13 research outputs found
Spherically confined H2+: 2σ +g and 2σ +u states
No full textThe molecular ion H2+ is studied under strong confinement conditions produced by a spherical barrier centered in the gravity center of the molecule. Results for the potential curves are obtained by diffusion Monte Carlo methods for the ground state (X) and the first excited state (A), and reported as functions of the internuclear distance d for different values of the confinement radius. Results show that the compressed states corresponding to both and present deep minima in their potential curves, due to the increased space for electron wave-functions when the protons are displaced from the barrier surface
The role of primordial atmosphere composition in organic matter delivery to early Earth
No full textA model of the atmospheric entry of sub-mm grains is employed to evaluate the effect of the chemical composition of the primordial Earth’s atmosphere on the grain heating, in the context of organic matter delivery. Calculations are performed with spherical, uniform grains of forsterite/fayalite composition as well with the recently proposed white soft mineral (WSM) grains. Different hypotheses on primordial atmosphere composition affect the scale height and the energy transfer. The present work shows that: the total gas budget of the atmosphere is not highly relevant as far as the determination of the heating associated with slowing to subsonic speed is concerned; accordingly, light components (which are expected to be present in a primordial atmosphere and more abundant in the upper one) may be the primary ones in the evaluation of momentum and heat transfer in such scenarios. Strong reduced heating is obtained in the case of an upper atmosphere rich in light components, showing that the composition of the primordial Earth atmosphere may represent the key issue in the delivery of thermolabile organic matter enclosed in sub-mm extraterrestrial grains
Confined H(1s) and H(2p) under different geometries
No full textIn this paper the diffusion Monte Carlo method is applied to the confined hydrogen atom with different confinement geometries. This approach is validated using the much studied spherical and cylindrical confinements and then applied to cubical and squared ones, for which data are not available, as new applications of the method relevant to solid state physics. The energy eigenvalues of the ground state and one low-lying excited state are reported as a function of the characteristic confinement length
Monte Carlo calculation of the potential energy surface for octahedral confined H 2+
No full textA rich literature has been produced on the quantum states of atoms and molecules confined into infinite potential wells with a specified symmetry. Apart from their interest as basic quantum systems, confined atoms and molecules are useful models for extreme high-pressure states of matter, spectroscopically active defects in solid lattices, and chemical species in molecular cages. A most important case is that of H2+ for which little or no results are available in the case of polyhedral confinement. The authors’ approach makes use of the Diffusion Monte Carlo (DMC) method. The advantage of this method is that previously developed codes are readily adapted to new, even complex, well geometries, and nuclear positions. In this paper, the potential energy surface (PES) of H2+ confined inside an octahedral well is reported for restricted D4h and D3d geometries and different well widths. The results are discussed using the concept of electron compression and the correlation with semi-confined atomic orbitals
Evaluation of CaSO4 micrograins in the context of organic matter delivery: Thermochemistry and atmospheric entry
No full textIn this paper, anhydrous calcium sulphate CaSO4 (anhydrite) is considered as a carrier material for organic matter delivery from Space to Earth. Its capability of incorporating important fractions of water, leading to different species like bassanite and gypsum, as well as organic molecules; its discovery on Mars surface and in meteorites; the capability to dissipate much energy by its chemical decomposition into solid (CaO) and gaseous (SO3) oxide, make anhydrite a very promising material in an astrobiological perspective. Since chemical cooling has been recently considered by some of the present authors for the case of Ca/Mg carbonates, CaSO4 can be placed into a class of 'white soft minerals' (WSM) of astrobiological interest. In this context, CaSO4 is evaluated here by using the atmospheric entry model previously developed for carbonates. The model includes grain dynamics, thermochemistry, stoichiometry, radiation and evaporation heat losses. Results are discussed in comparison with MgCO3 and CaCO3 and show that sub-mm anhydrite grains are potentially effective organic matter carriers. A Monte Carlo simulation is used to provide distributions of the sulphate fraction as a function of altitude. Two-zone model results are presented to support the isothermal grain hypothesis
Thermal decomposition rate of MgCO3 as an inorganic astrobiological matrix in meteorites
No full textCarbonate minerals, likely of hydrothermal origins and included into orthopyroxenite, have been extensively studied in the ALH84001 meteorite. In this meteorite, nanocrystals comparable with those produced by magnetotactic bacteria have been found into a carbonate matrix. This leads naturally to a discussion of the role of such carbonates in panspermia theories. In this context, the present work sets the basis of a criterion to evaluate whether a carbonate matrix in a meteor entering a planetary atmosphere would be able to reach the surface. As a preliminary step, the composition of carbonate minerals in the ALH84001 meteorite is reviewed; in view of the predominance of Mg in these carbonates, pure magnesite (MgCO3) is proposed as a mineral model. This mineral is much more sensitive to high temperatures reached during an entry process, compared with silicates, due to facile decomposition into MgO and gaseous carbon dioxide (CO2). A most important quantity for further studies is therefore the decomposition rate expressed as CO2 evaporation rate J (molecules/m2 s). An analytical expression for J(T) is given using the Langmuir law, based on CO2 pressure in equilibrium with MgCO3 and MgO at the surface temperature T. Results suggest that carbonate minerals rich in magnesium may offer much better thermal protection to embedded biological matter than silicates and significantly better than limestone, which was considered in previous studies, in view of the heat absorbed by their decomposition even at moderate temperatures. This first study can be extended in the future to account for more complex compositions, including Fe and Ca.I minerali carbonatici, probabilmente di origine idrotermale e inclusi nell'ortopropossenite, sono stati ampiamente studiati nel meteorite ALH84001. In questo meteorite, i nanocristalli paragonabili a quelli prodotti dai batteri magnetotattici sono stati trovati in una matrice carbonatica. Ciò porta naturalmente a una discussione sul ruolo di tali carbonati nelle teorie della panspermia. In questo contesto, il presente lavoro pone le basi di un criterio per valutare se una matrice carbonatica in una meteora che entra in un'atmosfera planetaria sarebbe in grado di raggiungere la superficie. Come fase preliminare, viene rivista la composizione dei minerali di carbonato nel meteorite ALH84001; in considerazione della predominanza di Mg in questi carbonati, la magnesite pura (MgCO3) viene proposta come modello minerale. Questo minerale è molto più sensibile alle alte temperature raggiunte durante un processo di entrata, rispetto ai silicati, a causa della facile decomposizione in MgO e anidride carbonica gassosa (CO2). Una quantità molto importante per ulteriori studi è quindi il tasso di decomposizione espresso come tasso di evaporazione della CO2 J (molecole / m2 s). Un'espressione analitica per J (T) è data usando la legge di Langmuir, basata sulla pressione di CO2 in equilibrio con MgCO3 e MgO alla temperatura superficiale T. I risultati suggeriscono che i minerali di carbonato ricchi di magnesio possono offrire una protezione termica molto migliore alla materia biologica incorporata rispetto a silicati e significativamente migliori del calcare, che era stato considerato in studi precedenti, in considerazione del calore assorbito dalla loro decomposizione anche a temperature moderate. Questo primo studio può essere esteso in futuro per tenere conto di composizioni più complesse, tra cui Fe e Ca
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
Computational Astrobiology in Bari University and ISTP-CNR
No full textIn this contribution, we will present some results from our latest studies in the field of computational astrobiology, which are devoted to improving our understanding of the origin and evolution of life in the Universe by means of theoretical and computational models. The phenomena that can be studied by means of numerical modeling range from the micro to the macro scale, from molecules to cosmic grains and communities of primordial organisms. These studies may help to develop a global view of the phenomena involved in the origin of life in the wider context of astrobiology
Kinetics of white soft minerals (WSMs) decomposition under conditions of interest for astrobiology: A theoretical and experimental study
No full textIn this paper, the thermal decomposition kinetics of a class of minerals that we call White Soft Minerals (WSMs) is studied by means of theoretical and experimental methods, in connection to the transport of extraterrestrial organic matter to Earth and the possible use of the decomposition reaction in the characterization of these minerals in space. WSMs include, under a single denomination, carbonates and sulphates of Mg, Fe, and Ca. To improve the present knowledge of the properties of such materials, we use the following techniques: kinetic models for chemical decomposition, atmospheric entry models, spectroscopy, and gravimetric analyses. Model results show that the atmospheric entry of WSM grains is strongly affected by their thermal decomposition. The decomposition reaction, being strongly endothermic, tends to significantly lower the grain temperature during the atmospheric entry, especially at high altitudes and for grazing entries. A previously proposed infrared spectroscopic technique to evaluate the degree of advancement of the reaction is found to be in good agreement with gravimetric measurements for calcium carbonate. The numerical model developed for the atmospheric entry scenarios is used to interpret experimental results. These main findings show that an additional contribution to the reaction enthalpy is needed to reproduce the experimental results, suggesting that the present theoretical model needs improvements such as the account of gas diffusion in the materials
Plasma Modeling and Prebiotic Chemistry: A Review of the State-of-the-Art and Perspectives
No full textWe review the recent progress in the modeling of plasmas or ionized gases, with compositions compatible with that of primordial atmospheres. The plasma kinetics involves elementary processes by which free electrons ultimately activate weakly reactive molecules, such as carbon dioxide or methane, thereby potentially starting prebiotic reaction chains. These processes include electron-molecule reactions and energy exchanges between molecules. They are basic processes, for example, in the famous Miller-Urey experiment, and become relevant in any prebiotic scenario where the primordial atmosphere is significantly ionized by electrical activity, photoionization or meteor phenomena. The kinetics of plasma displays remarkable complexity due to the non-equilibrium features of the energy distributions involved. In particular, we argue that two concepts developed by the plasma modeling community, the electron velocity distribution function and the vibrational distribution function, may unlock much new information and provide insight into prebiotic processes initiated by electron-molecule collisions
