Icarus 174, 15--30 (2005)
A survey of meteor spectra and orbits: evidence for three
populations of Na-free meteoroids
J. Borovicka, P. Koten, P. Spurny,
J. Bocek and R. Stork
Astronomical Institute
of the Academy of Sciences, 25165 Ondrejov Observatory, Czech
Republic
Abstract. We present a survey of 97 spectra of mainly sporadic
meteors in the
magnitude range +3 to −1, corresponding to meteoroid sizes 1–10 mm. For
the majority of the meteors, heliocentric orbits are known as well. We
classified the spectra according to relative intensities of the lines
of Mg, Na, and Fe. Theoretical intensities of these lines for a
chondritic composition of the meteoroid and a wide range of excitation
and ionization conditions were computed. We found that only a minority
of the meteoroids show chondritic composition. Three distinct
populations of Na-free meteoroids, each comprising ~10%
of sporadic meteoroids in the studied size range, were identified. The
first population are meteoroids on asteroidal orbits containing only Fe
lines in their spectra and possibly related to iron–nickel meteorites.
The second population are meteoroids on orbits with small perihelia (q
<~ 0.2 AU),
where Na was lost by thermal desorption. The third population of
Na-free meteoroids resides on Halley type cometary orbits. This
material was possibly formed by irradiation of cometary surfaces by
cosmic rays in the Oort cloud. The composition of meteoroids on Halley
type orbits is diverse, probably reflecting internal inhomogeneity of
comets. On average, cometary dust has lower than chondritic Fe/Mg
ratio. Surprisingly, iron meteoroids prevail among millimeter-sized
meteoroids on typical Apollo-asteroid orbits. We have also found
varying content of Na in the members of the Geminid meteoroid stream,
suggesting that Geminid meteoroids were not released from their parent
body at the same time.
Keywords: Meteors; Meteoroids; Spectroscopy; Comets,
composition; Asteroids, composition
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