On the sodium overabundance in cometary meteoroids Josep M. Trigo-Rodrı ´guez a,b, * , Jordi Llorca b,c a Institut de Cie `ncies de l’Espai (CSIC), Campus UAB, Facultat de Cie `ncies, Torre C-5, parells, 2a planta, 08193 Bellaterra (Barcelona), Spain b Institut d’Estudis Espacials de Catalunya (IEEC), Ed. Nexus, Gran Capita ` 2-4, 08034 Barcelona, Spain c Institut de Te `cniques Energe `tiques, Universitat Polite `cnica de Catalunya, Diagonal 647, 08028 Barcelona, Spain Received 30 September 2004; received in revised form 9 September 2005; accepted 1 May 2006 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 redis- tribution 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 knowl- edge of the ablation of volatile and moderately volatile phases of meteoroids. Ó 2006 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Meteor spectroscopy; Meteoroids chemical composition; Comets; Perseids; 109P/Swift-Tuttle; 55P/Tempel-Tuttle 1. Introduction: cometary versus asteroidal meteoroids. The role of Na The chemical composition and physical properties of meteoroids can be inferred from the spectroscopic and photometric study of their ablation columns in the atmo- sphere, called meteors. The flight of the meteoroid in the atmosphere produces a column of rarefied gas formed from a mixture of meteoric and atmospheric compounds, heated by the process of deceleration by continuous collisions of the incoming meteoroid with the atmospheric constituents. The kinetic energy lost produces a rise in the temperature of the meteoroid. When the temperature is higher than the evaporation temperatures of the mineral phases that form the meteoroid itself, the process known as ablation begins. Meteor spectroscopy is a valuable approach to the study of meteoroids because is able to provide the chemical composition and temperature characteristic of cometary particles whose size and velocity makes them unable to survive ablation. In fact, the interaction of cometary meteoroids with the atmosphere involves typical- ly geocentric velocities greater than 25 km s 1 . However, high meteoric fluxes coming from lower-velocity sources have been described, such as dust trails associated with comet 21P/Giacobinni-Zinner (Simek and Pecina, 1999) or with comet Pons Winnecke (Arlt et al., 1999). The range of masses of cometary particles varies from several kg for the biggest meteoroids detected by fireball networks (Ceplecha et al., 1998) to small fragments with minimum masses of 10 17 g as were detected by spacecraft in the coma of comet 1P/Halley (Jessberger et al., 1988; Fomenkova et al., 1992). Due to their fluffy structure and highly volatile nature, several processes such as solar radi- ation, solar wind bombardment and collisions with other 0273-1177/$30 Ó 2006 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2006.05.007 * Corresponding author. Tel.: +13108253202; fax: +13102063051. E-mail addresses: trigo@ieec.uab.es (J.M. Trigo-Rodrı ´guez), jordi. llorca@qi.ub.es (J. Llorca). www.elsevier.com/locate/asr Advances in Space Research 39 (2007) 517–525