The Identification of a Diogenite Meteoroid from Meteor Spectrum

The link between individual classes of meteorites and their parent bodies is an important question of the solar system history. Besides the reflectance spectroscopy of asteroids, the study of pre-encounter orbits of meteorites could reveal the direct relation between different types of asteroids and meteorite classes. Unfortunately, the orbits are known with good accuracy only for four ordinary chondrites [1]. The spectral observation of meteors can be used to derive the bulk chemical composition of meteoroids. The accuracy is, of course, much lower than in laboratory studies of meteorites. The radiation of meteors, consisting of emission spectral lines, is produced by the hot gaseous envelope around the meteoric body. The gas is formed by the vapors of the ablated material and the atmosphere. The lines commonly observed in meteor spectra belong to Fe, Na, Mg, Ca, Cr, and Mn. In good spectra the lines of Al, Si, Ti, Co, and Ni are observable. Lines of O and N originate in the atmosphere. The relative intensities of lines depend not only on the chemical composition of the meteoroid but also on many other factors [2,3]. They include the physical conditions in the radiating gas (e.g., temperature), the presence of two distinct spectral components [4], and the effect of incomplete evaporation. The last effect causes a difference between the chemical composition of the meteoroid and the radiating gas. Refractory elements (Ca, Al, Ti) are often depleted in the gas and their abundance in the gas varies even in the same meteor at different heights. To derive the chemical composition of the meteoroid from meteor spectrum, we have to eliminate all other factors. This can be done by physical modeling (for very good spectra only) or by comparing meteors of similar parameters (velocity, brightness, height). As a result, we can distinguish iron meteoroids and individual types of achondrites from chondrites. The identification of individual types of chondrites is not possible from meteor spectra. A search was performed in part of the Ondrejov meteor spectra archives. Most meteors were found to have a "normal" spectra, which are consistent with normal solar system abundances of the observed elements [2]. Most meteoroids are therefore either chondrites or of cometary origin. Among 53 meteors, only one iron meteoroid [5] and one probable diogenite was found. The abundances computed from meteor spectra are usually given relative to Fe. This is because Fe lines are by far most numerous in meteor spectra. The temperature and other parameters are based mostly on the appearance of the Fe spectrum and the abundances of other elements results naturally as ratios to Fe. Although Fe is not a convenient reference element from the mineralogical point of view, any recomputation of meteor results would lead to the increase of relative errors of the results. For the comparison with meteorites, it is therefore better to recompute the meteorite data as ratios to Fe. The meteor EN 041089 was observed on October 4, 1989, 23:33 UT. The meteoroid of original mass of about 0.7 kg was completely destroyed in the atmosphere. The spectrum of this meteor is unique in our sample by the low intensity of the Na line and high intensity of Mg line. It was found that the Na/Fe ratio was at least 7 times lower than in other meteors and the ratios of Mg, Si, and Cr to Fe at least two times higher. This is consistent with the composition of diogenites. The orbit of EN 041089 is remarkable by the orbital period of 3.955 +- 0.013 yr. This places the meteoroid in the 3:1 resonance with Jupiter. The scenario of delivering the basaltic achondrites from Vesta via the 3:1 resonance to the Earth [6] seems to be confirmed in this case. More details on the diogenite meteoroid EN 041089 will be given in the talk and can be also found in [3]. This is the first achondrite with reliable orbit. References [1] Brown P. et al. (1994) Nature, 367, 624-626. [2] Borovicka J. (1993) A and A, 279, 627-645. [3] Borovicka J. (1994) PASP Conf. Proc., in press. [4] Borovicka J. (1994) PSS, in press. [5] Ceplecha Z. (1966) Bull. Astr. Inst. Czech., 17, 195-206. [6] Binzel R. P. and Xu S. (1993) Science, 260, 186-191.