On the sodium overabundance in cometary meteoroids

Abstract

Relative chemical abundances and physical properties of cometary meteoroids can be deduced from meteor spectroscopy. The study of moderately volatile elements like Na can provide clues on the structure of cometary particles and processes suffered by these particles. Recent studies of spectra of photographic fireballs (produced by cm-sized meteoroids) suggest that Na is enhanced in a 1.5 factor relative to the chondritic value. The observed features in the ablation of Perseid and Leonid meteoroids suggest that the particle fragmentation behavior of an important part of the large meteoroids fits well with a dustball model where Na would be associated with an interstitial material joining mineral grains. We suggest that aqueous alteration in the parent bodies of these meteoroids can promote chemical redistribution of Na as has been widely observed in carbonaceous chondrites. Then, this element is mobilised towards interstitial fine-grain materials that are joining large mineral grains. These more friable materials (rich in Na) can be ablated preferentially in the first stages of ablation. Finally, we discuss briefly the interest in the development of meteor spectroscopy in UV and IR in order to improve our knowledge of the ablation of volatile and moderately volatile phases of meteoroids.