Accretive Conditions Research Articles

Effects of dissipating gas drag on planetesimal accretion in binary systems

AbstractWe numerically investigate the conditions for planetesimal accretion in the circumprimary disk under the perturbing presence of a companion star, with focus on the γ Cephei system. Gas drag is included with a dissipating time scale of 105years. We show at the beginning(within 103∼ 104years), gas drag damps the ΔVbetween planetesimals of same sizes and increases ΔVbetween planetesimals of different sizes. However, after increasing to high values(300∼800m/s), we find the ΔVbetween bodies of different sizes decrease to very low values (below 10m/s) in a few 105yrs(depending on the gas-dissipating time scaleTdamp, radial sizeRpand semi-major axisapof planetesimals). Hence, the high ΔVis somewhat short-lived, and runaway accretion can be turned on later. We conclude that the conditions for planetary formation in binary systems (even close binary systems) are much better than what we expected before.

IGR J08408-4503: A New Recurrent Supergiant Fast X-Ray Transient

The supergiant fast X-ray transient IGR J08408-4503 was discovered by INTEGRAL on May 15, 2006, during a bright flare. The source shows sporadic recurrent short bright flares, reaching a peak luminosity of 10^36 erg s^-1 within less than one hour. The companion star is HD 74194, an Ob5Ib(f) supergiant star located at 3 kpc in the Vela region. We report the light curves and broad-band spectra (0.1-200 keV) of all the three flares of IGR J08408-4503 detected up to now based on INTEGRAL and Swift data. The flare spectra are well described by a power-law model with a high energy cut-off at ~15 keV. The absorption column density during the flares was found to be ~10^21 cm^-2, indicating a very low matter density around the compact object. Using the supergiant donor star parameters, the wind accretion conditions imply an orbital period of the order of one year, a spin period of the order of hours and a magnetic field of the order of 10^13 G.

The effect of paleotopography, tectonics and sediment supply on quality of coal seams in continental basins of central and western Bohemia (Westphalian), Czech Republic

The Radnice Member, the most important coal-bearing unit of the continental basins in central and western Bohemia (Czech Republic), is used here to demonstrate the interplay of controls on mire formation and distribution in continental settings. Its Middle Westphalian strata are interpreted as tectonically active or erosively incised river valley infills principally controlled by tectonic subsidence and clastic input. Another important controlling factor was the paleotopography, with some hills up to 200 m above the valley floor, which was responsible for the distribution of siliciclastics within the dendritic depocenter. The interplay of these factors is described here in terms of base-level changes expressed in the form of A/S ratio (accommodation/sediment supply ratio, [Martinsen, O.J., Ryseth, A., Helland-Hansen, W., Flesche, H., Torkildsen, G., Idil, S., 1999. Stratigraphic base level and fluvial architecture: Ericson Sandstone (Campanian), Rock Springs Uplift, SW Wyoming, USA. Sedimentology, 46, 235–259]). Vertical changes in A/S ratio were related mostly to fluctuation of tectonic subsidence whereas lateral variations of this ratio between valleys or within them were connected with uneven clastic supply of individual valleys from their source areas. In valleys where clastic input overreached the accommodation space production (A/S < 1), clastics filled all the available accommodation and no or small mires disturbed by fluvial deposition developed. Valleys, where clastic input was smaller than increment of accommodation space (A/S > 1) due to high subsidence rate or low clastic input, sediments transported by axial drainage were confined to the upper valley parts. The lower and middle parts of the valleys which starved of clastic sedimentation provided ideal conditions for long-lasting peat accretion. As a result, two economically important groups of coals developed in the Radnice Member in the Kladno-Rakovník Basin: the older Radnice group and the younger Lubná group. A high rate of base-level rise was typical of the deposition of the Radnice group and led to the formation of extensive mires in lower parts of the valleys that had a restricted clastic influx. The mires of the Lubná group formed during a time when the base-level rise was still high but lower than during the deposition of the Radnice group. Mostly small but long-lasting mires developed in sedimentary shadows located along the valley margins or in smaller valleys away from the main clastic transport and fluvial dynamics.

Thermobarometry, Sm/Nd Ages and Geophysical Evidence for the Location of the Suture Zone Between Cuyania and the Western Proto-Andean Margin of Gondwana

Basement rocks comprising ortho- and paragneisses and schists whose tectono-metamorphic evolution is poorly known, are exposed in the Sierras de Umango, Maz-Espinal and Las Ramaditas, in the northwest of the La Rioja Province, Argentina. These units were included in the Maz, El Taco, El Zaino complexes, as well as the Tambillos Metamorphics that would be part of the northern end of the Cuyania terrane, a microcontinent derived from Laurentia that collided with Western Gondwana during the early Paleozoic, or belong to the active margin of the continent. To recognize rocks belonging to each of these tectonic units and to understand the history and physical conditions of accretion were some of the main goals of the multidisciplinary investigation whose preliminary results are presented here. Geochemical studies, trace and REE elements and Sm-Nd model ages allowed the recognition of several episodes of crustal accretion in these rocks. The oldest one occurred at ca. 2.2 Ga in an arc/back-arc environment along the eastern segment of the Sierra de Maz, and was possibly coeval with development of a early Proterozoic continental crust that acted as source to sediments of Maz Complex. The following episode of crustal accretion that formed rocks in this region was at ca. 1.4 Ga and is registered by tonalites emplaced in an extensional environment cropping out in the western flank of the Sierra del Espinal. In the Sierra de Umango, an arc/back-arc sequence registered an episode of crustal accretion during the Grenvillian Cycle (ca. 1.3 Ga). The last episode of crustal accretion detected in this area (800 Ma) is represented by an old alkaline volcanism in the Sierra de Umango. This episode could be representing the first stage of break-up of the Rodinia supercontinent during the Neoproterozoic. The metamorphic grade reached by these rocks is mostly represented by fabrics with mineral assemblages of intermediate to high pressures and high temperatures, typical of collisional environments. The oldest rock-forming fabrics tectono-metamorphic episode recognized is of middle Proterozoic age (ca. 1.04 to 0.969 Ga, garnet-whole-rock Sm/Nd age) being registered by metapelites from Maz Complex that attained temperatures of 650°C-6.3 kbar. A younger metamorphic event (463 Ma, garnet - whole-rock Sm/Nd age) is verified in metatonalites intrusive in these metapelites. Another metamorphic event at ca. 301 Ma (garnet-WR Sm/Nd age) was recognized in metasediments from El Taco Complex. Peak metamorphic conditions of this event, probably registering the last major tectonic episode that affected rocks of this area was established in 868°C-9.8 kbar. It is impossible to distinguish fabrics belonging to totally different tectonic episodes based on structural or metamorphic data. Therefore, distinction between major tectono-thermal events of totally different ages such as the high-T middle Proterozoic deformation and with N-NWestwards tectonic transport direction registered in the Sierras de Maz-Espinal and Umango from the youngest one (ca. 301 Ma) that attained the highest-P/T conditions, recognized in the Sierra de Las Ramaditas, had to be done on the basis of Sm/Nd ages. Geophysical evidence indicates the presence of extensive WNW-oriented lineaments that separate basements blocks of different magnetic and gravimetric signatures that are thought to represent ancient Grenvillian age suture zones. On the other hand, the northern segment of the Valle Fértil lineament that runs between Sierras de Umango and Maz-Espinal is at present interpreted as marking the eastern boundary of the Cuyania terrane. This is supported by isotopic data as well as the contrasting history of tectono-metamorphic events as determined for both of these segments of the NW Sierras Pampeanas.

A Planetesimal Accretion Zone in a Circumbinary Disk

We investigate the conditions for planetesimal accretion in a circumbinary disk. Until recently, it had been believed that only single solar-type stars might harbor planetary systems. On the other hand, circumbinary disks have been detected by infrared or radio observation. Planets may be formed in such disks. Binary systems give stronger gravitational perturbation against planetesimals orbiting nearer to the binary. Therefore, the relative velocities between planetesimals will be larger and when they exceed the escape velocity it is impossible for the planetesimals to accumulate into planets. We performed long-term numerical integrations of binary and planetesimal orbital motions in the framework of the coplanar elliptic restricted three-body problem and have found the upper limit of the orbital radius inside of which the relative velocity between the planetesimals exceeds the escape velocity. One of our results is that planetesimal accretion cannot occur in a zone within 13 AU from the barycenter of the binary system when the binary semimajor axis is 1 AU, the binary eccentricity 0.1, the total mass m1 + m2 = 1 M☉, and the mass ratio m2/(m1 + m2) = 0.2. In regions farther out than 13 AU, planetesimals can accrete. We also derive analytic expressions of the eccentricity of a planetesimal pumped up by the gravitational perturbation of the binary and the inner boundary radius of the planetesimal accretion zone according to the secular perturbation theory.

Morphodynamic modeling of an embayed beach under wave group forcing

The morphodynamic response of the nearshore zone of an embayed beach induced by wave groups is examined with a numerical model. The model utilizes the nonlinear shallow water equations to phase resolve the mean and infragravity motions in combination with an advection‐diffusion equation for the sediment transport. The sediment transport associated with the short‐wave asymmetry is accounted for by means of a time‐integrated contribution of the wave nonlinearity using stream function theory. The two‐dimensional (2‐D) computations consider wave group energy made up of directionally spread, short waves with a zero mean approach angle with respect to the shore normal, incident on an initially alongshore uniform barred beach. Prior to the 2‐D computations, the model is calibrated with prototype flume measurements of waves, currents, and bed level changes during erosive and accretive conditions. The most prominent feature of the 2‐D model computations is the development of an alongshore quasi‐periodic bathymetry of shoals cut by rip channels. Without directional spreading, the smallest alongshore separation of the rip channels is obtained, and the beach response is self‐organizing in nature. Introducing a small amount of directional spreading (less than 2°) results in a strong increase in the alongshore length scales as the beach response changes from self‐organizing to being quasi‐forced. A further increase in directional spreading leads again to smaller length scales. The hypothesized correlation between the observed rip spacing and wave group forced edge waves over the initially alongshore uniform bathymetry is not found. However, there is a correlation between the alongshore length scales of the wave group‐induced quasi‐steady flow circulations and the eventual alongshore spacing of the rip channels. This suggests that the scouring associated with the quasi‐steady flow induced by the initial wave groups triggers the development of rip channels via a positive feedback mechanism in which the small scour holes start attracting more and more discharge.

The Role of Soil Organic Carbon in Maintaining Surface Elevation in Rapidly Subsiding U.S. Gulf of Mexico Coastal Marshes

Subsidence is a primary factor governing marsh deterioration in Mississippi River deltaic plain coastal marshes. Marsh surface-water level relationships are maintained primarily through soil organic matter accumulation and inorganic sediment input. In this study we examined the role of soil organic matter accumulation in maintaining marsh elevation in a brackish Spartina patens marsh. Measured rates of soil organic accumulation were compared to plant biomass production and soil respiration (carbon dioxide and methane emission) at the study sites. The study demonstrated the importance of plant biomass production to soil organic carbon accumulation in maintaining viable Spartina patens marshes in sediment-deficient coastal environments. The role of Mississippi River freshwater reintroduction in maintaining conditions for organic accretion is discussed.

Formation of the Galilean Satellites: Conditions of Accretion

We examine formation conditions for the Galilean satellites in the context of models of late-stage giant planet accretion and satellite-disk interactions. We first reevaluate the current standard, in which the satellites form from a ‘‘ minimum mass subnebula ’’ disk, obtained by augmenting the mass of the current satellites to solar abundance and resulting in a disk mass containing about 2% of Jupiter’s mass. Conditions in such a massive and gas-rich disk are difficult to reconcile with both the icy compositions of Ganymede and Callisto and the protracted formation time needed to explain Callisto’s apparent incomplete differentiation. In addition, we argue that disk torques in such a gas-rich disk would cause large satellites to be lost to inward decay onto the planet. These issues have prevented us from identifying a self-consistent scenario for the formation and survival of the Galilean satellites using the standard model. We then consider an alternative, in which the satellites form in a circumplanetary accretion disk produced during the very end stages of gas accretion onto Jupiter. In this case, an amount of gas and solids of at least � 0.02 Jovian masses must still be processed through the disk during the satellite formation era, but this amount need not have been present all at once. We find that an accretion disk produced by a slow inflow of gas and solids, e.g., 2 � 10 � 7 Jovian masses per year, is most consistent with conditions needed to form the Galilean satellites, including disk temperatures low enough for ices and protracted satellite accretion times of � 10 5 yr. Such a ‘‘ gas-starved ’’ disk has an orders-of-magnitude lower gas surface density than the minimum mass subnebula (and for many cases is optically thin). Solids delivered to the disk build up over many disk viscous cycles, resulting in a greatly reduced gas-to-solids ratio during the final stages of satellite accretion. This allows for the survival of Galilean-sized satellites against disk torques over a wide range of plausible conditions.

The radio luminosity of persistent X-ray binaries

Summarizes all the reported detections of, and upper limits to, the radio emission from persistent (i.e. non-transient) X-ray binaries. A striking result is a common mean observed radio luminosity from the black hole candidates (BHCs) in the low/hard X-ray state and the neutron star Z sources on the horizontal X-ray branch. This implies a common mean intrinsic radio luminosity to within a factor of 25 (or less, if there is significant Doppler boosting of the radio emission). Unless coincidental, these results imply a physical mechanism for jet formation that requires neither a black hole event horizon nor a neutron star surface. As a whole the populations of atoll and X-ray pulsar systems are less luminous by factors of gt or approximately=5 and gt or approximately=10 at radio wavelengths than the BHCs and Z sources (while some atoll sources have been detected, no high-field X-ray pulsar has ever been reliably detected as a radio source). The authors suggest that all of the persistent BHCs and the Z sources generate, at least sporadically, an outflow with physical dimensions gt or=10/sup 12/ cm; that is, significantly larger than the binary separations of most of the systems. The authors compare the physical conditions of accretion in each of the types of persistent X-ray binary and conclude that a relatively low ( lt or=10/sup 10/ G) magnetic field associated with the accreting object, and a high ( gt or=0.1 Eddington) accretion rate and/or dramatic physical change in the accretion flow, are required for formation of a radio-emitting outflow or jet.

Hybrid Airfoil Design Procedure Validation for Full-Scale Ice Accretion Simulation

This paper presents results of the ice accretion tests performed to validate the hybrid airfoil design method. The hybrid airfoil design method was developed to facilitate the design of hybrid airfoils with full-scale leading edges and redesigned aft sections that simulate full-scale ice accretion simulation for a given ® range. Icing tests in the NASA Lewis Icing Research Tunnel were conducted with test conditions representative of e ight. A twodimensional half-scale hybrid airfoil was designed and built with a 20% plain e ap and a 5% upper and 20% lower full-scale leading-edge surface of a modern business jet wing section. This paper presents a comparison between the ice shapes accreted on the business jet and hybrid airfoil models during the tests. The test results show that ice accretion simulation could be predicted in terms of the droplet-impingement simulation alone and cone rm the assumption that the leading-edge ice accretion will be the same for the full-scale and hybrid airfoils if icing cloud properties, droplet impingement, local leading-edge e owe eld, model surface characteristics, and geometry are held constant. This assumption was found to be valid when tested under the most severe conditions of glaze ice accretion over a large time interval. A comparison between the actual ice shapes and those predicted by LEWICE 1.6 under similar conditions is also shown. The results suggest that the hybrid airfoil design method has signie cant application potential for tests where leading-edge ice accretion is desired because it provides an alternative to the myriad of issues related to ice accretion scaling.

Laboratory tests of a time-domain reflectometry system for frazil ice detection

A prototype, electromagnetic-based frazil ice detection system (patent pending) has been developed and tested under simulated frazil ice accretion conditions in an environmentally controlled flume. The system employs a time-domain reflectometer (TDR) and specially designed transmission line sensor to monitor the accretion of frazil ice by measuring the propagation time along the sensor when it is submerged. Changes in the round-trip travel time of the TDR pulse result from a decrease in the localized bulk dielectric constant as frazil ice accretes and displaces water around the sensor. Two frazil detection sensor configurations were tested, a parallel transmission line probe and a semicylindrical mesh coaxial probe. During 2 h long experiments, the TDR clearly indicated a decreasing probe propagation time as frazil ice continued to accrete. This is indicative of the decreasing bulk dielectric constant of the frazil ice and water mix. Continuous real-time data from the TDR were recorded. From these data, an estimate of volumetric ice fraction was calculated using a simple linear dielectric mixing equation. Volumetric ice fractions estimates for both probe configurations were calculated to increase from approximately 0.02 to 0.18 during the test. The system shows promise for detection and measurement of frazil ice growth and accretion in freshwater bodies.Key words: frazil ice, trash rack, water intake, icing detection, time-domain reflectometry, TDR.

On the Existence of a Primordial Cometary Belt between Uranus and Neptune

The existence of stable orbits in the interplanetary region between Uranus and Neptune over the lifetime of the Solar System has been reported by Holman (1997,Nature387, 785–788). A long-term integration of test particles in that zone resulted in a number of surviving bodies with semimajor axis roughly between 24 and 27 AU, and eccentricities and inclinations smaller than 10−2and 1°, respectively. According to up-to-date surveys, the total mass of this putative belt has been estimated as ≤ 10−3M⊕and it would be composed of objects with radius smaller than 50 km.In this work we assess the plausibility of the existence of a real population of objects in such a belt. Time-reversal arguments rule out the possibility of capture in such long-term stable orbits, for example, of objects escaped from the Kuiper belt. So if a real population exists nowadays, it should have undergone the conditions of planetesimal accretion in the region.Hence, we have studied the orbital evolution of test particles under different kinds of plausible primordial scenarios. The conditions considered were mutual collisions and gravitational encounters, planetary migration and the presence of an adjacent Pluto-sized object. Under none of these conditions have we obtained a surviving substantial population on the reported belt region, from which it is concluded that, at present time, it would be very unlikely to find a subtantial number of primordial objects in those long-term stable orbits.

Light elements in the Earth's outer core: A critical review

Abstract There is little doubt that densities for the Earth's outer core, inferred from seismology, require that it is constituted of an alloy of liquid iron and light elements. However, the nature of the light alloying elements is still uncertain as it depends in a large measure on the conditions of accretion of the Earth and formation of the core. The arguments brought forward for or against silicon, oxygen, sulphur, hydrogen and carbon are critically reviewed. There is no reason to consider that only one element is present in the outer core. Experimentally determined and/or calculated ternary and quaternary phase diagrams are needed to provide constraints on the nature of the light elements.

Hydroxycobalamin[c-lactam] Increases Total Coenzyme A Content in Primary Culture Hepatocytes by Accelerating Coenzyme A Biosynthesis Secondary to Acyl-CoA Accumulation

Hydroxycobalamin[c-lactam] (HCCL) treatment in rats results in decreased hepatic L-methylmalonyl-CoA mutase activity and increased hepatic total CoA content. To test the hypothesis that HCCL increases hepatic CoA biosynthesis secondary to propionyl- and methylmalonyl-CoA accumulation. CoA homeostasis was studied in primary culture rat hepatocytes. Conversion of [14C]pantothenic acid to [14C]CoA in the primary culture system was accelerated to rates 3-5 times control by acute incubation with the acyl-CoA-generating carboxylic acids pivalate (10 mmol/L) or propionate (10 mmol/L). HCCL (1 mg/L included from 24 to 72 h of culture) had no affect on the distribution of the hepatocyte CoA pool or total CoA content. However, culture in the presence of HCCL and propionate (2 mmol/L) resulted in accumulation of methylmalonyl-CoA and a 59% increase in total CoA content. The combination of HCCL (1 mg/L) and propionate (0.5 mmol/L), but not HCCL alone, increased the rate of [14C]pantothenic acid conversion to [14C]CoA by 150%. Degradation of [14C]CoA in the primary culture hepatocyte system was unaffected by chronic exposure to 2 mmol/L propionate, but was increased 90% by the combination of HCCL and 2 mmol/L propionate. Thus, in the presence of a source of propionyl-CoA, HCCL treatment results in methylmalonyl-CoA accumulation and accelerated CoA biosynthesis. The increased CoA biosynthesis leads to increased hepatocyte total CoA content, which may contribute to cellular metabolic homeostasis under conditions of acyl-CoA accretion.

Capture Probability of Colliding Planetesimals: Dynamical Constraints on Accretion of Planets, Satellites, and Ring Particles

We evaluate accretion rate and capture probability of planetesimals in Keplerian motion by three-body trajectory integration and analytic calculation to examine conditions for accretion of planets, satellites, and ring particles. Fragmentation or cratering of colliding bodies is not considered, but the changes of orbits caused by inelastic rebound with given restitution coefficient are calculated without the assumption of inevitable accretion. Numerical results show that with such low relative random velocities that v/ve ≤ 0.1 (ve is the escape velocity), slight inelastic dissipation of kinetic energy is enough to lead most colliding bodies to accretion, but substantial kinetic, energy needs to be dissipated for capture when v ≥ ve. In relatively high-velocity collisions between planetesimals in heliocentric orbits or between satellites around a planet, capture probability, C, which is defined by the ratio of the capture rate to the collision rate, is expressed in terms of the restitution coefficients in normal and tangential direction, εn and εt, respectively, and a certain critical value, εcr, as follows: (i) C = 1 if εn, εt < εcr, (ii) C = (ε2cr - ε2n)/(ε2t - ε2n.) if εn < εcr ≤t εt. (iii) C = (ε2cr - ε2t)/(ε2n - ε2t) if εt < εcr ≤ εn. (iv) C = 0 if εcr ≤ εn, εt. The critical value εcr is a function of v /ve and rp(rp is the ratio of the sum of radii of colliding bodies to the Hill sphere radius), and εcr ≃ 0.7 when v = ve and rp ⪡ 1. These analytic results and the numerical results obtained by trajectory integrations show quite a good agreement for v /ve ≥ 0.2. We find that tidal effects of the central body is not significant as long as v/ve ≥ 0.2 and rp⪡ 1, and we may apply the two-body approximation for capture probability in such cases. Using these results, together with experimental data for the restitution coefficient of ice, we estimate a critical mass of icy satellites above which they accrete by gravitational force. We also investigate the capture probability for collisions between planetary ring particles where rp ≃ 1 and tidal effects are more important. We find that accretion is possible even inside the Roche limit and the capture probability decreases abruptly for rp ≃23, when the target body protrudes out of the Hill sphere. This suggests that in Saturn's B ring gravitational accretion seems quite difficult unless the restitution coefficient is sufficiently small, while accretion of small particles onto large bodies may be possible in A ring unless the material density of particles is substantially smaller than 1 g cm-3.

Recruitment of coralline crusts and filamentous turf algae in the Galapagos archipelago: effect of simulated scour, erosion and accretion

Recruitment of crustose coralline and filamentous turf algae onto lava boulders was investigated in a shallow (< 3 m) lagoon in Academy Bay, Galapagos archipelago. The treatments simulated conditions of sediment accretion (physical addition of sediment), sediment erosion (removal of sediment by water motion) and scour (physical abrasion). Crustose corallines were early colonizers of lava boulders under all treatments but were overgrown by algal turfs within the duration of the experiment (58 days) in all treatments except that simulating scour. Biomass of turfs was significantly lower in environments of scour. The amount of sediment trapped in turfs differed little between treatments over the period of the experiment. The weight of sediment trapped was up to five times the biomass of the algae. Small-scale patchiness in spatial distribution of algal turf and crustose coralline communities in Academy Bay may reflect the depositional environment. Studies of crustose coralline and filamentous turf algae indicated encrusting corallines were associated with environments of scour where filamentous turf-forming species were more common to environments of sediment accretion and erosion.

Accretion, metamorphism, and brecciation of ordinary chondrites: Evidence from petrologic studies of meteorites from Roosevelt County, New Mexico

We have made petrologic studies of a suite of 29 newly discovered ordinary chondrites from Roosevelt County, New Mexico, including an H3 chondrite that is rich in solar wind gases and contains clasts of melt rock and graphite‐magnetite‐rich material, and an L5 chondrite with a unique igneous inclusion. Our data and published work show that mean CaO concentrations of low‐Ca pyroxene can be used to distinguish petrologic types in H, L, and LL groups. Mean CaO concentrations in olivine show significant variations among equilibrated chondrites, but these are not correlated with petrologic type; varied cooling rates could be responsible. The degree of heterogeneity of FeO concentrations in olivines of types 4–6 is also not correlated with degree of metamorphism; it may be controlled more by postmetamorphic brecciation and mixing of material than by metamorphism. Mean FeO concentrations of silicates show average increases of 3–5% from type 4 to type 6 in each group. This increase and the accompanying increase in bulk concentrations of siderophile elements appear to predate parent body formation. Correlation of nebula‐controlled bulk chemistry and planetary metamorphic effects could only have been produced under special conditions of accretion. One possible origin is runaway growth, which allowed parent bodies to form by accretion kilometer‐sized planetesimals to larger bodies, accompanied by differential movement of chemically distinct large and small bodies to produce radially zoned parent bodies that were subsequently metamorphosed by internal heating. Alternatively, ordinary chondrites may have been metamorphosed in a variety of planetesimals, each of which had a uniform chemical composition and metamorphic temperature. Correlations between composition and temperature may have arisen if both were correlated with another parameter, such as heliocentric distance.

Steady flow from an injection well in leaky aquifers

Wells are used to solve several water management problems. In this study, hydraulics of fully penetrating injection wells in two semi-confined aquifers overlain by a thick soil slab have been analysed. Equations for steady piezometric head distribution in the two semi-confined aquifers arising due to constant rate of injection into (1) the lower aquifer, (2) the upper aquifer, and (3) both aquifers, have been derived. These solutions are developed under conditions of accretion from the land surface. In absence of accretion, when the well is fully penetrating both aquifers, it is analytically shown that the ratio of fluxes through the aquifers is equal to their transmissivity ratio. Sample computations of the three cases ascertain the validity of the solutions.

UNDISTORTED FROUDE MODEL FOR SURF ZONE SEDIMENT TRANSPORT

Small scale movable bed wave tank experiments were carried out according to undistorted Froude model laws with the sediment fall time, H/wT, as the governing parameter for scaling the model sediment. Four questions addressed in this study included: (a) the ability to reproduce larger scale model results for both erosional and accretive conditions, (b) the effects of more realistic concave upward initial beach profiles instead of the more usual planar initial slopes, (c) the criterion for onshore-offshore sediment transport, and (d) the capability of the model to simulate post-storm recovery. Based on a comparison with large scale results of Saville (1957), it was found that the model provided good agreement for erosive conditions. For accretive conditions, the results were less conclusive although the general patterns of profile change were similar. The final beach profiles resulting from concave upward initial profiles were found to be substantially different from those for an initially planar profile. It appears that the initially planar profile unrealistically affects the breaker type and results in a more pronounced longshore bar and offshore slopes that are steeper than found in nature. Tests conducted to evaluate the criterion separating onshore-offshore transport suggested a higher value of the fall time parameter, H/wT, than was originally proposed by Dean (1973); this is interpreted to be due to scale effects in most of the model data used in the original development. Tests to simulate post-storm recovery were affected by the presence of "reflection bars" associated with a partial standing wave system. The reflection bars appear to strongly affect the sediment transport limiting the post-storm profile recovery. The most effective recovery was induced by continually changing wave conditions to maintain the wave breakpoint slightly landward of the bar crest.

Unequilibrated ordinary chondrites: A tentative subclassification based on volatile-element content

For unequilibrated ordinary chondrites (= UOC), two measures of primitiveness are available: volatile content, in principle reflecting accretion conditions from the solar nebula, and metamorphism, reflecting reheating in the parent bodies. These two measures do not always correlate, and we have therefore developed a tentative classification scheme based on volatile content that complements the Sears et al. (1980) scheme based on metamorphism. Like the latter, it subdivides type 3 chondrites on a scale of 3.0 to 3.9; the notation 3.4/0 indicates a meteorite that is subtype 3.4 according to metamorphism and 3.0 according to volatile content. The classification is based mainly on C and Xe—two elements that are little affected by shock-induced reheating—and to a lesser extent on Ar 36,Bi,In, and Tl. Of 22 meteorites considered, the majority have concordant classifications (±0.2) on the two scales. However, 5 meteorites are richer in volatiles than their metamorphic grade indicates: Sharps 3.4/0, ALHA 77011 3.5/0, Ngawi 3.6/3, ALHA 77299 3.7/4, and Mezö-Madaras 3.7/3. It remains to be seen whether these differences indeed denote a more primitive nature. Some new clues to the formation of chondrites may eventually come from Xe and C. Their concentrations in UOC's vary by more than 5×, but the Xe C ratio remains nearly constant at 3.4 × 10 −3 of the solar-system ratio. Even the ratios for other chondrite classes differ only slightly from that for UOC's, e.g., C3O (1.5×) and E3,4 (0.4×). Either the 4 factors determining this ratio ( T, t, P, and internal surface area of the carbon) varied in complementary fashion, or—more probably—they varied only slightly in the entire source region of chondrites.