SPECTRAL ANALYSIS OF THE MARTIAN METEORITE ALH 84001.  J. L. Bishop1 and C. M. Pieters2, 1DLR Institute for Planetary Exploration, Rudower Chaussee 5, 12489 Berlin, Germany, 2Department of Geological Sciences, Brown University, Providence RI 02912, USA.

Published in Meteoritics, 31, pp. A15-A16.

The pyroxene-rich martian meteorite ALH 84001 has been chemically and mineralogically analyzed [1] and found to differ substantially from other martian meteorites [1,2]. Initial spectral analyses of ALH 84001 included strong near-IR absorptions near 1 and 2 µm that are characteristic of orthopyroxene [3]. These features are observed in spectra of multiple surfaces of a chip, as well as in spectra of a <125-µm particle-sized powder, although the bands are stronger and the continuum is more positive in the powder spectrum. Characteristic orthopyroxene mid-IR spectral features are observed near 9.1, 11.4, and 20 µm in all the ALH 84001 spectra [3]; however, the mid-IR spectral features in the chip and powder spectra exhibit differences that are likely due to particle size and texture, as well as inhomogeneity or variability in the sample mineralogy.

When comparing the spectral properties of the martian meteorite ALH 84001 to those of laboratory minerals [4], there are important similarities and differences. The orthopyroxene mineral hypersthene [4] contains major-element abundances similar to those measured by Mittlefehldt [1] for the orthopyroxene component of ALH 84001. Although the mid-IR reflectance spectra of hypersthene are consistent with some aspects of the spectra of a chip of ALH 84001, the spectral features observed for hypersthene alone (cleavage face and a powdered sample [4]) cannot explain all the spectral features observed for ALH 84001. Spectral analysis of the chip suggests the presence of a clinopyroxene in ALH 84001, while spectral analysis of the powder spectrum suggests the presence of a high-Ca pyroxenoid-like mineral.

Many of the spectral features of minerals in the mid-IR are heavily dependent on particle size [4], and for mixtures the spectral features are heavily dependent on the relative particle size distributions [5]. This influence of particle size on the mid-IR spectral character can be seen in recent reflectance spectra of natural pyroxene-bearing soils [5] and of pyroxene soils prepared in the laboratory from multiple pyroxene powders having different size fractions[6]. Additional spectra of selected pyroxene soil samples have shown that sample texture and measurement geometry also influence the mid-IR spectral features for fine-grained materials. Further analysis of particle size, texture, and spectral measurement geometry of pyroxene-bearing mixtures should provide new insights for the interpretation of the spectral properties of the martian meteorite ALH 84001.

Spectral analyses of martian meteorites provide important information about the mineralogy of Mars, as well as clues that may be useful in deconvolving remote sensing spectra containing both the fine-grained surface soil and other surface rocks. Recent emittance measurements of the martian meteorite Zagami indicate the presence of pyroxene spectral features [7]; however, the spectral character of the martian meteorites as a group has not yet been examined in detail in the mid-IR. Near-infrared spectral analyses of the martian meteorite EETA 79001, including detailed mathematical modeling of two pyroxene components in conjunction with pyroxene-hematite mixture modeling for spectra of the surface of Mars, support the idea that pyroxenes are present in some regions on Mars along with a ferric Fe-bearing component[8].

References:  [1] Mittlefehldt D. (1994) Meteoritics, 29, 214. [2] Treiman A. (1995) JGR, 100, 5329. [3] Bishop J. et al. (1994) Meteoritics, 29, 444. [4] Salisbury et al. (1991) Infrared Spectra of Minerals, Johns Hopkins Univ. [5] Bishop J. et al. (1996) GCA, 60, 765. [6] Pieters C. et al. (1993) LPS XXIV, 1147. [7] Hamilton and Christiansen (1996) LPS XXVII. [8] Mustard and Sunshine (1995) Science, 267, 1623.