Dust Accretion Research Articles

Aeolian dust contribution to the formation of alpine soils at Amdo (Northern Tibetan Plateau)

The Tibetan Plateau has one of the largest distributions of alpine meadow in the world. Due to the cold climate and dominance of physical weathering conditions, coarse or massive debris prevails. Thus, pedogenesis is controlled by both the temperature and parent material. In this study, the alpine soils (Gelic Cambisol) at altitudes of 4768–4980m in the Amdo zone of the northern Tibetan Plateau were investigated. We found that the parent materials throughout the alpine soil profiles were rich in clayey and silty minerals. Here, we focused on the origin of the soil forming materials of alpine meadows. We examined the grain size distribution, mineral compositions, trace element concentrations, and Sr–Nd isotopic compositions of the alpine soils and their potential source materials in various grain size fractions. Based on a comparison of many fingerprints, our results indicated that a red clay-rich bed at the bottom of the alpine soil profile is a residual paleo-weathering crust. Nevertheless, the parent materials in the alpine soils did not have a clear inherited relationship with the weathering products of the in situ bedrock. Correspondingly, we confirmed that the parent materials in the alpine soils have an aeolian origin. The alluvial sediments, and widespread beach and lacustrine delta sediments in the Amdo zone are the main source of alpine soil materials. However, it was not clear whether sediments are the main source of aeolian dust. Aeolian dust, with a short transport distance, is one of the main components (<20μm and 20–300μm) of alpine soils. Correspondingly, we confirmed that the minor coarse components (>300μm) of alpine soils had a local origin, i.e., from adjacent bedrock outcrops. In contrast, long-range transport of dust had a minor role in the formation of alpine soils. In the region investigated, the aeolian dust began accumulating in the middle Holocene. On the Tibetan Plateau, one positive effect of aeolian processes is that the aeolian dust accretion is a source of parent materials and nutrients for alpine soils.

THE CHEMICAL COMPOSITION OFρPUPPIS AND THE SIGNS OF ACCRETION IN THE ATMOSPHERES OF B–F-TYPE STARS

We investigated the chemical composition of ρ Pup using high-resolution spectral observations taken from the Very Large Telescope and the IUE archives and also spectra obtained at the 1.8 m telescope of the Bohyunsan observatory in Korea. The abundances of 56 chemical elements and the upper limits of Li and Be abundances were determined. The abundance pattern of ρ Pup was found to be similar to that of Am-type stars. The possibility of the influence of the accretion of interstellar gas and dust on the abundance patterns of B–F-type stars is discussed. The plots of the relative abundances of chemical elements in the atmospheres of ρ Pup and δ Sct versus the second ionization potentials of these elements show the correlations. The discontinuities at 13.6 and 24.6 eV—the ionization potentials of hydrogen and helium, respectively, are also exhibited in these plots. These discontinuities can be explained by interaction of the atoms of interstellar gas, mainly hydrogen and helium atoms, with the atoms of stellar photospheres (so-called charge-exchange reactions). Note that only the jumps near 13.6 and 24.6 eV were pointed out in previous investigations of relative abundances versus the second ionization potentials for Am-type stars. The dependencies of the relativemore » abundances of chemical elements on the second ionization potentials of these elements were investigated using the published abundance patterns of B–F-type stars. The correlations of relative and absolute abundances of chemical elements, second ionization potentials, and projected rotational velocities are clearly detected for stars with effective temperatures between 7,000 and 12,000 K. If the correlation of relative abundances and second ionization potentials can be explained by the accretion of interstellar gas on the stellar surfaces, the investigation of these correlations can provide valuable information on the density and velocities of interstellar gas in different regions of the Galaxy and also on the influence of this phenomenon on stellar evolution. The dependencies of the relative abundances of chemical elements on the condensation temperatures of these elements were also found in the atmospheres of ρ Pup, δ Sct, and other B–F-type stars. Ten possible λ Boo-type stars were detected. The effective temperatures of these objects are between 10,900 and 14,000 K.« less

Temperature dependence of sodium and ionized calcium resonance lines perturbed by helium

Traces of heavy metals in cool DZ white dwarf stars may be attributed to the accretion of circumstellar dust thought to originate from tidal disruption of rocky parent bodies. Spectra of such stars therefore provide a unique opportunity to study the composition of extrasolar planetary systems. The determination of metal abundances from stellar spectra depends on stellar atmospheric parameters and an accurate prior knowledge of the collision broadening of the line profiles by the most common constituents of the stellar atmosphere. For this purpose, we present theoretical absorption spectra of Na and Ca+ broadened by He for the conditions prevailing in cool white dwarfs.

The Siding Spring cometary encounter with Mars: A natural experiment for the Martian atmosphere?

AbstractOn 19 October 2014 comet C/2013 A1 will encounter Mars. The planet is anticipated to pass through the coma resulting in a greater than four order‐of‐magnitude increase in the accretion of dust with 430 tonnes of dust with diameters between 1 µm and 12.4 mm surviving atmospheric passage. At high altitude, the dust would impact temperature and may affect limb dust extinction and cloud formation. The UV photolysis of the organic carbon content of the dust, 1.9 to 4.6 tonnes, would have a negligible effect on atmospheric methane. Should C/2013 A1 brighten, increases in upper atmospheric accretion of coma particles will exceed the background dust and the population of small coma particles may be constrained from orbital measurements. For M1 &lt; –1.3, methane produced might be measurable by the Sample Analysis at Mars Tunable Laser Spectrometer instrument onboard Curiosity.

The dust content of QSO hosts at high redshift

Infrared observations of high-z quasar (QSO) hosts indicate the presence of large masses of dust in the early universe. When combined with other observables, such as neutral gas masses and star formation rates, the dust content of z~6 QSO hosts may help constraining their star formation history. We have collected a database of 58 sources from the literature discovered by various surveys and observed in the FIR. We have interpreted the available data by means of chemical evolution models for forming proto-spheroids, investigating the role of the major parameters regulating star formation and dust production. For a few systems, given the derived small dynamical masses, the observed dust content can be explained only assuming a top-heavy initial mass function, an enhanced star formation efficiency and an increased rate of dust accretion. However, the possibility that, for some systems, the dynamical mass has been underestimated cannot be excluded. If this were the case, the dust mass can be accounted for by standard model assumptions. We provide predictions regarding the abundance of the descendants of QSO hosts; albeit rare, such systems should be present and detectable by future deep surveys such as Euclid already at z>4.

Lunar weather measurements at three Apollo sites 1969–1976

AbstractThe first lunar weather stations, matchbox‐sized, 270 g Apollo Dust Detector Experiments about 100 cm above the surface of the Moon near Apollo 12, 14, and 15 landing sites, measured dust accretion, charged particle radiation, and temperature changes—three environmental factors proved during Apollo to affect technical systems deployed on the Moon. Degradation of seven horizontal solar cells was measured every lunar daytime from 1969 to 1976. The anomalously intense August 1972 solar particle event (SPE) degraded three covered cells by less than 1%, while two cells desensitized by intense preirradiation showed no measurable effects. Although independent studies estimated the long‐term fluence bombarding the cells was less than half that of the August SPE, long‐term gradual degradation of five covered cells (normalized to 2000 days) was an order of magnitude greater, between 4% and 10%. If the long‐term effects were totally caused by dust, with articulated caveats including simulated (maria) Minnesota Lunar Simulant‐1 dust particles with diameters 20 to 38 µm, this provides the first direct measured long‐term net accretion of dust with an upper limit of order 100 µg cm−2 yr−1, equivalent to a layer 1 mm thick in 1000 years, but it may be significantly less. Two bare cells were abruptly degraded by 7% during the August SPE, however long‐term they measured additional damage of 29% and 24%, indicating a long‐neglected suite of low‐energy radiation, posing risks for bare materials exposed on the surface of the Moon.

Micrometeorite flux on Earth during the last ~50,000 years

AbstractFlux of micrometeorites is estimated by using cosmic spherule counts from a seafloor area of 2.50 m2 from the Indian Ocean. The spherules are recovered from sediment samples in close‐spaced locations from the Indian Ocean after sieving 293 kg of sediment. The terrestrial age of the spherules has a range of 0–~50,000 years. The spherules have a size range of 57–750 µm (average size 265 ± 92 µm). The diameter of the spherules increases from scoriaceous‐barred‐cryptocrystalline‐glassy types. The time‐averaged flux of the spherules is 160 t/yr, a sizeable mass (&gt;60%) resides in the &gt;300 µm fraction; the slope of distribution is similar to that of Deep‐Sea Spherules but significantly different from other collections which have lower average diameters. It is observed here, a significant population of cosmic dust resides in the larger sizes which can be recovered by sampling large areas in time and space. The spherule textures are similar to that of unbiased collections from the polar regions, indicating that the textural types of cosmic dust that have been raining on the Earth during the last 50 kyr have been constant regardless of size. Major element chemistry of a majority of the spherules show elemental ratios that are close to a CM or CI chondritic parent body; a single spherule (0.2% of the population) suggests an achondritic parent body. Unbiased collections spanning large areas temporally and spatially enlarge the inventory of the Earth‐crossing meteoroid complex and provide valuable inputs for models on cosmic dust accretion.

Paleoenvironmental reconstruction based on the morphology and distribution of secondary carbonates of the loess-paleosol sequence at Süttő, Hungary

Micro- and macroscale secondary carbonates were investigated from the loess-paleosol sequence of Süttő, Hungary. As secondary carbonates are formed in pedosedimentary environments, they designate how the microecosystem adapts to dust accretion. Besides pedogenic aspects, the hints of diagenesis can be tracked, especially concerning leaching effects. The main goal of this study was to make an attempt whether the distribution-related signals of secondary carbonates combined with their genetic properties can be used for paleoenvironmental reconstruction. The elaborated method is wet sieving of bulk loess/paleosol samples in order to: 1) separate secondary carbonates to describe morphological properties, partly with the help of scanning electron microscopy; and 2) provide a depth-related semiquantitative distribution. The semiquantitative distribution is ordered to the marine isotope stage units of the Süttő sequence (MIS 6 to 2). The MIS 6 section is characterized by presumably slower dust accumulation and raises the possibility of a multiphase leaching history throughout the profile. Lower dust accumulation rates with increasing aridity are assumed upwards in MIS 5. MIS 4 is typified by arid conditions and the alternation of lower and higher dust accumulations. Certain properties of paleosol development and frost deformation related to secondary carbonates can be distinguished in MIS 3. Dust accumulation rates are presumed to be consistent upwards in this section. During MIS 2, different sedimentation stages are distinguished, but generally they show a decreasing trend upwards to the modern soil. This work serves as a complementary method which may help to refine the paleoenviromental signals of the Süttő loess-paleosol sequence to determine relative sedimentary phases. The limitations of the method have to be taken into consideration, as the results are semiquantitative.

Effects of atmospheric entry heating on the noble gas and nitrogen content of micrometeorites

Fragments of the carbonaceous chondrite Orgueil were subjected to pulse-heating sequences in order to simulate the heating conditions experienced by micrometeorites (MMs) upon entry into Earthʼs atmosphere. By increasing the experimental run times from 2 to 120 s at a fixed temperature of 1350 °C, the different textures of natural MMs (from non-vesicular fine-grained particles to melted cosmic spherules) were reproduced, and the noble gas (He, Ne, Ar) and nitrogen abundances and isotope ratios of the MM analogues were subsequently determined by CO2 laser extraction-static mass spectrometry analysis. The starting material shows a heterogeneous He–Ne–Ar–N signature, consistent with the mineralogical heterogeneity of CI chondrites and the inhomogeneous distribution of various noble gas and nitrogen components among meteoritic minerals. Nonetheless, our experiments demonstrate that moderately to strongly heated Orgueil fragments retain only a few percent of their initial noble gas and nitrogen inventories, indicating that atmospheric entry heating results in extensive degassing of meteoritic dust particles. The evolution of the noble gas and nitrogen isotope ratios may, in part, be explained by equilibration with the atmosphere; however, the decreasing δ15N values may also indicate preferential degradation of a 15N-rich component by thermal processing of chondritic matter. Furthermore, the efficient loss of helium and cosmogenic neon during heating will lead to an underestimate of the 3He and 21Ne exposure ages of MMs, as well as to large uncertainties for cosmic dust accretion rates derived from extraterrestrial 3He abundances in deep-sea sediments or polar ice cores. While the relative proportions of infalling cometary and asteroidal dust on Earth are unknown, the contribution of noble gases, nitrogen, and water from cosmic dust to the terrestrial volatile inventory appears negligible.

Photophoresis boosts giant planet formation

In the core accretion model of giant planet formation, a solid protoplanetary core begins to accrete gas directly from the nebula when its mass reaches about 5 earth masses. The protoplanet has at most a few million years to reach runaway gas accretion, as young stars lose their gas disks after 10 million years at the latest. Yet gas accretion also brings small dust grains entrained in the gas into the planetary atmosphere. Dust accretion creates an optically thick protoplanetary atmosphere that cannot efficiently radiate away the kinetic energy deposited by incoming planetesimals. A dust-rich atmosphere severely slows down atmospheric cooling, contraction, and inflow of new gas, in contradiction to the observed timescales of planet formation. Here we show that photophoresis is a strong mechanism for pushing dust out of the planetary atmosphere due to the momentum exchange between gas and dust grains. The thermal radiation from the heated inner atmosphere and core is sufficient to levitate dust grains and to push them outward. Photophoresis can significantly accelerate the formation of giant planets.

SIGNS OF ACCRETION IN THE ABUNDANCE PATTERNS OF THE COMPONENTS OF THE RS CVn-TYPE ECLIPSING BINARY STAR LX PERSEI

We present spectroscopic observations of LX Per carried out using the Korean Bohyunsan Observatory Echelle Spectrograph (BOES) with spectral resolving power R = 80, 000. The spectrograph was attached to the 1.8 m telescope. The fit of synthetic spectra to the observed spectrum of the system allowed us to find the component parameters and the abundances of chemical elements in the atmospheres of the components. The strong Ca II H and K emissions are confirmed; we also found emission lines in the Ca II reversals' triplet absorptions at the wavelengths of 8498, 8542, and 8662 A in the spectrum of the cooler component of LX Per. A unique photometric solution using the distorted light curves of three different epochs was made. The spot model light curves were fitted to the 1981, 1982, and 1983 observations successfully by adjusting only spot parameters. We could infer that the variation of spot location and size was the main reason for the changing shape of light curves. The main feature of the abundance patterns of both components was the apparent deficiency of heavy (Z > 30) elements. Only elements with strong lines, namely Y and Ba, were detected. Correlations of relative abundances of chemicalmore » elements with condensation temperatures and second ionization potentials of these elements, which can be explained by the accretion of dust and gas, were found.« less

Atmospheric dust contribution to the budget of U-series nuclides in soils from the Mount Cameroon volcano

The atmospheric dust contribution to U-series nuclides in soils is evaluated by analyzing two soil and paleo-soil (<10ka) profiles from the Mount Cameroon volcano where Sr–Nd–Pb fingerprints are clearly affected by Saharan dust accretion. The comparison of measured U-series disequilibria between bulk soils and associated parent lavas suggests that dust inputs can contribute up to 25% of the U and Th budget in soils whereas chemical weathering affects a maximum of 7% of the U budget. The uniform U–Th isotopic composition of the Saharan dust end-member inferred from the soils and the paleo-soils suggests a relative stability of the Saharan dust isotopic composition during the last 10ka.

Aeolian controls of soil geochemistry and weathering fluxes in high-elevation ecosystems of the Rocky Mountains, Colorado

When dust inputs are large or have persisted for long periods of time, the signature of dust additions are often apparent in soils. The of dust will be greatest where the geochemical composition of dust is distinct from local sources of soil parent material. In this study the influence of dust accretion on soil geochemistry is quantified for two different soils from the San Juan Mountains of southwestern Colorado, USA. At both study sites, dust is enriched in several trace elements relative to local rock, especially Cd, Cu, Pb, and Zn. Mass-balance calculations that do not explicitly account for dust inputs indicate the accumulation of some elements in soil beyond what can be explained by weathering of local rock. Most observed elemental enrichments are explained by accounting for the long-term accretion of dust, based on modern isotopic and geochemical estimates. One notable exception is Pb, which based on mass-balance calculations and isotopic measurements may have an additional source at one of the study sites. These results suggest that dust is a major factor influencing the development of soil in these settings and is also an important control of soil weathering fluxes. After accounting for dust inputs in mass-balance calculations, Si weathering fluxes from San Juan Mountain soils are within the range observed for other temperate systems. Comparing dust inputs with mass-balanced based flux estimates suggests dust could account for as much as 50–80% of total long-term chemical weathering fluxes. These results support the notion that dust inputs may sustain chemical weathering fluxes even in relatively young continental settings. Given the widespread input of far-traveled dust, the weathering of dust is likely and important and underappreciated aspect of the global weathering engine.

Density Effects in Sodium and ionized Calcium perturbed by helium in Cool White Dwarf Stars

Traces of heavy metal in cool DZ white dwarfs may be attributed to the accretion of circumstellar dust. This dust is thought to originate from the tidal disruption of some rocky material, they provide a unique opportunity to study the composition of extra-solar planetary systems. The determinations of precise atmospheric parameters and abundances require accurate description of the line profiles of the identified features. We present absorption spectra of Na-He and Ca + -He for the conditions prevailing in cool white dwarfs.

Nd isotope evidence for dust accretion to a soil chronosequence in Hainan Island

Hainan soils contain a mixture of material derived from in situ weathering of parent material plus atmospheric inputs dominated by continental dust. We use Nd isotope geochemistry to evaluate the impact of continental dust on a chronosequence of Hainan soils. The results demonstrate that dust has a profound effect on soils in 180 ka and older in tropical Hainan Island. Dust is the dominant source of Nd in the oldest, most intensely weathered soils, with radiogenic dust-derived isotopic signatures (ε Nd (0) = − 6.46) and significant high mass fraction of dust-derived Nd (75%). The f dust Nd values for soils show an increasing trend with time, implying that dust accumulates with the ages of soils. We also calculate a long term average dust accretion rates of 14.90 and 12.39 mg cm − 2 ka − 1 at the 180 and 300 ka sites; whereas the rates of the old sites are low, indicating underestimates of time-averaged dust deposition rates over million year time scales. On the basis of the geomorphic stability sites, we think that the decrease of dust inputs due to climate changes and the loss of dust due to weathering are the dominant loss mechanisms for dust and lead to underestimates of time-averaged dust deposition rates in the old sites. The results also underscore the potential for neodymium isotopes to constrain the origin of soils and paleosols. ► Soils show radiogenic dust-derived isotopic signatures (ε Nd (0) = − 6.46). ► Significant high mass fraction of dust-derived Nd identifies the presence of dust. ► The increasing trend of f dust Nd values for soils indicates dust accumulations with time. ► We calculate long term dust accretion rates to quantify the deposition flux. ► The results underscore the potential for Nd isotope to constrain the origin of soils.

A coagulation–disintegration model of Oort–Hulst cluster-formation

We propose and analyse a model for the collision of highly energetic particles in which collisions cause the complete fragmentation of the particles into dust and in which particles nucleate and grow through the accretion of dust. This is inspired by the work of Oort and van Hulst. We analyse a variety of aggregation and disintegration kernels, using asymptotic techniques to illustrate the form of solutions, and find steady states and self-similar behaviour.

HOW NOT TO BUILD TATOOINE: THE DIFFICULTY OF IN SITU FORMATION OF CIRCUMBINARY PLANETS KEPLER 16b, KEPLER 34b, AND KEPLER 35b

We study planetesimal evolution in circumbinary disks, focusing on the three systems Kepler 16, 34 and 35 where planets have been discovered recently. We show that for circumbinary planetesimals, in addition to secular forcing, eccentricities evolve on a dynamical timescale, which leads to orbital crossings even in the presence of gas drag. This makes the current locations of the circumbinary Kepler planets hostile to planetesimal accretion. We then present results from simulations including planetesimal formation and dust accretion, and show that even in the most favourable case of 100% efficient dust accretion, in situ growth starting from planetesimals smaller than ~10 km is difficult for Kepler 16b, Kepler 34b and Kepler 35b. These planets were likely assembled further out in the disk, and migrated inward to their current location.

Experimental investigation of the nebular formation of chondrule rims and the formation of chondrite parent bodies

We developed an experimental setup to test the hypothesis that accretionary rims around chondrules formed in the solar nebula by accretion of dust on the surfaces of hot chondrules. Our experimental method allows us to form dust rims around chondrule analogs while levitated in an inert-gas flow. We used micrometer-sized powdered San Carlos olivine to accrete individual dust particles onto the chondrule analogs at room temperature (20°C) and at 1100°C. The resulting dust rims were analyzed by means of two different techniques: non-destructive micro computer tomography, and scanning electron microscopy. Both methods give very similar results for the dust rim structure and a mean dust rim porosity of 60% for the hot coated samples, demonstrating that both methods are equally well suited for sample analysis. The chondrule analog’s bulk composition has no measurable impact on the accretion efficiency of the dust. We measured the chemical composition of chondrule analog and dust rim to check whether elemental exchange between the two components occurred. Such a reaction zone was not found; thus, we can experimentally confirm the sharp border between chondrules and dust rims described in the literature. We adopted a simple model to derive the degree of post-accretionary compaction for different carbonaceous chondrites. Moreover, we measured the rim porosity of a fragment of Murchison meteorite, analyzed it with micro-CT and found rim porosities with this technique that are comparable to those described in the literature.

MINIMUM DUST ABUNDANCES FOR PLANETESIMAL FORMATION VIA SECULAR GRAVITATIONAL INSTABILITIES

We estimate minimum dust abundances required for secular gravitational instability (SGI) to operate at the midplane dust layer of protoplanetary disks. For SGI to be a viable process, the growth time of the instability T_grow must be shorter than the radial drift time of the dust T_drift. The growth time depends on the turbulent diffusion parameter alpha, because the modes with short wavelengths are stabilized by turbulent diffusion. Assuming that turbulence is excited via the Kelvin-Helmholtz or streaming instabilities in the dust layer, and that its strength is controlled by the energy supply rate from dust accretion, we estimate the diffusion parameter and the growth time of the instability. The condition T_grow < T_drift requires that the dust abundance must be greater than a critical abundance Z_min, which is a function of the Toomre parameter Q_g and aspect ratio h_g / r of the gas disk. For a wide range of parameter space, the required dust abundance is less than 0.1. A slight increase in dust abundance opens a possible route for the dust to directly collapse to planetesimals.

Chondritic ingredients: I. Usual suspects and some oddballs in the Leoville CV3 meteorite

Abstract– Reduced CV3 chondrites are relatively pristine rocks and prime candidates for studies exploring processes that predated planet formation. We closely examined the petrographic features and trace elemental composition of different CV3 constituents in the accretionary breccia Leoville. The petrographic results are presented here. Our sample (2.2 cm2) is not brecciated. The main ingredient—about 65 area%—is fine‐ to coarse‐grained ferromagnesian type I chondrules. Minor constituents (in order of 2‐D abundance) include refractory inclusions, Al‐rich chondrules, and very fine‐crystalline clasts of moderately volatile composition. Type II chondrules and metal nuggets occur sporadically. The chondrule–matrix ratio is approximately 3:1. Medium‐ and coarse‐grained chondrules exhibit porphyritic textures, probably caused by incomplete melting, and frequent, partial or continuous, recrystallized dust rims. The fine‐grained population most likely represents randomly sectioned dust rims. The rim material and some of the medium‐grained objects are relatively troilite‐rich. Iron‐nickel metal is rare. In addition, almost all constituents show strikingly ragged or convoluted outlines. Only a few, rim‐less components exhibit smooth contours. Evidence for incomplete melting and the formation of recrystallized or igneous rims in carbonaceous chondrites is well established, suggesting that both processes were widespread events. The observed features in Leoville support this conclusion. In addition, our findings indicate that surface abrasion in a turbulent dust‐filled regime may have taken place after the consolidation of dust rims. Alternatively, the irregular, convoluted nature of at least the rimmed chondrules may have been inherent to the dust accretion event and was not erased by subsequent heating.