87 research outputs found
Thermal evolution and sintering of chondritic planetesimals IV. Temperature dependence of heat conductivity of asteroids and meteorites
Understanding the compaction and differentiation of the planetesimals and
protoplanets from the Asteroid Belt and the terrestrial planet region of the
Solar System requires a reliable modeling of their internal thermal evolution.
An important ingredient for this is a detailed knowledge of the heat
conductivity of the chondritic mixture of minerals and metal in planetesimals.
The temperature dependence of the heat conductivity is evaluated here from the
properties of its mixture components by a theoretical model. This allows to
predict the temperature dependent heat conductivity for the full range of
observed meteoritic compositions and also for possible other compositions. For
this purpose, published results on the temperature dependence of heat
conductivity of the mineral components found in chondritic material are fitted
to the model of Callaway for heat conductivity in solids by phonons. For the
Ni,Fe-alloy published laboratory data are used. The heat conductivity of
chondritic material then is calculated by means of mixing-rules. The role of
micro-cracks is studied which increase the importance of wall-scattering for
phonon-based heat conductivity. The model is applied to published data on heat
conductivity of individual chondrites. The experimental data for the dependence
of the heat conductivity on temperature can be reproduced rather well by the
model if the heat conductivity is calculated for the composition of the
meteorites. It is found that micro-cracks have a significant impact on the
temperature dependence of the heat conductivity because of their reduction of
phonon scattering length.Comment: 18 pages, 7 figures, accepted by Astronomy & Astrophysic
Thermal history modeling of the L chondrite parent body
The radius of the L chondrite parent body, its formation time, and its
evolution history are determined by fitting theoretical models to empirical
data of radioisotopic chronometers for L chondrites. A simplified evolution
model for the L chondrite parent body is constructed considering sintering of
the initially porous material, temperature dependent heat conductivity, and an
insulating regolith layer. Such models are fitted to thermochronological data
of five meteorites for which precise data for the Hf-W and U-Pb-Pb
thermochronometers have been published. A set of parameters for the L chondrite
parent body is found that yields excellent agreement (within error bounds)
between a thermal evolution model and thermochonological data. Empirical
cooling rate data also agree with the model results within error bounds such
that there is no conflict between cooling rate data and the onion-shell model.
Two models are found to be compatible with the presently available empirical
data: One model with a radius of 115 km and a formation time of 1.89 Ma after
CAI formation, another model with 160 km radius and formation time of 1.835 Ma.
The central temperature of the smaller body remains well below the Ni,Fe-FeS
eutectic melting temperature and is consistent with the apparent non-existence
of primitive achondrites related to the L chondrites. For the bigger model
incipient melting in the central core region is predicted which opens the
possibility that primitive achondrites related to L chondrites could be found.Comment: 22 pages, 11 figures, accepted by Astronomy & Astrophysic
Organic matter in interstellar dust lost at the approach to the heliosphere: Exothermic chemical reactions of free radicals ignited by the Sun
Aims. We tackle the conundrums of organic materials missing from interstellar
dust when measured inside the Solar System, while undoubtedly existing in the
local interstellar cloud (LIC), which surrounds the Solar System.
Methods. We present a theoretical argument that organic compounds sublimate
almost instantaneously by exothermic reactions, when solar insolation triggers
the recombination of free radicals or the rearrangement of carbon bonds in the
compounds.
Results. It turns out that the triggering temperature lies in the range of
2050 K by considering that sublimation of organic materials takes place
beyond the so-called filtration region of interstellar neutral atoms. We find
that in-situ measurements of LIC dust in the Solar System result in an
overestimate for the gas-to-dust mass ratio of the LIC, unless the sublimation
of organic materials is taken into account. We also find that previous
measurements of interstellar pickup ions have determined the total elemental
abundances of gas and organic materials, instead of interstellar gas alone.
Conclusions. We conclude that LIC organic matter suffers from sublimation en
route to the heliosphere, implying that our understanding of LIC dust from
space missions is incomplete. Since space missions inside the orbit of Saturn
cannot give any information on the organic substances of LIC dust, one must
await a future exploration mission to the inner edge of the Oort cloud for a
thorough understanding of organic substances in the LIC. Once our model for the
sublimation of interstellar organic matter by exothermic chemical reactions of
free radicals is confirmed, the hypothesis of panspermia from the diffuse
interstellar medium is ruled out.Comment: 9 pages, 6 figures, to appear in Astronomy & Astrophysic
Результаты теоретических и опытных работ по изучению механизма работы буровых компоновок со смещенным центром масс поперечного сечения
Актуальность работы: необходимость повышения эффективности бурения геологоразведочных скважин в сложных горно-геологических условиях, в том числе связанных с естественным искривлением скважин. Цель исследования: разработка методики использования и конструкций буровых компоновок со смещенным центром масс (тяжести) поперечного сечения, позволяющих повысить эффективность бурения геологоразведочных скважин. Методы исследования: аналитические исследования, опытно-конструкторские работы и экспериментальные опытные работы. Результаты. Разработаны теоретические положения, методика применения и конструкции буровых компоновок со смещенным центром тяжести поперечного сечения; проведены производственные испытания различных конструкций буровых компоновок со смещенным центром тяжести поперечного сечения при различных способах бурения. Выводы. На основе разработанной модели движения буровых компоновок со смещенным центром тяжести поперечного сечения получены аналитические зависимости для расчета величин эксцентриситета поперечного сечения буровых компоновок, обеспечивающие их вращение вокруг оси скважины (вид Ф1), а также длину вовлекаемого в режим вращения Ф1 участка колонны, что позволяет создавать компоновки бурильной колонны, способные работать в более благоприятном режиме и обеспечивать повышение эффективности бурового процесса. На основе стандартных снарядов со съемным керноприемником типоразмера HQ разработаны и изготовлены трубы со смещенным центром тяжести, которые испытаны на производственных скважинах в составе компоновки, в которой размещено три трубы со смещенным центром тяжести поперечного сечения. Результаты испытаний показали, что в составе высокосбалансированных бурильных колонн снаряда со съемным керноприемником эффективно применение труб со смещенным центром тяжести: достигается снижение интенсивности естественного искривления скважин, снижается вибрация и затраты мощности на работу бурильной колонны.Relevance of the research is the necessity to increase the efficiency of drilling prospecting wells in difficult mining-and-geological conditions, including those connected with a natural curvature of wells. The aim of the research is to develop a technique of using and designs of boring configurations with the displaced cross section mass center which allow increasing the efficiency of drilling the prospecting wells. Research methods: analytical researches, developmental works and experimental skilled works. Results. The authors have developed the theoretical regulations, a technique of application and a design of boring configurations with the displaced cross section mass center and carried out the production tests of various designs of boring configurations of with the displaced cross section mass center when drilling. Conclusions. Based on the developed model of movement of boring configurations with the displaced center of gravity of cross section the authors obtained the analytical dependences for calculating the sizes of eccentricity of boring configuration cross section providing their rotation round a well axis (Ф1 type), as well as the length of the column part involved in the rotation mode Ф1 that allows developing the configurations of a boring column capable of operating in more favorable mode and providing the increase of boring efficiency. Based on standard shells with the removable core receiver of a standard size of HQ the pipes with the displaced cross section mass center were developed and produced. They were tested on production wells as a part of configuration in which three pipes with the displaced cross section mass center were placed. The results of the tests showed that it is efficient to apply the pipes with the displaced cross section mass center as a part of the high-balanced boring columns as the decrease in intensity of natural curvature of wells is reached, vibration and costs of power for boring column operation decrease
Eastern Mediterranean volcanism during Marine Isotope Stages 9 to 7e (335–235 ka): insights based on cryptotephra layers at Tenaghi Philippon, Greece
Thermal evolution and sintering of chondritic planetesimals III. Modelling the heat conductivity of porous chondrite material
The construction of models for the internal constitution and the temporal
evolution of large planetesimals, the parent bodies of chondrites, requires
information on the heat conductivity of the complex mixture of minerals and
iron metal found in chondrites. It is attempted to evaluate the heat
conductivity of a multi-component mineral mixture and granular medium from the
heat conductivities of its mixture components. Random mixtures of solids with
chondritic composition and packings of spheres are numerically generated. The
heat conduction equation is solved in high spatial resolution for a test cube
filled with such matter. From the heat flux through the cube the heat
conductivity of the mixture is derived. The model results for porous material
are consistent with data for compacted sandstone, but are at odds with
measurements for H and L chondrites. The discrepancy is traced back to shock
modification of the currently available meteoritic material by impacts on the
parent body over the last 4.5 Ga. This causes numerous micro-cracks that act as
additional barriers for heat transfer. The void structure in meteorites is
different from that which probably existed in the pristine material of the
parent bodies. The results obtained for the heat conductivity of the pristine
material are used for calculating models for the evolution of the H chondrite
parent body which are fitted to the cooling data of a number of H chondrites.
The fit to the data good.Comment: 19 pages, 8 figures, accepted by Astronomy & Astrophysic
The lunar Dhofar 1436 meteorite: <sup>40</sup>Ar‐ <sup>39</sup>Ar chronology and volatiles, revealed by stepwise combustion and crushing methods
The lunar meteorite Dhofar 1436 is dominated by solar wind type noble gases. Solar argon is equilibrated with “parentless” 40Ar commonly known as lunar orphan argon. Ar‐Ar isochron analyses determined the lunar trapped 40Ar/36Ar ratio to 2.51 ± 0.04, yielding a corrected plateau age of 4.1 ± 0.1 Ga, consistent with the lunar Late Heavy Bombardment period. Lunar trapped and radiogenic argon components are all released at high temperatures (1200–1400 °C). Surprisingly, solar noble gases and lunar trapped argon can largely be released by crushing. Initial crushing steps mainly release elementally fractionated solar wind gases, while in advanced crushing steps, cosmogenic components dominate. Cosmogenic noble gases indicate irradiation at the lunar surface; they are less fractionated than solar wind species. We favor a scenario in which both solar and a large fraction of cosmogenic gases were acquired before the 4.1 Ga event, which caused shock metamorphism and formation of the regolith breccia. Sintering and agglutination along grain boundaries resulted in mobilization of solar wind, reimplanted, radiogenic, and cosmogenic noble gases, and resulted in their partial homogenization, fractionation, and retrapping in voids and/or defects accessible by crushing. An alternative scenario would be complete reset of the K‐Ar system 4.1 Ga ago accompanied by loss of all previously accumulated solar and cosmogenic noble gases. Later, the precursor of Dhofar 1436 became lunar regolith and accumulated solar and cosmogenic noble gases and reimplanted 40Ar before its final formation of the polymict impact breccia. The C abundance of the step‐combusted Dhofar 1436 is 555.3 ppm, with δ13C of −28‰ to +11‰. Nitrogen contents released by crushing and combustion are 3.2 ppm and 20.8 ppm, respectively. The lightest nitrogen composition (δ15N = −79‰) is likely due to release from voids of shock metamorphic phases and is rather a result of the mobilization of nitrogen components that accumulated prior to the 4.1 Ga event
Photophoretic separation of metals and silicates: the formation of Mercury like planets and metal depletion in chondrites
Mercury's high uncompressed mass density suggests that the planet is largely
composed of iron, either bound within metal (mainly Fe-Ni), or iron sulfide.
Recent results from the MESSENGER mission to Mercury imply a low temperature
history of the planet which questions the standard formation models of impact
mantle stripping or evaporation to explain the high metal content. Like
Mercury, the two smallest extrasolar rocky planets with mass and size
determination, CoRoT-7b and Kepler-10b, were found to be of high density. As
they orbit close to their host stars this indicates that iron rich inner
planets might not be a nuisance of the solar system but be part of a general
scheme of planet formation. From undifferentiated chondrites it is also known
that the metal to silicate ratio is highly variable which must be ascribed to
pre-planetary fractionation processes. Due to this fractionation most
chondritic parent bodies - most of them originated in the asteroid belt - are
depleted in iron relative to average solar system abundances. The astrophysical
processes leading to metal silicate fractionation in the solar nebula are
essentially unknown. Here, we consider photophoretic forces. As these forces
particularly act on irradiated solids, they might play a significant role for
the composition of planetesimals forming at the inner edge of protoplanetary
discs. Photophoresis can separate high thermal conductivity materials (iron)
from lower thermal conductivity solids (silicate). We suggest that the
silicates are preferentially pushed into the optical thick disk. Subsequent
planetesimal formation at the edge moving outwards leads to metal rich
planetesimals close to the star and metal depleted planetesimals further out in
the nebula
COSIMA-Rosetta calibration for in-situ characterization of 67P/Churyumov-Gerasimenko cometary inorganic compounds
20 pages, 3 figures, 5 tablesInternational audienceCOSIMA (COmetary Secondary Ion Mass Analyser) is a time-of-flight secondary ion mass spectrometer (TOF-SIMS) on board the Rosetta space mission. COSIMA has been designed to measure the composition of cometary dust grains. It has a mass resolution m/{\Delta}m of 1400 at mass 100 u, thus enabling the discrimination of inorganic mass peaks from organic ones in the mass spectra. We have evaluated the identification capabilities of the reference model of COSIMA for inorganic compounds using a suite of terrestrial minerals that are relevant for cometary science. Ground calibration demonstrated that the performances of the flight model were similar to that of the reference model. The list of minerals used in this study was chosen based on the mineralogy of meteorites, interplanetary dust particles and Stardust samples. It contains anhydrous and hydrous ferromagnesian silicates, refractory silicates and oxides (present in meteoritic Ca-Al-rich inclusions), carbonates, and Fe-Ni sulfides. From the analyses of these minerals, we have calculated relative sensitivity factors for a suite of major and minor elements in order to provide a basis for element quantification for the possible identification of major mineral classes present in the cometary grains
- …