Not available / text

Les chondrites carbonées (CCs) sont des objets primitifs accrétés lors de la formation du Système Solaire. Composées en grande partie de chondres, de matrice et d’inclusions réfractaires, elles ont enregistré les hétérogénéités chimiques, isotopiques et minéralogiques de la nébuleuse solaire. Contrairement aux autres classes de chondrites, la grande majorité des CCs sont primitives (types pétrologiques 1 à 3). Elles n’ont donc pas subi de métamorphisme important sur leur corps parent. Toutefois, un groupe de CCs, les CKs, montre un métamorphisme thermique intense (types pétrologiques 4 à 6). Ces chondrites sont caractérisées par des matrices recristallisées, des olivines équilibrées à ∼Fa31, un degré d’oxydation important (olivines riches en NiO, rapport métal/magnétite proche de zéro), des teneurs en éléments réfractaires lithophiles intermédiaires aux CVs et aux COs, ou encore des compositions isotopiques en oxygène se situant dans le champ défini par les CVs et les COs. Les CKs ont été peu étudiées jusqu’au début des années 90, car peu nombreuses (seulement 210 classifiées au 6 décembre 2011) et de petite taille (masse médiane ∼33,5g). Leurs compositions isotopiques et chimiques laissent supposer l’existence d’un lien génétique avec les CV3. Les découvertes récentes de nouvelles CKs depuis 1990, et notamment de CK3 par le biais de collectes systématiques au Sahara et en Antarctique, permettent l’étude détaillée de l’évolution métamorphique des CKs, notamment à la transition 3–4. Ce travail a pour but de caractériser les conditions dans lesquelles s’est déroulé cet épisode métamorphique, et grâce à l’observation de plusieurs CK3–4, d’étudier la relation CV-CK. La caractérisation détaillée de l’évolution métamorphique de 19 CKs dont 5 CK3 a permis de confirmer que les différences observées entre les divers composants chondritiques (abondance, minéralogie, texture) des CVs et des CKs peuvent être expliquées par un épisode thermique secondaire de HT-BP (∼300–650°C) en conditions oxydantes (∼NNO). De plus, l’analyse de profils de diffusions dans les chondres des CKs indique des durées de métamorphisme intermédiaires à celles communément invoquées pour du choc (de quelques secondes à quelques jours) et pour la désintégration d’éléments à courte durée de vie (plusieurs millions d’années). Une série d’expériences réalisées en four 1 atmosphère avec contrôle de la fugacité d’oxygène nous a permis de reproduire les textures caractéristiques des CKs et d’obtenir une teneur en fer d’équilibre des olivines des CVs, valeur proche de celle mesurée dans les CKs. Cela semble donc confirmer que les CKs sont des CVs rééquilibrées. Par conséquent, la classification actuelle de ces chondrites en deux groupes distincts devrait être modifiée afin de rendre compte de l’existence de cette série métamorphique CV-CK continue. Nous proposons de considérer le chauffage radiatif comme cause possible du métamorphisme des CKs. Un modèle numérique nous a permis de confirmer que des météoroïdes carbonés avec des périhélies situés entre 0,07 et 0,15 UA peuvent être chauffés à des températures pouvant aller jusqu’à 780°C. Les tailles pré-atmosphériques estimées pour les CV-CK (de quelques centimètres à 2,5 mètres) sont compatibles avec ce type de processus. La fragmentation d’un corps parent homogène de type CV (possiblement l’astéroïde à l’origine de la famille d’Eos) pourrait former des météoroïdes qui, sous l’effet de phénomènes de résonances, seraient redirigés vers l’intérieur du Système Solaire et pourraient ainsi être métamorphisés par chauffage radiatif. Ce type de processus thermique secondaire n’étant efficace que pour de petits fragments d’astéroïdes, il ne doit pas être considéré comme un processus corps-parent stricto sensu. / Carbonaceous chondrites (CCs) are primitive objects accreted during the earliest stage of the Solar System formation. Mainly composed of chondrules, matrix and refractory inclusions, CCs recorded chemical, isotopic and mineralogical heterogeneities of the solar nebula. Unlike other chondrite classes, most CCs are primitive (petrologic types 1 to 3), i.e., they have not been affected by thermal parent-body processes. However, CK chondrites suffered an intense metamorphism (petrologic types 4 to 6). The CK group is characterized by recrystallized matrices, equilibrated olivines (∼Fa31), a high level of oxidation (Ni-rich olivines, metal/magnetite ratio close to zero), low contents of refractory inclusions, refractory lithophile abundances intermediate between CV and CO groups, and oxygen isotope compositions overlapping the CV and CO groups. CKs have been poorly studied until the 1990’s, in part due to the small number of classified samples (210 as of December 6th, 2011), and their small masses (median mass∼33.5g). Isotopic and major element compositions support a genetic link with CV3s. Since1990, recent discoveries of CKs, in particular of CK3s recovered by systematic Antarctic and Saharan collects, allow a detailed study of the CK metamorphic evolution, especially at the 3–4 transition. The objective of this study is the characterization of the conditions of metamorphism of CKs, and through analyses of several CK3–4 samples, the study of the CV-CK relationship. The detailed characterization of the metamorphic evolution of 19 CKs, including 5 CK3, confirms that observed differences between chondritic components in CVs and CKs (abundance, mineralogy, texture) can be explained by a secondary HT-BP thermal process (∼300–650°C) under oxidizing conditions (∼NNO). Moreover, durations of metamorphism obtained by the analysis of diffusion profiles in CK chondrules are intermediate between those commonly admitted for shock (few seconds to several days) and for short-lived radionuclides decay (several million years). An experimental study, using a 1-atmosphere furnace with controlled oxygen fugacity, provides additional arguments for the CV-CK relationship. We reproduced characteristic CK textures and obtained olivine iron contents of equilibrated CVs close to those measured in CKs. These experiments confirm that CKs can be considered as reequilibrated CVs. Thus, the current classification of CVs and CKs in two distinct groups should be modified in order to account for the existence of the CV-CK continuous metamorphic series from type 3 to 6. We propose to consider radiative heating as a possible cause of metamorphism for CKs. Numerical thermal modeling indicates that carbonaceous meteoroids with low perihelia (between 0.07 and 0.15 AU) can be heated at temperatures up to 780°C. Pre-atmospheric sizes estimated for CVs and CKs (from a few centimeters to 2.5 meters) support this thermal process. Fragmentation of an homogeneous CV-type parent body (possibly the parent asteroid at the origin of the Eos family) could be the source of meteoroids which, due to resonances, move toward the Sun and thus be metamorphosed by radiative heating. This secondary thermal process, affecting only small asteroid fragments, should not be considered as a parent-body process in the sense that it did not occur on the asteroid before its disruption.

Thesis. 1976. B.S.--Massachusetts Institute of Technology. Dept. of Mechanical Engineering. / Microfiche copy available in Archives and Engineering. / Includes bibliographical references. / by Joseph B. Hamilton. / B.S.

Les météoroïdes, issus de petits corps du système solaire produisent des phénomènes lumineux (flashs d'impact) lorsqu'ils percutent le sol de la Lune. Ces événements lumineux transitoires sont observables depuis le sol en utilisant des télescopes de taille moyenne (typiquement > 200 mm de diamètre) à l'aide des caméras CCD rapides de haute sensibilité. Dans cette thèse, nous décrivons la configuration instrumentale, la procédure d'observation et d'analyse qui a été implémentée à l'Observatoire Universitaire de l'Oukaimden pour l'observation régulière des flashs lunaires. Les premiers impacts lunaires observés et confirmés depuis un observatoire situé en Afrique et dans le monde arabe sont analysés dans ce manuscrit. Nous discutons les caractéristiques de cinq flashs et les paramètres physiques attribués aux impacteurs associés. Nous présentons une première estimation du flux d'impact à l'issue de cette phase de surveillance des impacts lunaires depuis nos observatoires. Nous présentons également le développement et le test d'une stratégie pour déterminer avec précision les coordonnées des impacts observés depuis la Terre. Cette précision de séléno-localisation pourra être utilisée au profit de futures missions sismologiques qui utiliseront les impacts météoritiques pour explorer l'intérieur lunaire. / Meteoroids, coming from small bodies of the solar system, produce luminous phenomena (impact flashes) when they strike the lunar surface. These transient light events are observable from the ground using medium-sized telescopes (typically> 200 mm in diameter) by using high-speed CCD cameras. In this thesis, we describe the instrumental configuration, observation and analysis procedure that was implemented at the Oukaimden Observatory for regular observation of lunar flashes. The first lunar impacts observed and confirmed from an observatory located in Africa and the Arab world are analyzed in this manuscript. We discuss the characteristics of five flashes and the physical parameters attributed to the associated impactors. We present an initial estimate of the impact flux from this first phase of monitoring lunar impacts from our observatories. We also present the development and testing of a strategy to accurately determine the coordinates of impacts observed from Earth. This precision of seleno-localization can be used for future seismological missions that will use meteoroid impacts to explore the lunar interior.

<p>This thesis concerns the fate of the meteoric smoke in the Middle Atmosphere, and its effect on ice phenomena such as noctilucent clouds (NLC) and polar stratospheric clouds (PSC). </p><p>The potential role of NLC as tracer for mesospheric processes and variability, and as a tool for monitoring this remote and inaccessible region, has generated substantial interest within the scientific community. The nucleation of ice in such a dry environment is not trivial. Supersaturation is considered too low for homogeneous nucleation. Hence, pre-existing condensation nuclei are deemed necessary, with smoke particles having long been considered the most likely candidate. Here we show that the atmospheric circulation transports meteoric smoke particles away from the polar region before they coagulate large enough to efficiently act as ice condensation nuclei. We also show that the charging of meteoric smoke, in combination with deviations from the mean thermal state, may solve this dilemma by significantly altering the ice nucleation properties of smoke. Thus, while it is highly questionable whether neutral smoke can provide sufficient amounts of condensation nuclei for ice formation at the polar summer mesopause, charged meteoric smoke proves to be a promising candidate to explain mesospheric ice phenomena as we observe them.</p><p> We further show that the bulk of the meteoric material is transported to the Arctic winter stratosphere, yielding significantly higher concentrations of meteoric smoke in the region of PSC nucleation than has previously been believed. Our new predictions of meteoric smoke in this region may thus shed new light on open questions relating to PSC nucleation.</p>

This thesis concerns the fate of the meteoric smoke in the Middle Atmosphere, and its effect on ice phenomena such as noctilucent clouds (NLC) and polar stratospheric clouds (PSC). The potential role of NLC as tracer for mesospheric processes and variability, and as a tool for monitoring this remote and inaccessible region, has generated substantial interest within the scientific community. The nucleation of ice in such a dry environment is not trivial. Supersaturation is considered too low for homogeneous nucleation. Hence, pre-existing condensation nuclei are deemed necessary, with smoke particles having long been considered the most likely candidate. Here we show that the atmospheric circulation transports meteoric smoke particles away from the polar region before they coagulate large enough to efficiently act as ice condensation nuclei. We also show that the charging of meteoric smoke, in combination with deviations from the mean thermal state, may solve this dilemma by significantly altering the ice nucleation properties of smoke. Thus, while it is highly questionable whether neutral smoke can provide sufficient amounts of condensation nuclei for ice formation at the polar summer mesopause, charged meteoric smoke proves to be a promising candidate to explain mesospheric ice phenomena as we observe them. We further show that the bulk of the meteoric material is transported to the Arctic winter stratosphere, yielding significantly higher concentrations of meteoric smoke in the region of PSC nucleation than has previously been believed. Our new predictions of meteoric smoke in this region may thus shed new light on open questions relating to PSC nucleation.

Linear, small deflection plate theory is used to study the stress at the contact axis and the deflection of an infinite plate caused by the impact of an axisymmetric cometary meteoroid. The analysis assumes that momentum exchange is the primary mechanism, that the time of exchange is instantaneous, and that the momentum of the meteoroid is negligible after impact. The stress at the origin is reduced to a single definite integral and the deflection to the Hankel inversion integral, both requiring definition of the particular projectile before further evaluation. A particular cometary meteoroid is mathematically represented in the analysis by its projected momentum per unit area onto the plate. The three specific shapes studies are the usual projectile shapes used in hypervelocity laboratories - cylinder, cone, and sphere - even though the analysis is not intended for the high-strength, high-density laboratory projectiles. Projectile comparisons based on equal mass, diameter, and total momentum indicate that frangible, low-strength cone projectiles cause significantly higher stresses and larger displacements of the plate at short times after impact than similar sphere and cylinder projectiles. / Master of Science

No description available.

A meteor observed with the naked eye is colloquially called a shooting star. The streak of light is generated by an extra-terrestrial particle, a meteoroid, entering the Earth’s atmosphere. The term meteor includes both luminosity detectable by optical means and ionization detectable by radar. The radar targets of meteor head echoes have the same motion as the meteoroids on their atmospheric flight and are relatively independent of aspect angle. They appear to be compact regions of plasma created at around 100 km altitude and have no appreciable duration. This thesis reviews the meteor head echo observations carried out with the tristatic 930 MHz EISCAT UHF radar system during four 24h runs between 2002 and 2005, and a 6h run in 2003 with the monostatic 224 MHz EISCAT VHF radar. It contains the first strong observational evidence of a submillimeter-sized meteoroid breaking apart into two distinct fragments. This discovery promises to be useful in the further understanding of the interaction processes of meteoroids with the Earth’s atmosphere and thus also the properties of interplanetary/interstellar dust. The tristatic capability of the EISCAT UHF system makes it a unique tool for investigating the physical properties of meteoroids and the meteor head echo scattering process. The thesis presents a method for determining the position of a compact radar target in the common volume of the antenna beams and demonstrates its applicability for meteor studies. The inferred positions of the meteor targets are used to estimate their velocities, decelerations, directions of arrival and radar cross sections (RCS) with unprecedented accuracy. The head echoes are detected at virtually all possible aspect angles all the way out to 130° from the meteoroid trajectory, limited by the antenna pointing directions. The RCS of individual meteors simultaneously observed with the three receivers are equal within the accuracy of the measurements with a very slight trend suggesting that the RCS decreases with increasing aspect angle. A statistical evaluation of the measurement technique shows that the determined Doppler velocity agrees with the target range rate. This demonstrates that no contribution from slipping plasma is detected and that the Doppler velocities are unbiased within the measurement accuracy. The velocities of the detected meteoroids are in the range of 19-70 km/s, but with very few detections at velocities below 30 km/s. The thesis compares observations with a numerical single-body ablation model, which simulates the physical processes during meteoroid flight through the atmosphere. The estimated meteoroid masses are in the range of 10-9 - 10-5.5 kg. / Meteorer är ljusfenomen på natthimlen som i vardagligt tal kallas fallande stjärnor. Ljusstrimmorna alstras av meteoroider, små partiklar på banor genom solsystemet, som kolliderar med jordens atmosfär. Förutom ljus genererar meteoroider regioner av joniserat plasma, som är detekterbara med radar. Meteoriska huvudekon tycks komma från kompakta radarmål på ungefär 100 km höjd och rör sig genom atmosfären med de infallande meteoroidernas hastighet. Huvudekons signalstyrka förefaller oberoende av vinkeln mellan radarmålens rörelseriktning och riktningen från vilken radiovågorna infaller och sprids. Avhandlingen sammanfattar huvudekoobservationer från fyra 24-timmarsmätningar mellan 2002 och 2005 med det trestatiska 930 MHz EISCAT UHF-radarsystemet och en 6-timmarsmätning under 2003 med den monostatiska 224 MHz EISCAT VHF-radarn. Avhandlingen innehåller den första observationella bekräftelsen på att en meteoroid av sub-millimeterstorlek faller sönder i två distinkta fragment i atmosfären. Upptäckten är betydelsefull för studier av meteoroiders växelverkansprocesser med atmosfären och interplanetärt/interstellärt stofts materialegenskaper. EISCAT UHF-systemet består av tre vitt åtskilda mottagarstationer, vilket gör det till ett unikt mätinstrument för studier av meteoroiders egenskaper och hur radiovågor sprids från de radarmål som ger upphov till huvudekon. Avhandlingen presenterar en metod med vilken ett radarmåls position kan bestämmas om det detekteras simultant med de tre mottagarna. Metoden används till att med hög noggrannhet beräkna meteorers radartvärsnitt samt meteoroiders hastighet och atmosfärsinbromsning. De detekterade huvudekona genereras av meteoroider med i princip alla av mätgeometrin tillåtna rörelseriktningar i förhållande till radarstrålen, ända ut till 130° från radiovågornas spridningsriktning. Enskilda meteorers radartvärsnitt är likvärdiga inom mätnoggrannheten i de tre mottagarstationernas dataserier, men en svag trend antyder att radartvärsnittet minskar med ökande vinkel mellan meteoroidernas rörelseriktning och spridningsriktningen. En statistisk utvärdering av mättekniken visar att den uppmätta dopplerhastigheten stämmer överens med radarmålens flygtidshastighet. Detta innebär att dopplerhastigheterna är väntevärdesriktiga och opåverkade av bidrag från det spår av plasma som meteoroiderna lämnar efter sig. De uppmätta hastigheterna är 19-70 km/s, men bara ett fåtal detekterade meteoroider är långsammare än 30 km/s. Meteoroidmassorna är uppskattade till 10-9 – 10-5.5 kg genom jämförelser av observationerna med simuleringar av meteoroiders färd genom atmosfären i en numerisk ablationsmodell.

This thesis presents results based on data collected with the 930 MHz EISCAT UHF radar system and three SKiYMet specular meteor radars. It describes in detail a method for meteoroid orbit calculation. The EISCAT UHF system comprises three identical 32 m parabolic antennae: one high-power transmitter/receiver and two remote receivers. Precise meteoroid deceleration and radar cross section are determined from 410 meteor head echoes simultaneously observed with all three receivers between 2002 and 2005, during four 24h runs at the summer/winter solstice and the vernal/autumnal equinox. The observations are used to calculate meteoroid orbits and estimate meteor visual magnitudes. None of the observed meteors appear to be of extrasolar or asteroidal origin; comets, particularly short period (&lt;200 years) ones, may be the dominant source for the particles observed. About 40% of the radiants are associated with the north apex sporadic meteor source and 58% of the orbits are retrograde. The geocentric velocity distribution is bimodal with a prograde population centred around 38 km/s and a retrograde population peaking at 59 km/s. The absolute visual magnitudes of meteors are estimated to be in the range of +9 to +5 using a single-object numerical ablation model. They are thus observable using intensified CCD cameras with telephoto lenses. The thesis also investigates diurnal meteor rate differences and sporadic meteor radiant distributions at different latitudes using specular meteor trail radar measurements from 68°N, from 55°N and from 8°S. The largest difference in amplitude of the diurnal flux variation is at equatorial latitudes, the lowest variation is found at high latitudes. The largest seasonal variation of the diurnal flux is observed with the high-latitude meteor radar. The investigations show a variation in the sources with both latitude and time of day. The EISCAT UHF system and the high-latitude meteor radar are located close to the Arctic Circle. Such a geographical position means that zenith points towards the North Ecliptic Pole (NEP) once every day all year round. This particular geometry allows the meteoroid influx from the north ecliptic hemisphere to be compared throughout the year as the ecliptic plane coincides with the local horizon. Considering only the hour when NEP is closest to zenith, the EISCAT UHF head echo rate is about a factor of three higher at summer solstice than during the other seasons, a finding which is consistent with the high-latitude meteor radar measurements. / Avhandlingens resultat är baserade på mätningar med den trestatiska EISCAT UHF-radarn och tre SKiYMet meteorradarsystem. En metod för meteoroidbanberäkning presenteras i detalj. EISCAT UHF-systemet består av tre identiska, 32 m stora parabolantenner: en högeffektssändare/mottagare och två fjärrstyrda mottagare. Under fyra 24-timmarsmätningar vid vår-/höstdagjämning och sommar-/vintersolstånd mellan 2002 och 2005 detekterades 410 meteoriska huvudekon simultant med alla tre mottagare. Dessa trestatiska meteorers atmosfärsinbromsning och radartvärsnitt har fastställts mycket noggrant och använts till att beräkna meteoroidernas banor samt uppskatta meteorernas luminositeter. Ingen av de observerade meteoroiderna verkar vara av interstellärt eller asteroidursprung. Deras troligaste ursprung är kometer, framför allt kortperiodskometer (&lt;200 år). Ungefär 40% av meteorradianterna kan associeras till norra apex, ett källområde för sporadiska meteorer, och totalt är 58% av partiklarnas banor retrograda. Meteoroidernas geocentriska hastighetsfördelning har två lokala maxima: ett för den prograda populationen vid 38 km/s och ett för den retrograda vid 59 km/s. Genom att anpassa datat till en numerisk ablationsmodell som simulerar meteoroidernas färd genom atmosfären har de detekterade meteorernas absoluta visuella magnituder uppskattats till mellan +9 och +5. Detta innebär att de är observerbara med bildförstärkta, teleskopiska CCD-kameror. Avhandlingen diskuterar även hur sporadiska meteorers dygns- och säsongsinflöde beror på geografisk latitud och meteorradianternas distribution på himmelssfären. Detta utreds med hjälp av spårekon detekterade under perioden 1999-2004 med tre meteorradarsystem på latituderna 68°N, 55°N och 8°S. Dygnsinflödet varierar mest på låga latituder och minst på höga. Ju högre latitud, desto mer förändras däremot dygnsinflödet över året. Avhandlingen visar att de dominerande källområdena varierar med säsong, över dygnet och med latitud. Både EISCAT UHF-systemet och meteorradarn på 68°N är belägna nära polcirkeln. Detta innebär att norra ekliptiska polen (NEP) är i zenit en gång per dygn, året om. Vid just denna tidpunkt sammanfaller ekliptikan med den lokala horisonten, vilket möjliggör att det observerade meteorinflödet från norra ekliptiska hemisfären kan jämföras över året. Under timmen då NEP är närmast zenit har EISCAT UHF uppmätt ett ungefär tre gånger högre meteorinflöde vid sommarsolståndet än under de andra säsongerna, vilket överensstämmer med resultaten från meteorradarn på 68°N.