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1.
Bulk compositions of aggregate meteoroids made of the originally accreted dust with its highly varied in mineral content and chemistry and considerable grain size variations do not have a chondritic bulk composition. Deviations from CI element abundances reflect indigenous variations within and among comet nuclei. These unmodified meteoroids that are heterogeneous in all their properties are fundamentally different from meteoroids with a CI bulk composition that are fine-grained, equigranular materials and chemically and mineralogically homogeneous. Collection and data reduction bias exists but the compositions of individual fast meteors are entirely constrained by the measured main component meteor abundances.  相似文献   

2.
A detailed analysis of a photographic spectrum of a Geminid fireball obtained in December 14, 1961 at the Ondrejov Observatory is presented. We have computed a synthetic spectrum for the fireball and compared it with the observed spectrum assuming chemical equilibrium in the meteor head. In this way we have determined relative chemical abundances in meteor vapors. Comparing the relative chemical abundances of this Geminid meteoroid with those obtained from meteoroids associated with comets 55P/Tempel-Tuttle and 109P/Swift-Tuttle we found no significant chemical differences in main rock-forming elements. Despite of this similarity, the deepest penetration of the Geminid meteoroids and their ability to reach high rotation rates in space without fragmentation suggest that thermal processing is affecting their physical properties. We suggest that as consequence of space weathering a high-strength envelope is produced around these particles. In this picture, heating processes of the mineral phases could result in the peculiar properties observed during atmospheric entry of the Geminid meteoroids, especially their strength, which is evidenced by its resistance to ablation. Finally, although one meteoroid cannot be obviously considered as representative of the composition of its parent body, we conclude that 3200 Phaethon is able to produce millimetre-size debris nearly chondritic in composition, but the measured slight overabundance of Na would support a cometary origin for this body.  相似文献   

3.
High dispersion photographic spectra of three Leonid and five Perseid meteors are used to derive relative abundances of nine chemical elements in the radiating meteoric vapors and in the meteoroids. Al and Ca were found to be incompletely evaporated in the main spectral component at 5000 K but completely evaporated in the second component at 10,000 K. Si lines are present in both components which enhances the reliability of determination of the Si abundance. The composition of the meteoroids was found to be more similar to comet Halley than to chondritic meteoroids. Fe, Cr, and Mn are depleted and Si, Na, and H are enhanced relative to Mg in comparison with CI chondrites.  相似文献   

4.
Assuming that similar organic components as in comet 81P/Wild 2 are present in incoming meteoroids, we try to anticipate the observable signatures they would produce for meteor detection techniques. In this analysis we consider the elemental and organic components in cometary aggregate interplanetary dust particles and laboratory analyses of inter- and circumstellar carbon dust analogues. On the basis of our analysis we submit that (semi) quantitative measurements of H, N and C produced during meteor ablation will open an entire new aspect to using meteoroids as tracers of these volatile element abundances in active comets and their contributions to the mesospheric metal layers.  相似文献   

5.
We present a survey of 97 spectra of mainly sporadic meteors in the magnitude range +3 to −1, corresponding to meteoroid sizes 1-10 mm. For the majority of the meteors, heliocentric orbits are known as well. We classified the spectra according to relative intensities of the lines of Mg, Na, and Fe. Theoretical intensities of these lines for a chondritic composition of the meteoroid and a wide range of excitation and ionization conditions were computed. We found that only a minority of the meteoroids show chondritic composition. Three distinct populations of Na-free meteoroids, each comprising ∼10% of sporadic meteoroids in the studied size range, were identified. The first population are meteoroids on asteroidal orbits containing only Fe lines in their spectra and possibly related to iron-nickel meteorites. The second population are meteoroids on orbits with small perihelia (q?0.2 AU), where Na was lost by thermal desorption. The third population of Na-free meteoroids resides on Halley type cometary orbits. This material was possibly formed by irradiation of cometary surfaces by cosmic rays in the Oort cloud. The composition of meteoroids on Halley type orbits is diverse, probably reflecting internal inhomogeneity of comets. On average, cometary dust has lower than chondritic Fe/Mg ratio. Surprisingly, iron meteoroids prevail among millimeter-sized meteoroids on typical Apollo-asteroid orbits. We have also found varying content of Na in the members of the Geminid meteoroid stream, suggesting that Geminid meteoroids were not released from their parent body at the same time.  相似文献   

6.
One of the goals of comet research is the determination of the chemical composition of the nucleus because it provides us with the clues about the composition of the nebula in which comet nuclei formed. It is well accepted that photo-chemical reactions must be considered to establish the abundances of mother molecules in the coma as they are released from the comet nucleus or from distributed dust sources in the coma. However, the mixing ratios of mother molecules in the coma changes with heliocentric distance. To obtain the abundances in the nucleus relative to those in the coma, we must turn our attention to the release rates of mother molecules from the nucleus as a function of heliocentric distance. For this purpose, we assume three sources for the coma gas: the surface of the nucleus (releasing mostly water vapor), the dust in the coma (the distributed source of several species released from dust particles), and the interior of the porous nucleus (the source of many species more volatile than water). The species diffusing from the interior of the nucleus are released by heat transported into the interior. Thus, the ratio of volatiles relative to water in the coma is a function of the heliocentric distance and provides important information about the chemical composition and structure of the nucleus. Our goal is to determine the abundance ratios of various mother molecules relative to water from many remote-sensing observations of the coma as a function of heliocentric distance. Comet Hale-Bopp is ideal for this purpose since it has been observed using instruments in many different wavelength regions over large ranges of heliocentric distances. The ratios of release rates of species into the coma are than modeled assuming various chemical compositions of the spinning nucleus as it moves from large heliocentric distance through perihelion. Since the heat flow into the nucleus will be different after perihelion from that before perihelion, we can also expect different gas release rates after perihelion compared to those observed before perihelion. Since not all the data are available yet, we report on progress of these calculations. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

7.
We analyze the chemical composition and abundances of comets based on in situ measurements of Comet 1P/Halley and remote sensing observations of several recent bright comets including Hale-Bopp (C/1995 O1) and Hyakutake (C/1996 B2), in light of the elemental abundances of the solar system. Nitrogen is underabundant in comets relative to the solar system because nitrogen tends to be in N2, which is chemically relatively inert. While many details remain uncertain, some gross features are emerging. The abundance of water : silicates: carbonaceous molecules (CO, CO2, and hydrocarbons) by mass is approximately 1 : 1 : 1. Furthermore, the mass abundance of ice : dust (silicates and hydrocarbon polycondensates) is about1 : 1. We compare a list of identified comet molecules with molecules detected in the interstellar medium, although a comparison with their relative abundances, particularly in the ice phase, would be more meaningful. However, ice-phase abundances are not yet available. One can expect a variation of the abundances of carbon-bearing molecules in comets to be associated with their place of origin in the solar nebula. However, we also note that comets are heterogeneous. Thus, observed differences may be related to the place of origin, heterogeneity of the nucleus, or acquired through evolution. The molecular and elemental compositions of the coma are most likely not the same as those in the nucleus. This is particularly true for volatile ices and their gases and for the dust-to-ice and dust-to-gas ratios. Analyses must carefully consider the three sources of gas: Water from the surface of the nucleus, gases more volatile than water from the interior of the nucleus, and gases from the sublimation of the dust distributed in the coma. Topography on the surface of the nucleus may cause important evolutionary differences in the dust-to-gas mass ratio. Relatively inactive areas on the surface of the nucleus are probably associated with convex topography. Gas sublimated from convex areas (hills and mountains) diverges more strongly relative to gas sublimated from concave areas, which can entrain dust more efficiently. Thus, the entrainment of dust from convex areas is poor and dust may fall back to the surface of the nucleus creating a dust mantle, which further inhibits outgassing.  相似文献   

8.
Abstract— Depending on their velocity, entry angle and mass, extraterrestrial dust particles suffer certain degrees of heating during entry into Earth's atmosphere, and the mineralogy and chemical composition of these dust particles are significantly changed. In the present study, pulse-heating experiments simulating the atmospheric entry heating of micrometeoroids were carried out in order to understand the mineralogical and chemical changes quantitatively as well as to estimate the peak temperature experienced by the particles during entry heating. Fragments of the CI chondrites Orgueil and Alais as well as pyrrhotites from Orgueil were used as analogue material. The experiments show that the volatile elements S, Zn, Ga, Ge, and Se can be lost from 50 to 100 μm sized CI meteorite fragments at temperatures and heating times applicable to the entry heating of similar sized cosmic dust particles. It is concluded that depletions of these elements relative to CI as observed in micrometeorites are mainly caused by atmospheric entry heating. Besides explaining the element abundances in micrometeorites, the experimentally obtained release patterns can also be used as indicators to estimate the peak heating of dust particles during entry. Using the abundances of Zn and Ge and assuming their original concentrations close to CI, a maximum heating of 1100–1200 °C is obtained for previously analyzed Antarctic micrometeroites. Thermal alteration also strongly influenced the mineralogy of the meteorite fragments. While the unheated samples mainly consisted of phyllosilicates, these phases almost completely transformed into olivine and pyroxene in the fragments heated to ≥800 °C. Therefore, dust particles that still contain hydrous minerals were probably never heated to temperatures ≥800 °C in the atmosphere. During continued heating, the grain size of the newly formed silicates increased and the composition of the olivines equilibrated. Applying these results quantitatively to Antarctic micrometeorites, typical peak temperatures in the range of 1100–1200 °C during atmospheric entry heating are deduced. This temperature range corresponds to the one obtained from the volatile element concentrations measured in these micrometeorites and points to an asteroidal origin of the particles.  相似文献   

9.
The question whether the solar chemical composition is typical for solar-type stars is analysed by comparing the Sun with different stellar samples, including a sample of stars with very similar parameters, solar twins. Although typical in terms of overall metallicity for stars of solar age and galactic orbit, the solar atmosphere is found to have abundances, as compared with solar twins, that indicate that its gas has once been affected by dust formation and dust separation. It is concluded that this may be related to the formation of the solar planetary system and its special properties.  相似文献   

10.
The cometary Leonid meteoroids represent a size range in between largest carbon-richIDPs and the smallest CI meteorites. Their dustball structure and chemistry offer anopportunity to constrain hierarchical dust accretion inferred from petrologic studies ofaggregate and cluster IDPs. The Leonid shower meteoroids of known ``comet ejection'ages provide an opportunity to study space weathering of cometary dust over periodsof up to several hundred years. The meteors and aggregate and cluster IDPs displaycontinuous thermal modification of organics and volatile element (Na, K-bearing phases), that occur as discrete minerals and amorphous solids each different response during kinetically controlled ablation. Leonid meteoroids are not excessively Na-rich. The occurrences of Leonid meteors can now be accurate predicted and combined withknowledge better models for the settling rates, collections of surviving dust becomea comet nucleus-sampling mission. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

11.
The Earth's extraterrestrial dust flux includes a wide variety of dust particles that include FeNi metallic grains. During their atmospheric entry iron micrometeoroids melt and oxidize to form cosmic spherules termed I‐type spherules. These particles are chemically resistant and readily collected by magnetic separation and are thus the most likely micrometeorites to be recovered from modern and ancient sediments. Understanding their behavior during atmospheric entry is crucial in constraining their abundance relative to other particle types and the nature of the zodiacal dust population at 1 AU. This article presents numerical simulations of the atmospheric entry heating of iron meteoroids to investigate the abundance and nature of these materials. The results indicate that iron micrometeoroids experience peak temperatures 300–800 K higher than silicate particles explaining the rarity of unmelted iron particles which can only be present at sizes of <50 μm. The lower evaporation rates of liquid iron oxide leads to greater survival of iron particles compared with silicates, which enhances their abundance among micrometeorites by a factor of 2. The abundance of I‐types is shown to be broadly consistent with the abundance and size of metal in ordinary chondrites and the current day flux of ordinary chondrite‐derived MMs arriving at Earth. Furthermore, carbonaceous asteroids and cometary dust are suggested to make negligible contributions to the I‐type spherule flux. Events involving such objects, therefore, cannot be recognized from I‐type spherule abundances in the geological record.  相似文献   

12.
A new approach is considered to the problem of ionization of the inner comas of comets connected with two phenomena: meteor-like process due to cometary molecules — interplanetary meteoroids impacts and explosion-type process due to high-velocity collisions between cometary dust grains and interplanetary meteoroids. It is found that the efficiency of explosive ionization exceeds the efficiency of meteor ionization approximately 100 times. The explosive ionization may be possible mechanism for anomaly ionization of the inner comae of dusty comets like Halley 1986 III with the dust to gas production rate ratio more than 0.1.  相似文献   

13.
We present a model-atmosphere analysis for the bright ( V ∼13) star ZNG-1, in the globular cluster M10. From high-resolution ( R ∼40 000) optical spectra we confirm ZNG-1 to be a post-asymptotic giant branch (post-AGB) star. The derived atmospheric parameters are T eff=26 500±1000 K and log  g =3.6±0.2 dex . A differential abundance analysis reveals a chemical composition typical of hot post-AGB objects, with ZNG-1 being generally metal poor, although helium is approximately solar. The most interesting feature is the large carbon underabundance of more than 1.3 dex. This carbon deficiency, along with an observed nitrogen enhancement relative to other elements, may suggest that ZNG-1 evolved off the AGB before the third dredge-up occurred. Also, iron depletions observed in other similar stars suggest that gas–dust fractionation in the AGB progenitor could be responsible for the observed composition of these objects. However, we need not invoke either scenario since the chemical composition of ZNG-1 is in good agreement with abundances found for a Population II star of the same metallicity.  相似文献   

14.
Jupiter and Saturn produce important gravitational impulses on meteoroids released by comet 109P/Swift-Tuttle. The meteoroids from this comet once released follow retrograde orbits that during their periodic approaches to these planets (within 1.6 and 0.9 A.U., respectively) are impulsed gaining orbital energy. This perturbation effect is translated into a net inward shift in the node of the perturbed meteoroids. Such geometry with Jupiter occurred in 2004 over a meteoroid trail ejected by this comet during the 1862 A.D. return of the comet to perihelion. In order to study the predicted outburst produced by one-revolution meteoroids, the Spanish Photographic Meteor Network (SPMN) performed an extensive campaign. As a part of this observational effort here are presented 10 accurate meteoroid orbits. We discuss their origin by comparing them with the theoretical orbital elements of the dust trails intercepting the Earth during the 2004 Perseid return.  相似文献   

15.
It is shown that on theoretical grounds the relative abundances of the elements in the interstellar gas phase should be correlated with the wavelength of maximum interstellar polarization max. If these correlations can be determined by observations, then there is the possibility to determine the relative abundances of the heavier elements within the mantles as well as within the cores of interstellar dust grains, at least in principle.The observational data available up to now confirm the existence of such correlations between max and the interstellar gas phase abundances of titanium, iron, magnesium, and carbon. Statements about the chemical composition of the dust particles are not yet possible. For this there are observations of the interstellar gas depletion needed, especially in such lines of sight where max has extreme values.Paper presented at a Workshop on The Role of Dust in Dense Regions of Interstellar Matter, held at Georgenthal, G.D.R., in March 1986.  相似文献   

16.
Abstract— Observations of long‐lasting persistent trains following the entry of some meteoroids into the Earth's atmosphere are suggested to arise in part from the interaction between meteoroid components and the atmosphere and in the heterogeneous recombination reaction of atmospheric O atoms with NO. The latter occurs on the surfaces of dust left by the explosive fragmentation of larger meteoroids. A strong role is attributed to reactions of troilite (FeS), a meteorite component, with the atmosphere at elevated temperatures. The suggestions made in this paper complement previous work that suggested that long‐lived emissions results from a variety of species made in the shock of larger meteoroids.  相似文献   

17.
Recent progress on the interrelation between meteor streams and comets is reviewed both on dynamical and physical aspects. The topics include the recent concept of the structure of meteor streams, resulted success of the prediction of the meteor storms, and the recent observational situation on the cometary dust grains and meteoroids. Two possible explanations for the origin of the meteoroids together with the implication for the relation between the birthplace of the parent comets and the meteoroids are discussed.  相似文献   

18.
Alan E. Rubin 《Icarus》2011,213(2):547-558
Chondrite groups can be distinguished on the basis of their abundances of refractory lithophile elements (RLE). These abundances are, in part, functions of the mass fraction of Ca-Al-rich inclusions (CAIs) within the chondrites. Carbonaceous chondrites contain the most CAIs and the highest RLE abundances; they also contain modally abundant fine-grained matrix material that consists largely of modified nebular dust. The amount of dust varied throughout the solar nebula: enstatite and ordinary chondrites formed in low-dust regions in the inner part of the nebula, R chondrites formed in higher-dust zones at somewhat greater heliocentric distances, and carbonaceous chondrites formed in even dustier regions farther from the Sun. The amount of ambient dust peaked in the region where CV and CK chondrites accreted; these chondrites have abundant matrix, the highest modal abundances of CAIs, and the highest bulk RLE contents. Substantial amounts of nebular dust occurred in highly porous multi-millimeter-to-centimeter-size dustballs that were on the order of 100 times more massive than CAIs. Radial drift processes in the nebula affected these dustballs to approximately the same extent as the CAIs; both types of objects were aerodynamically concentrated in the same nebular regions. These regions maintained approximately the same relative amounts of dust through the periods of chondrule formation and chondrite accretion.  相似文献   

19.
David ?apek  Ji?í Borovi?ka 《Icarus》2009,202(2):361-370
A considerable depletion of sodium was observed in Geminid meteoroids. To explain this phenomenon, we developed a quantitative model of sodium loss from meteoroids due to solar heating. We found that sodium can be lost completely from Geminid meteoroids after several thousands of years when they are composed of grains with sizes up to ∼100 μm. The observed variations of sodium abundances in Geminid meteor spectra can be explained by differences in the grain sizes among these meteoroids. Sodium depletions are also to be expected for other meteoroid streams with perihelion distances smaller than ∼0.2 AU. In our model, the meteoroids were represented by spherical dust-balls of spherical grains with an interconnected pore space system. The grains have no porosity and contain usual minerals known from meteorites and IDP's, including small amount of Na-bearing minerals. We modeled the sequence of three consecutive processes for sodium loss in Geminid meteoroids: (i) solid-state diffusion of Na atoms from Na-bearing minerals to the surface of grains, (ii) thermal desorption from grain surfaces and (iii) diffusion through the pore system to the space. The unknown material parameters were approximated by terrestrial analogs; the solid-state diffusion of Na in the grains was approximated by the diffusion rates for albite and orthoclase.  相似文献   

20.
The heliocentric radial distribution of the flux of hyperbolic cosmic dust particles, as measured by the Pioneer 8 and 9 spacecraft, is closely related to the radial variation of the spatial density of source or “parent” meteoroids. Within the limits of the experimental and theoretical uncertainties the spatial density of parent meteoroids, as deduced from the hyperbolic cosmic dust data, is found to be increasing with increasing heliocentric distance in the neighborhood of one a.u. Other recent experimental evidence confirms this result. The new results also suggest that the ratio of the areal density of submicron sized craters to the areal density of millimeter sized craters will be less on the north-south faces of lunar rocks than on the east-west faces of the same rocks. The changeinratio is not as large as previously thought, however. Finally it is noted that the solar system is not presently contributing significant amounts of dust to the interstellar medium though it may once have done so.  相似文献   

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