A chemical model of meteoric ablation

Abstract

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 (10−17–10−13g) meteoroids, but makes a minor contribution to the overall ablation rate.

Kinetic studies of atmospherically relevant silicon chemistry : Part I: Silicon atom reactions

“…In the atmosphere, meteoric ablation produces Si atoms, which will be oxidised to SiO very rapidly by O 2 : at the ablation peak around 90 km in the MLT region 4 HSiOH. An investigation of these reactions is reported in the second part of this study.…”

“…Ablation models predict that silicon will evaporate from molten meteoroids as a mixture of Si, SiO and SiO 2 , a fraction of which will then be ionised by hyperthermal collisions with atmospheric molecules. 4 Indeed, Si + has been observed by rocket-borne mass spectrometers in the MLT, 5 although most silicon species below 95 km are expected to be neutral as a result of rapid reactions of Si + with O 3 and O 2 , followed by dissociative recombination with electrons. 6,7 Silicon, iron and magnesium ablated from meteoroids are quickly oxidized and most likely recondense to form nanometre-sized particles known as meteoric smoke particles (MSPs).…”

Analysis of ALTAIR 1998 meteor radar data

“…Regarding item 2, the accuracy of the derived deceleration rates depends on the accuracy of the polynomial fits to the velocities, where in this study we have used least squares quadratic fits of the 3-D velocities versus Q (path integral of the air density). With respect to item 3, we think that our present model (and that of Vondrak et al [2008]) are closer approximations to reality than most earlier models. However, a frequently stated problem with dynamical mass measurements is the need for assuming a value for the density r s .…”

“…The constant s assumption would be appropriate if, for instance, the meteoroid mass loss was dominated by "sputtering." [12] In an alternative model [e.g., Vondrak et al, 2008;Janches et al, 2009;Lebedinets, 1973], the mass loss is dominated by thermal evaporation of the meteor constituents. The instantaneous evaporation rate for each constituent is determined by the instantaneous temperature.…”

“…[13] The consensus of most current theoretical studies of the ablation and slowing down of small meteors in the atmosphere [Lebedinets, 1973;Janches et al, 2009;Vondrak et al, 2008] is that (1) very rapid heating occurs due to collisions with air molecules, moderated by energy losses due to blackbody emission from the surface and due to evaporation. (2) The heating leads to vaporization of meteor constituents (generally preceded by melting).…”