41 research outputs found
Atmospheric entry of sub-millimetre-sized grains into Mars atmosphere: white soft mineral micrometeoroids
No full textIn this work, we study the passage through the Martian atmosphere of micrometeorites with a white soft mineral (WSM) composition, which have been proposed as transporters of organic molecules in the solar system. The atmospheric entry model includes the dynamics of the atmospheric entry and the physico-chemical aspects of the thermal decomposition process. The results show that, due to the reduced entry speed, Mars may have been a promising collector of matter in this form. In particular, the chemical decomposition process is much more effective than in the case of the Earth's atmosphere in maintaining a moderate temperature of the micrometeorite during most of the entry process
Electric field induced dissociation of a confined hydrogen molecule
No full textThe effect of a static electric field ionization on the neutral hydrogen molecule H2 confined in a spherical potential
well is studied, as a simple model for the chemical activation of molecular species in a medium. Quantum
diffusion Monte Carlo is employed with complete account of electron correlation. Field-induced ionization and
dissociation are discussed, for different values of the confinement radius and electric field strength. This study
allows to highlight the mechanism of electric field initiation of chemical reactions in fluids at different pressures,
without the details of a specific chemical environment
Theoretical analysis of the atmospheric entry of sub-mm meteoroids of MgxCa1−xCO3 composition
No full textCurrent models allow to reliably simulate mechanical and thermal phenomena associated with a mi- crometeor passage through the Earth’s atmosphere. However, these models have rarely been applied to materials other than those most common in meteorites, such as silicates and metals. A particular case that deserves attention is the one of micrograins made of minerals, in particular carbonates, which have been associated, in meteorites, with organic molecules. Carbonates are known for their decomposition in vacuum at moderate temperatures, and they might contribute to the thermal protection of organic mat- ter. In this work, a model with non isothermal atmosphere, power balance, evaporation, ablation, radia- tion losses and stoichiometry, is proposed. This paper includes the very first calculations for meteoroids with a mixed carbonate composition. Results show that the carbonate fraction of these objects always go to zero at high altitudes except for grazing entries, where the reached temperature is lower and some carbonate remains unreacted. For all entry conditions, peculiar temperature curves are obtained due to the decomposition process. Furthermore, a significant impact of decomposition cooling on the tempera- ture peak is observed for some grazing entry cases. Although specific solutions used in these calculations can be improved, this work sets a definite model and a basis for future research on sub-mm grains of relatively volatile minerals entering the Earth’s atmosphere
Quantum states and static field ionization of a cylindrical confined hydrogen atom: A diffusion Monte Carlo study
No full textIn this article, the diffusion Monte Carlo (DMC) method is applied to the study of the quantum states of a hydrogen atom confined into a cylindrical potential well. We present an independent reproduction of previous studies based on different methods, in particular the energy eigenvalues for ground and selected excited states and the polarizability of the ground state, both for finite and infinite cylinders. The static field ionization of ground and excited states of the confined atom is discussed, including the determination of the potential energy surface and equilibrium position of the proton. This study provides a further demonstration of the versatility of the DMC method for this and analogous problems.The quantum diffusion Monte Carlo method is applied to a hydrogen atom confined in a cylindrical potential well. Ionization process is studied for ground and excited states. The potential energy surfaces of the nucleus feature interesting characteristics as the electric filed increases. imag
Thermal decomposition of MgCO3 during the atmospheric entry of micrometeoroids
No full textIn this paper, a first study of the atmospheric entry of carbonate micrometeoroids, in an astrobiological perspective, is performed. Therefore an entry model, which includes two-dimensional dynamics, non-isothermal atmosphere, ablation and radiation losses, is build and benchmarked to literature data for silicate micrometeoroids. A thermal decomposition model of initially pure magnesium carbonate is proposed, and it includes thermal energy, mass loss and the effect of changing composition as the carbonate grain is gradually converted into oxide. Several scenarios are obtained by changing the initial speed, entry angle and grain diameter, producing a systematic comparison of silicate and carbonate grain. The results of the composite model show that the thermal behaviour of magnesium carbonate is markedly different from that of the corresponding silicate, much lower equilibration temperatures being reached in the first stages of the entry. At the same time, the model shows that the limit of a thermal protection scenario, based on magnesium carbonate, is the very high decomposition speed even at moderate temperatures, which results in the total loss of carbon already at about 100 km altitude. The present results show that, although decomposition and associated cooling are important effects in the entry process of carbonate grains, the specific scenario of pure MgCO3 micrograin does not allow complex organic matter delivery to the lower atmosphere. This suggests us to consider less volatile carbonates for further studies
Diffusion Monte Carlo calculations of the polarizability of a confined hydrogen atom: benchmarking and application to high symmetry wells
No full textWe present a non-perturbative direct method to calculate the polarizability of a hydrogen atom confined in a three-dimensional potential well of any geometry. The calculation is based on the diffusion Monte Carlo method. The advantage of the method is simplicity of implementation and immediate adaptability to any well shape. The method is validated for the well-studied spherically confined hydrogen atom, and demonstrated in the case of two other geometries (cube and octahedron), for which this paper provides the first set of results. Although demonstrated here for the confined hydrogen atom, the method can be immediately applied to any single-electron system placed in a three-dimensional potential well of any type and geometry. Results for a hydrogen atom confined in potential wells of cubic or spherical symmetry suggests that the polarizability in these cases is a universal function of the volume of the well. This result can simplify calculations for real situations such as in quantum dots
Micrometeoroids as Carriers of Organics: Modeling of the Atmospheric Entry and Chemical Decomposition of Sub‐Millimeter Grains
No full textOne of the most exciting perspectives in astrochemistry lies in the part played by molecules in space, since they might have a crucial role in the Earth’s chemical evolution and origin of life. Life-related molecules may reach the Earth’s surface embedded in solid particles: the mineral composition of these grains may provide a thermal protection against the high temperatures during the atmospheric entry process. While evaluating several mineral phases, the most interesting candidates are those present on the surface of several bodies of our Solar System and have an association with organics on Earth: carbonates, mainly of magnesium and calcium, and calcium sulfates. This chapter reviews recent studies performed using computer models: they include the dynamics of the atmospheric entry, the kinetics of the chemical reactions involved, and heat transfer processes. Results demonstrate that the thermal decomposition reaction of the materials considered provides a check of their feasibility as organics carriers and partially mitigates the heating in the first stage of the entry process. Another important aspect is the primordial atmospheric composition: the actual nature of the main atmospheric molecules affects significantly the grain heating, showing that this overlooked feature of meteoroid entry models may play an important role
Computer simulations of biotic chiral selection scenarios
No full textThe biotic scenario of the selection of biological homochirality is one of the most interesting applications of computer modelling to astrobiology. These scenarios have been studied for more than 70 years, yet there are plenty of studies to better assess them, in particular in the development of models of the selective extinction process. In this paper, we review former studies performed by biology-grounded models of this process and present a new class of computer programs: they further demonstrate the complexity of the selective extinction dynamics and the role played into it by non-trivial chemical-physical concepts. Indeed, the results display large and persistent differences between the populations of the two different chiral types, made possible by the freedom of individual populations to fluctuate wildly while the total population is stabilized by the limited availability of chemical energy. Such strong differences ultimately lead to the selective extinction of one of the two types. This way, computer simulations provide increasing evidence in favour of the biotic scenario
Quantum states of confined hydrogen plasma species: Monte Carlo calculations
No full textThe diffusion Monte Carlo method with symmetry-based state selection is used to calculate the quantum energy states of H2+ confined into potential barriers of atomic dimensions (a model for these ions in solids). Special solutions are employed, permitting one to obtain satisfactory results with rather simple native code. As a test case, 2πu and 2Πg states of H2+ ions under spherical confinement are considered. The results are interpreted using the correlation of H2+ states to atomic orbitals of H atoms lying on the confining surface and perturbation calculations. The method is straightforwardly applied to cavities of any shape and different hydrogen plasma species (at least one-electron ones, including H) for future studies with real crystal symmetries
Static field ionization of the spherically confined hydrogen atom
No full textThe ionization of the hydrogen atom confined in a spherical potential well and subjected to a static electric field is studied, using the diffusion Monte Carlo (DMC) method. Atomic ionization within a potential well is found to be a stationary, gradual, and reversible process. The value of the electric field at the onset of ionization is of the order of 0.1 atomic units, and depends on the symmetry of the atomic wave function and on the confinement dimension. By decreasing the confinement sphere, the difference between the bound and ionized states disappears, showing that strict confinement leads to pressure ionization of the atom. The off-center case is studied characterizing the potential energy surface (PES), and the transition between field-induced and pressure-induced ionization is confirmed. Except for very weak fields, the minimum of the PES is reached when the proton is in contact with the boundary of the well
