify the isotope effect. We outline how this new isotope proxy can be used to gain new insights into fundamental aspects of the sulfur biogeochemical cycle, including additional constraints on seawater sulfate budget and processes in sedimentary sulfide formation. The isotope systematics discussed here cannot explain the much larger variation of Δ³³S and Δ³⁶S observed in Archean rock records. Furthermore, Phanerozoic samples we have studied show a characteristic Δ³³S and Δ³⁶S relationship that differs from those measured in Archean rocks and laboratory photolysis experiments. Thus, high precision analysis of Δ³³S and Δ³⁶S can be used to distinguish small non-zero Δ³³S and Δ³⁶S produced by mass-dependent processes from those produced by mass-independent processes in Archean rocks and extraterrestrial materials.