Thermodynamic and transport properties of reacting air including ablated species

The entry of meteoroids into the Earth's atmosphere at high speeds produces a bow shock wave and the high temperatures in the shock layer induce an intense heat flux that melts and vaporizes the body. The shock is then structured in two regions: the 'ablation layer' close to the meteoroid surface and constituted by a vapor in equilibrium with the liquid film at the meteoroid surface, and the air shock layer, separated by an interface, whose thickness depends on the meteoroid's dimensions and its entry conditions (velocity and altitude). Across the layer the temperature changes from around 3000 K at the surface of the body to about 20 000 K at the interface [1], reaching very high temperatures at the shock front. Any chemical model of meteoric ablation [1-3] should accurately characterize the ablation layer and the interface, deriving the equilibrium composition, the thermodynamic properties and also the transport coefficients for the estimation of the flow characteristics during hypersonic entry, such as friction and surface heat load. The model should describe the transition between the vapor layer, the composition reproducing the elemental fractions characteristic of the meteoroid, and the interface region, where the complexity of the chemistry increases due to the mixing with air components and the properties of the resulting plasma depend on the fraction of the ablated species in the mixture. The chemical and mineralogical nature of the meteorites (chapter 5) is the basis of their classification and indicates that for chondrites (stony meteorites) the most abundant phases are silicates, producing a differential ablation profile (see figure 4.2 of chapter 4) that shows the dominant ablation of Si, Fe and Mg at an altitude of around 90 km. In this chapter recent efforts to derive accurate thermodynamic and transport properties of silicon compounds, SiO2 or SiC, regarded as models for chondriticmeteorites, are reported and the role of ablated silicon species in affecting the properties of air is also investigated, allowing a description of the interface region. The properties are calculated in a wide range of temperatures [3 × 103-5 × 104 K], i.e. using as lower limit the temperature at the melting surface of the meteoroid body. Advanced chemical models are considered, including molecular species such as C3, O3, Si2, Si3, Si2N, SiN, NO2, ..., potentially minority species but in some cases important at low temperatures, and also molecular positive and negative ions. The multiply charged atomic ions are included up to the fourth ionization level to ensure the soundness of results for high temperatures, where the plasma is fully ionized. The calculations are performed with the web-access EquilTheTA tool [4] and core databases, accessed by thermodynamic and transport computational modules, collecting physical-chemical data and transport cross sections for atomic and molecular species. These databases have been extended to include accurate internal partition functions of atomic and molecular silicon-based species and binary collision dynamical information for interactions involving silicon-carbon, silicon- oxygen and silicon-nitrogen compounds. The thermodynamic and transport properties of plasmas containing silicon-based chemical components represent fundamental information, not only for the simulation of meteoroid thermal ablation during atmosphere entry, but also for the experimental investigation of meteorites. In fact, the composition of the plasma formed in laser-ablation techniques allows, under the assumption of local thermodynamic equilibrium, the reconstruction of synthetic emission spectra that are useful for the elemental analysis of meteorites as well as terrestrial rocks [5-7] through a calibration-free approach [8]. Furthermore, this knowledge offers theoretical support for the design of ablative thermal protection systems for space vehicles [9, 10], as well as arc welding [11] for the production of silica powder.