A chemical model of meteoric ablation T. Vondrak1, J. M. C. Plane1, S. Broadley1, and D. Janches2 1School of Chemistry, University of Leeds, Leeds, UK 2NorthWest Research Associates, CoRA Division, Boulder, Colorado, USA
Abstract. Most of the extraterrestrial dust entering the Earth's atmosphere ablates to
produce metal vapours, which have significant effects on the aeronomy of the
upper mesosphere and lower thermosphere. A new Chemical Ablation Model
(CAMOD) is described which treats the physics and chemistry of ablation, by
including the following processes: sputtering by inelastic collisions with
air molecules before the meteoroid melts; evaporation of atoms and oxides
from the molten particle; diffusion-controlled migration of the volatile
constituents (Na and K) through the molten particle; and impact ionization
of the ablated fragments by hyperthermal collisions with air molecules.
Evaporation is based on thermodynamic equilibrium in the molten meteoroid
(treated as a melt of metal oxides), and between the particle and
surrounding vapour phase. The loss rate of each element is then determined
assuming Langmuir evaporation. CAMOD successfully predicts the meteor head
echo appearance heights, observed from incoherent scatter radars, over a
wide range of meteoroid velocities. The model also confirms that
differential ablation explains common-volume lidar observations of K, Ca and
Ca+ in fresh meteor trails. CAMOD is then used to calculate the
injection rates into the atmosphere of a variety of elements as a function
of altitude, integrated over the meteoroid mass and velocity distributions.
The most abundant elements (Fe, Mg and Si) have peak injection rates around
85 km, with Na and K about 8 km higher. The more refractory element Ca
ablates around 82 km with a Na:Ca ratio of 4:1, which does therefore not
explain the depletion of atomic Ca to Na, by more than 2 orders of
magnitude, in the upper mesosphere. Diffusion of the most volatile elements
(Na and K) does not appear to be rate-limiting except in the fastest
meteoroids. Non-thermal sputtering causes ~35% mass loss from the
fastest (~60–70 km s−1) and smallest (10−17–10−13
g) meteoroids, but makes a minor contribution to the overall ablation rate.
Citation: Vondrak, T., Plane, J. M. C., Broadley, S., and Janches, D.: A chemical model of meteoric ablation, Atmos. Chem. Phys., 8, 7015-7031, doi:10.5194/acp-8-7015-2008, 2008.