Shower Meteoroids: Constraints From Interplanetary Dust Particles And Leonid Meteors

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.     

The origins of I‐type spherules and the atmospheric entry of iron micrometeoroids

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.     

First results from video spectroscopy of 1998 Leonid meteors

Abstract— We report spectroscopic observations of meteors made from the FISTA aircraft on 1998 November 17 as a part of the Leonid multi-instrument aircraft campaign. Low-resolution spectra of 119 meteors of apparent visual magnitudes from +3 to ?4, corresponding to meteoroid masses from 10^?6 to 10^?3 kg, were obtained. After analyzing a representative sample of the spectra and comparing them to the spectra of Perseid meteors from the Ondrejov archive, the following conclusions were reached: Leonid meteoroids are very loose and disintegrate easily in the atmosphere. This leads to much faster evaporation of volatile Na than of other elements, an effect which is not observed in the Perseid meteors. Relative bulk abundances of Mg, Fe, Ca, and Na in Leonid meteors are nearly CI-chondritic within the uncertainty of the method (factor of 3). Smaller meteoroids tend to be poorer in Na, which is true also for Perseid meteors. Most meteoric vapor emissions could be reasonably well explained with the temperature of 4500 K. High-temperature meteoric emissions (Ca⁺, Mg⁺) are present only in bright meteors. Leonid spectra are very rich in atmospheric emissions of O, N, and N₂, even at high altitudes and in faint meteors. These emissions are therefore not connected with the meteor shock wave. Thermal continuum is also present in the spectra. Organic material was not revealed.     

Detection of Leonids meteoric dust in the upper atmosphere by polarization measurements of the twilight sky

A new method for the meteoric dust detection in the upper atmosphere based on the polarimetric observations of the twilight sky is proposed. Polarization measurements are effective for detection of the meteoric dust scattering on the background consisting basically of troposphere multiple scattering. The method is based on the observed and explained polarization properties of the sky background during different stages of twilight. It is used to detect the mesosphere dust after the Leonids maximum in 2002, estimate its altitude range and to investigate its evolution—slow decrease of the altitude. The polarization method takes into account the multiple scattering and sufficient contribution of moonlight scattering background and turns out to be more sensitive than existing analogs used in the present time.     

Carbon in Meteoroids: Wild 2 Dust Analyses, IDPs and Cometary Dust Analogues

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.     

Sources of insoluble inclusions in stratospheric sulfate particles

Abstract– Properties of aerosol collected in the stratosphere from altitudes of 20–45 km are reviewed. Removal of the soluble material from predominantly sulfate particles collected at 20 km revealed the presence of insoluble individual particles, or small groups of them, typically 40–50 nm in diameter. The size distribution of components of chain aggregates found above 35 km was almost identical, suggesting that rupture of the chains by condensing sulfuric acid, as they fell into the sulfate layer from above, was the source of the inclusions. Particles collected above 35 km on thin films of metal all showed the presence of a partially volatile liquid. On a copper surface, the liquid was stabilized, and of greater extent than the solid component. Three observations suggest that the upper stratospheric particles and their associated liquid were partly or wholly organic and derived from cometary dust too small to be heated on entering the atmosphere. These are: (1) the presence of a liquid that reacts with copper and the similarity to the behavior of particles collected on copper during a manned space flight, (2) their morphological similarity to published photographs of particles collected in the mesosphere from rockets, (3) the consistency with recent spacecraft observations of the size distribution of components sub‐10 μm aggregates in cometary dust and the presence within them of carbon compounds.     

Natural Variations in Comet-Aggregate Meteoroid Compositions

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.     

FORMS OF NONTERRESTRIAL DUST ON THE EARTH

The author carried out a study of pulverised cosmic matter extracted from the soil at the fall locality of the Sikhote Alin iron meteorite shower. Three forms of dust were distinguishable: meteoritic, sharp-angled, irregular particles from the break-up of the meteorite; meteoric, spherical, magnetic particles from ablation; and micro meteorites. Meteoritic and meteoric dust was also discovered in the soil of the regions of fall of the Boguslavka and Yardymly iron meteorites. Experiments made by the author for the purpose of obtaining artificial meteoric dust from meteoritic matter of various types have shown that the meteoric dust obtained from stony meteorites is composed of spherules similar to those extracted from the soil in the areas of fall of the Sikhote Alin, Boguslavka and Yardymly iron meteorites. Cosmic dust, the particles of which are usually called micrometeorites, due to their small size, are not subjected to the influence of temperature as they pass through the Earth's atmosphere and they reach the Earth's surface unaltered. It is proposed that meteoric and cosmic dust comprises the largest part of the cosmic matter falling onto the Earth:     

The variability of H2O fluxes in the earth's atmosphere

Water is the only substance that occurs not only in all three phases, gaseous, fluid, and solid in the Earth's atmosphere but also in transitions between them like in its “polymer forms” - clusters, clathrates, aerosols. Although the importance of the solid and fluid forms is popularly understood, there is less general understanding of the role of water vapor and especially for its polymer forms than for example, of carbon dioxide (C0₂) or ozone (03). The spatial and temporal distributions of the various phases of water in the atmosphere are in fact very important factors to climate, weather, the biosphere, the homogeneous and inhomogeneous chemistry of the atmosphere, as well as to the propagation of electromagnetic waves used in transatmospheric (global) navigation and communication systems and for relevant remote sensing measurements. In this context the atmosphere acts like a temporal and spatial variable, frequency dependent filter. Despite the fact that above the tropopause, i.e. in the stratosphere and mesosphere, there is less than 0.1 % of the total water vapor content of the Earth's atmosphere its influence is very significant for the physics and chemistry of the upper atmosphere and hence for the climate. The question is discussed: How variable is the total hydrogen budget of the Earth's atmosphere with time?     

Very small bodies in the solar system: The Earth's atmosphere as detector

Particles of mass less than about 1 gm are a minor fraction of the total matter impinging on the Earth averaged over millennia time scales. However, these particles dominate during a single particular year and produce the most obvious evidence of incoming extra-terrestrial matter in the form of ablation trails in the atmosphere which are visible at night as meteors.Observations of meteors give astronomical information on the composition, structure, and cometary associations of the particles. The composition is deduced from optical spectra of meteors, whilst telescopic studies of the trails during formation give information on the physical structure of the particles. Any cometary associations are deduced from measurement of meteor orbits determined photographically, using television, or by radar.Meteors occur in the atmosphere at heights from about 70 to 120 km. Optical observations are restricted to night-time and usually under conditions of low moonlight. A typical television based detector can record +8^M meteors with a sporadic rate of 15–20 per hour and velocities accurate to about 3%. The luminosity of the trail is strongly dependent on the velocity of the meteoroid (to about the third power).Radar observations of meteors are unrestricted by weather or time of day, and can readily detect meteors at least two orders of magnitude smaller in mass than those detectable optically. Again the observations are heavily biased toward the higher velocities as the electron line density varies approximately asV ^3.5. However, the higher the velocity of the meteoroid the greater the height of the meteor trail, and the reduced probability of radar detection due to rapid diffusion of the trail. Thus radar observations tend to select meteors in the intermediate velocity range 30–40 km s^–1.     

A Jupiter fragmented comet: Cause of the K/T boundary record

The extended period of mass extinctions around the K/T boundary correlating with extraterrestrial amino acids in the sediment record constitutes strong evidence of a cometary cause. The input of extraterrestrial matter over 10⁵ yr supports the hypothesis of a giant comet, fragmented into subcomets on close encounter with Jupiter, and subsequently perturbed into Earth-crossing orbits. Copious amounts of dust were emitted via this and possibly successive fragmenting encounters, and via normal cometary evaporation. The dynamics of dust from the disintegrating comet fragments favours retention in Earth-crossing orbits of the sub-micron fraction of organic composition. The shroud of dust accreted in the Earth's upper atmosphere varied with time and imposed climatic stresses that caused species extinctions over 10⁵ yr. While the iridium peak in the sediments coincides with the Chicxulub crater impactor, other iridium detail suggests that some of the impactor material was reinjected into space and in part re-accreted by Earth from the interplanetary orbits.     

Noble gas space exposure ages of individual interplanetary dust particles

Abstract— The He, Ne, and Ar compositions of 32 individual interplanetary dust particles (IDPs) were measured using low‐blank laser probe gas extraction. These measurements reveal definitive evidence of space exposure. The Ne and Ar isotopic compositions in the IDPs are primarily a mixture between solar wind (SW) and an isotopically heavier component dubbed “fractionated solar” (FS), which could be implantation‐fractionated solar wind or a distinct component of the solar corpuscular radiation previously identified as solar energetic particles (SEP). Space exposure ages based on the Ar content of individual IDPs are estimated for a subset of the grains that appear to have escaped significant volatile losses during atmosphere entry. Although model‐dependent, most of the particles in this subset have ages that are roughly consistent with origin in the asteroid belt. A short (<1000 years) space exposure age is inferred for one particle, which is suggestive of cometary origin. Among the subset of grains that show some evidence for relatively high atmospheric entry heating, two possess elevated ²¹Ne/²²Ne ratios generated by extended exposure to solar and galactic cosmic rays. The inferred cosmic ray exposure ages of these particles exceeds 10⁷ years, which tends to rule out origin in the asteroid belt. A favorable possibility is that these ²¹Ne‐rich IDPs previously resided on a relatively stable regolith of an Edgeworth‐Kuiper belt or Oort cloud body and were introduced into the inner solar system by cometary activity. These results demonstrate the utility of noble gas measurements in constraining models for the origins of interplanetary dust particles.     

THE TUNGUSKA EVENT AND THE METEORS WITH TERMINAL FLARES

The comparison of the Tunguska body explosion with the effect of terminal flares of meteors and fireballs leads us to the conclusion that these events are of a similar nature but differ only by their scale. We consider that the dynamics of progressive breaking and evaporation of meteoric bodies during their entry into the terrestrial atmosphere could explain the terminal burst. An extremely porous body model for the Tunguska meteorite was analysed and rejected as unsatisfactory. The realistic values of the initial velocity (～30 km/sec) and of the inclination angle for the Tunguska's trajectory (5–15°) give orbital elements not in contradiction with the cometary origin of the Tunguska body.     

Accretion and early degassing of the Earth: Constraints from Pu-U-I-Xe Isotopic systematics

Abstract— A review of problems related to Xe isotopic abundances in meteorites and terrestrial materials leads to four postulates which should be taken into account to build a model of the Earth's accretion and early evolution. 1. The pre-planetary accretion time scale was shorter than the ¹²⁹I half-life, 17 Ma, so the initial ratio of ¹²⁹I/¹²⁷I had not been decreased considerably when planetary accretion started; therefore, this must also be the case for the ²⁴⁴Pu abundance. 2. The initial relative abundance of involatile refractory ²⁴⁴Pu in proto-planetary materials should be the same as in chondrites, that is, ²⁴⁴Pu/²³⁸U = 0.0068; this value corresponds to initial ²⁴⁴Pu 0.30 ppb in the bulk silicate earth. In contrast, I is a highly volatile element; its initial abundance, accretion history and even the present-day mean concentrations in principal terrestrial reservoirs are poorly known. 3. There is much less fission Xe in the upper mantle, crust, and atmosphere than is predictable from the fission of ²⁴⁴Pu (Xe(Pu)) based on the above argument. Therefore, Xe(Pu) has been mainly released from these reservoirs. 4. A mechanism for Xe(Pu) escape from the complementary upper mantle-crust-atmosphere reservoirs, for example, atmospheric escape via collisions of a growing Earth with large embryos and/or hydrodynamic hydrogen flux, etc., operated during the Earth's accretion. These postulates have been used as a background for a balance model of homogeneous Earth accretion which envisages: growth of the Earth due to accumulation of planetesimals; fractionation inside the Earth and segregation of the core; degassing via collision and fractionation; and escape of volatiles from the atmosphere. During the post-accretion terrestrial history, the processes described by the model are continuous fractionation, degassing and recycling of the upper mantle and crust. The lower mantle is considered as an isolated reservoir. Depending on the scenario invoked, the accretion time scale varies within the limits of 50–200 Ma. In the light of recent experimental data, the latter value is inferred to the most realistic version which explains a high Xe(U)/Xe(Pu) ratio in the upper mantle. Contrary to previous suggestions, the ¹²⁹I-¹²⁹Xe subsystem is considered to be meaningless with regard to the terrestrial accretion time scale. The terrestrial inventory of ¹²⁹Xe(I) is controlled by the initial abundance of volatile elements (including I and Xe) in proto-terrestrial materials and the subsequent degassing history of the Earth. The residence time of a volatile element (e.g., Xe) in the bulk mantle (bm) during accretion, < t (Xe)_bm>, is approximated by the ratio of < t (Xe)_bm> m _bm(t)/φ_{bm, mf} ≤ 10 Ma, where m _bm(t) is the mantle mass, and φ_{bm, mf} is the rate of metal/silicate fractionation, which provided segregation of the core; φ_{bm, mf} is determined by involatile siderophile element abundances in the upper mantle. This relationship implies a link between the abundance of involatile siderophile and volatile incompatible elements. A short <t(Xe)_bm> reflects a high degassing rate due to extremely high φ_{bm, mf} 10²⁰ g/year. A small ratio of the atmospheric amount of Xe over the total amount of this gas in prototerrestrial materials, ≤0.01, is in accord with the process of Xe escape and fractionation in the primary Earth atmosphere.     

The entry heating and abundances of basaltic micrometeorites

Basaltic micrometeorites (MMs) derived from HED‐like parent bodies have been found among particles collected from the Antarctic and from Arctic glaciers and are to date the only achondritic particles reported among cosmic dust. The majority of Antarctic basaltic particles are completely melted cosmic spherules with only one unmelted particle recognized from the region. This paper investigates the entry heating of basaltic MMs in order to predict the relative abundances of unmelted to melted basaltic particles and to evaluate how mineralogical differences in precursor materials influence the final products of atmospheric entry collected on the Earth's surface. Thermodynamic modeling is used to simulate the melting behavior of particles with compositions corresponding to eucrites, diogenites, and ordinary chondrites in order to evaluate degree of partial melting and to make a comparison between the behavior of chondritic particles that dominate the terrestrial dust flux and basaltic micrometeroids. The results of 120,000 simulations were compiled to predict relative abundances and indicate that the phase relations of precursor materials are crucial in determining the relative abundances of particle types. Diogenite and ordinary chondrite materials exhibit similar behavior, although diogenite precursors are more likely to form cosmic spherules under similar entry parameters. Eucrite particles, however, are much more likely to melt due to their lower liquidus temperatures and small temperature interval of partial melting. Eucrite MMs, therefore, usually form completely molten cosmic spherules except at particle diameters <100 μm. The low abundance of unmelted basaltic MMs compared with spherules, if statistically valid, is also shown to be inconsistent with a low velocity population (12 km s^?1) and is more compatible with higher velocities which may suggest a near‐Earth asteroid source dominates the current dust production of basaltic MMs.     

Microbeam Analysis of Four Chondritic Interplanetary Dust Particles for Major Elements,Carbon and Oxygen

Abstract— Chemical compositions determined using electron excited x-rays are reported for four interplanetary dust particles collected in the stratosphere. These analyses include measurements of carbon and oxygen abundances which are important elements in these primitive materials. Spot analyses show very heterogeneous compositions on a micrometer scale although average composition approaches that of C1 carbonaceous chondrites. While the spot analyses show intermediate compositions between cometary dust and carbonaceous chondrites, the heterogeneity more closely resembles that of comet Halley dust particles.     

Chemical abundances determined from meteor spectra: I. Ratios of the main chemical elements

Abstract— Relative chemical abundances of 13 meteoroids were determined by averaging the composition of the radiating gas along the fireball path that originated during their penetration into the Earth's atmosphere. Mg, Fe, Ni, Cr, Mn, and Co abundances, relative to Si, are similar to those reported for CI and CM carbonaceous chondrites and interplanetary dust particles. In contrast, relative abundances of Ca and Ti in meteor spectra indicate that these elements suffer incomplete evaporation processes. The chemical composition of all meteoroids studied in this work differs from that of 1P/Halley dust.     

Water and related chemistry in the solar system. A guaranteed time key programme for Herschel

“Water and related chemistry in the Solar System” is a Herschel Space Observatory Guaranteed-Time Key Programme. This project, approved by the European Space Agency, aims at determining the distribution, the evolution and the origin of water in Mars, the outer planets, Titan, Enceladus and the comets. It addresses the broad topic of water and its isotopologues in planetary and cometary atmospheres. The nature of cometary activity and the thermodynamics of cometary comae will be investigated by studying water excitation in a sample of comets. The D/H ratio, the key parameter for constraining the origin and evolution of Solar System species, will be measured for the first time in a Jupiter-family comet. A comparison with existing and new measurements of D/H in Oort-cloud comets will constrain the composition of pre-solar cometary grains and possibly the dynamics of the protosolar nebula. New measurements of D/H in giant planets, similarly constraining the composition of proto-planetary ices, will be obtained. The D/H and other isotopic ratios, diagnostic of Mars’ atmosphere evolution, will be accurately measured in H₂O and CO. The role of water vapor in Mars’ atmospheric chemistry will be studied by monitoring vertical profiles of H₂O and HDO and by searching for several other species (and CO and H₂O isotopes). A detailed study of the source of water in the upper atmosphere of the Giant Planets and Titan will be performed. By monitoring the water abundance, vertical profile, and input fluxes in the various objects, and when possible with the help of mapping observations, we will discriminate between the possible sources of water in the outer planets (interplanetary dust particles, cometary impacts, and local sources). In addition to these inter-connected objectives, serendipitous searches will enhance our knowledge of the composition of planetary and cometary atmospheres.     

Television spectra of meteors

The first results of the television observations of meteors at the Ondejov Observatory are presented. It is shown that three spectral components may be distinguished in meteors: cool meteoric, hot meteoric and hot atmospheric. The intensity ratio of these components varies strongly even in meteors of the same velocity and within the records of single meteors. This is evidence for variations in the ablation process and in the formation of the shock wave. The so called calcium anomaly is in fact only a demonstration of these variations.     

Micrometeorites and Their Implications for Meteors

Micrometeorites (MMs) are extraterrestrial dust particles, in the size range 25–400 μm, recovered from the Earth’s surface. They have experienced a wide range of heating during atmospheric entry from completely molten spherules to particles heated to temperatures <300°C that have retained low temperature minerals. The majority of MMs have mineralogies, textures and compositions that strongly resemble components from chondritic meteorites suggesting these correspond to sporadic, low geocentric velocity meteors. Changes in MMs due to entry heating, however, have implications for meteoric processes in general that may allow the observed behaviour of meteors to be directly related to the material properties of their meteoroids.