MAGNETITE AS POSSIBLE TEMPLATE FOR THE
SYNTHESISOFCHIRALORGANICSINCARBONACEOUS CHONDRITES.
Q. H. S. Chan1 and M. E. Zolensky1. 1NASA Johnson Space
Center, Houston, TX 77058, USA. Email: hschan@nasa.gov.
Introduction: The main goal of the Japanese Aerospace Exploration Agency (JAXA) Hayabusa-2 mission is to visit and return to Earth samples of a C-type asteroid (162173) 1999 JU3 inorder to understand the origin and nature of organic materials inthe Solar System.
Life on Earth shows preference towards the set of organics
with particular spatial arrangements, this ‘selectivity’ is a crucial
criterion for life. With only rare exceptions, life ‘determines’ touse the left- (L-) form over the right- (D-) form of amino acids,resulting in a L-enantiomeric excess (ee). Recent studies haveshown that L-ee is found within the α-methyl amino acids in meteorites [1, 2], which are amino acids with rare terrestrial occurrence, and thus point towards a plausible abiotic origin for ee.
One of the proposed origins of chiral asymmetry of amino acids in meteorites is their formation with the presence of asymmetric catalysts [3]. The catalytic mineral grains acted as a surface atwhich nebular gases (CO, H2 and NH3) were allowed to condense
and react through Fisher Tropsch type (FTT) syntheses to form
the organics observed in meteorites [4]. Magnetite is shown to bean effective catalyst of the synthesis of amino acids that arecommonly found in meteorites [5]. It has also taken the form as
spiral magnetites (a.k.a. ‘plaquettes’), which were found in various carbonaceous chondrites (CCs), including C2s Tagish Lakeand Esseibi, CI Orgueil, and CR chondrites [e.g., 6, 7, 8]. In addition, L-ee for amino acids are common in the aqueously alteredCCs, as opposed to the unaltered CCs [1]. It seems possible thatthe synthesis of amino acids with chiral preferences is correlated
to the alteration process experienced by the asteroid parent body,and related to the configuration of spiral magnetite catalysts.
Since C-type asteroids are considered to be enriched in organic
matter, and the spectral data of 1999 JU3 indicates a certain degree of aqueous alteration [9], the Hayabusa-2 mission serves asa perfect chance to attest this argument.
In order to understand the distribution of spiral magnetites
among different meteorite classes, as well as to investigate theirspiral configurations and correlation to molecular asymmetry, weobserved polished thin sections of CCs using scanning electron
microscope (SEM) imaging. Individual magnetite grains werepicked, embedded in epoxy, thin-sectioned using an ultramicrotome, and studied with electron backscatter diffraction(EBSD) in order to reconstruct the crystal orientation along thestack of magnetite disks.
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