In-situ polarimetric analysis of organic materials in Allende and Murchison meteorites

Murchison meteorite collected at Murchison, Victoria, Australia in 1969, belongs to carbonaceous chondrite (CV2), which contains appreciable amounts of organic materials including various kinds of amino-acid compounds. Cronin and Pizzarello (1997) reported excesses of left-handed versions of four amino acids with 7-9 %, which triggered controversy over the origin of homochirality in organismic substances as enantiomerically pure. We have conducted to clarify the chirality of amino acids in the organic compounds found in the CV chondrites by a new technique using a high-sensitive polarimeter. Previous studies have proceeded with chiral evaluations of the amino-acid compounds prepared by a solvent extraction separation from a whole sample. Such conventional method has a loss of position and precursor information on individual grain of organic material in the meteorite. In this study, in-situ polarimetric analysis of the organic substance in Murchison (CM2) and Allende (CV3) meteorites has been performed grain by grain using a high sensitivity polarimeter with a high-spacial resolution. For these measurements, we prepared a thin wedge-shaped plate from the meteorite samples, of which end has the thickness less than 50 μm available for a transmitted micro-IR spectroscopy and an optical-activity measurement. A micro-IR mapping over the sample revealed two-dimensional distributions of organic materials. Figure 1 shows the distributions of hydrocarbon and hydrous silicate minerals, which have an appearance similar to that obtained in Bell CM2 chondrite (Kebukawa 2010). In response to mapping results, the chirality of the specific grain has been determined using the polarimeter (Fig. 2). Conclusively, this method for the thin sample allows for an in-situ polarimetric analysis of organic materials in the meteorites.