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Meteoric Ablation

Metallic species appearing in the mesosphere region arise from the ablation of cosmic dust particles in the Earth's atmosphere. Because of their high velocity the dust particles undergo a rapid frictional heating at heights below 150 km by collisions with air molecules and subsequently starts to vaporise. The estimates of the total mass influx vary broadly between 40 - 240 metric tons per day. The mean entry velocity of particles is about 20 km s-1.The mass of most frequent particles lies in the interval 5 - 50 micrograms (radius ~80 - 180 micrometers for a density of 2000 kg m-3.

The vertical profile of the imported metals is an important parameter in the modelling of the metal chemistry in the MLT region. Most of the models developed for estimating the metal input into the atmosphere assume that relative abundances in the ablated material is equal to their concentration in meteorites and the mass loss rate is derived from vapour pressure of the thermally ablated material calculated by the Clausius-Clapeyron equation:

log10 P(T) = CA - (CB/T)

A fractionation of the released material due to higher volatility of alkali metals and their oxides relatively to refractory elements such as calcium was suggested recently (McNeil W.J., Lai S., Murrad E., J. Geophys. Res. 1998, 103, 10899). We are developing a differential ablation model including the calculation of the equilibria in the melted particle, equilibrium between the melt and the vapour and in the gas phase. We have chosen MAGMA chemical equilibrium code by Fegeley, Cameron and Schaefer (Earth Planet. Sci. Lett.1987, 82, 207; Icarus 169, 2004, 216) which reliably predicts melt-vapour fractionation in a broad range of silicate melts. Our model is based on the following assumptions:
  • Equilibrium in melt and melt-gas calculated simultaneously
  • Oxides evaporate stoichiometrically
  • Gas equilibrium distribution of metals into MxOy
  • Isothermal particle of spherical shape
  • Free molecular flow regime along the whole path
  • Liquid - gas equilibrium
  • No mass loss before melting
  • No non-thermal processes (sputtering)
  • Mass loss controlled by Langmuir evaporation

The model indicates a differential ablation of the particle mass. It gives the peak release rate of alkali metals (K, Na) from the most abundant particle (5 micrograms, 20 km s-1) approximately 10 km higher above the maximum for Mg and Fe. The dependence of the ablated elemental fractions on the particle velocity show that for velocities < 20 km s-1 most of calcium is not released into atmosphere. Particles of the most probable velocity of 20 km s-1 release a significant amount of iron if their initial mass exceeds ~5.10-11 kg.

Calcium is ablated only from particles heavier than ~10-9 kg. The ablation profile obtained for the whole mass and velocity distribution predicts the centroid height of the alkali metal ablation ~100 km and ~90 km for Fe and Mg.

The model in the near future will be extended by the inclusion of the non-thermal sputtering processes. We will explore also the possibility of kinetic constraints on the vaporisation of meteoroids. The work on the application of the model on other solar system planets is also under way.


Mass distribution of extraterrestrial particles derived from impact craters on the Long Duration Exposure Facility (McBride et al. Adv. Space Res. 1999, 23, 73).


Velocity distribution of extraterrestrial particles derived from impact craters on the Long Duration Exposure Facility (McBride et al. Adv. Space Res. 1999, 23, 73).


Ablation rate of selected metals from a particle of the velocity of 20 km s-1 and mass of 5 micrograms.


Fraction of elements released from a 5 micrograms particle. Dependence on particle velocity.


Fraction of elements released from particles of velocity of 20 km s-1. Dependence on the particle mass.


Ablation profile integrated over the whole distribution of masses and velocities

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