A large fireball was witnessed on April 23, 2019 at 9:09 p.m. local time by residents in Costa Rica. As the meteor entered the Earth’s atmosphere, it broke apart and deposited meteorites centered upon the small town of Aguas Zarcas. Early reports indicated that this meteorite belongs to a special group called carbonaceous chondrites which are typically rich in organic compounds. In fact, this meteorite had a “Murchison-like” odor. The strewn field was found to yield hundreds of individual fragments totaling 27 kg (59.5 lbs) of which approximately 11 kg (24.3 lbs) were recovered before it rained in the area. Based upon its mineralogy, elemental abundances, and O-isotope composition, Aguas Zarcas is classified as a CM2 carbonaceous chondrite. (See Photo 1) The recent fall and rapid recovery of Aguas Zarcas provides a rare opportunity to investigate this exotic carbon-rich meteorite. Not since the fall of Murchison CM2 on Sept 28, 1969 in Victoria Australia, nearly fifty years ago, have scientists been able to study such a pristine specimen of a CM2 carbonaceous chondrite. (See Photo 2)
A recent paper by D. P. Glavin, J. E. Elsila, H. L. McLain et al entitled: Extraterrestrial Amino Acids in the CM2 Aguas Zarcas and Murchison Carbonaceous Chondrites Asteroid Science 2019 (LPI Contrib. No. 2189) and available online at https://www.hou.usra.edu/meetings/asteroidscience2019/pdf/2037.pdf confirms the presence of extraterrestrial organic compounds namely amino acids.
Amino acids typically contain a central alpha-carbon from which four other chemical groups or moieties are attached. All amino acids contain an amino group (-NH2) which is a weak base, a carboxylate group (-COOH), which is weakly acidic and a neutral atom of hydrogen. The other moiety the R, emanating from the central carbon defines each individual amino acid. (See figure 1)
Since there are four different chemical groups originating from the central alpha-carbon, amino acids exhibit a property called chirality or handedness. Note, glycine is the only amino acid that does not exhibit chirality since the fourth chemical group is also an atom of hydrogen. The right handed version is termed the D-enantiomer and the left handed version is called the L-enantiomer. (See figure 2) Initial studies of the Murchison meteorite indicated an equal mixture of D & L forms of amino acids that formed in space. However, Cronin and Pizzarello in 1997 discovered an excess of the L-enantiomer amino acids in the Murchison meteorite. This finding has profound implications for the origin of life on Earth.
Amino acids are the building blocks of proteins in living organisms. Living organisms on Earth utilize only the L-amino acids in the synthesis of proteins. With an excess of the L-enantiomer in carbon-rich meteorites, life may have been bias toward the form that was in excess.
D. P. Glavin and colleagues report the first amino acid analyses of the Aguas Zarcas meteorite. The authors separated out the individual amino acids and analyzed them to determine their mass, chemical structure, isotopic composition, and concentration within the meteorite. The analysis was then compared to the Murchison meteorite. A variety of two to six-carbon amino acids were identified in the Aguas Zarcas meteorite with abundances ranging from ~0.1 to 20 nmol/g (Table 1). Two rare, non-protein amino acids alpha-aminoisobutyric acid and isovaline were elevated providing evidence that these amino acids are extraterrestrial in origin. Even including small terrestrial amino acid contributions to Aguas Zarcas, the total amino acid abundance of Aguas Zarcas sample UA 2741 (~50 nmol/g) is roughly one third that of the Murchison meteorite (~162 nmol/g). The relative distribution of alpha-amino acids found in Aguas Zarcas sample UA 2741 and Murchison is quite similar, however it is notable that the Aguas Zarcas meteorite is depleted in beta-amino acids (beta-alanine and D,L-beta-amino-n-butyric acid) relative to Murchison. The lower beta-alanine abundances in Aguas Zarcas may indicate that this meteorite experienced less parent body aqueous alteration compared to Murchison.
Carbon isotope values (13C) of amino acids that fall outside of the typical terrestrial range prove that many of the amino acids in Aguas Zarcas and Murchison are extraterrestrial in origin. The 13C values of the protein amino acids may have resulted from terrestrial amino acid contamination. However, the 13C values of non-protein D- and L-isovaline indicate the measured L-isovaline excess of approximately 10% is non-terrestrial in origin.
The discovery of extraterrestrial amino acids including an excess of L-isovaline in the Aguas Zarcas meteorite provides evidence of an early solar system formation bias towards L-amino acids prior to the origin of life. The identification and confirmation of extraterrestrial organic molecules in the Aguas Zarcas CM2 carbonaceous chondrite has just begin. Stay tuned for more discoveries on the horizon.
Photos of Aguas Zarcas & Murchison
References
• D. P. Glavin, J. E. Elsila, H. L. McLain et al entitled: Extraterrestrial Amino Acids in the CM2 Aguas Zarcas and Murchison Carbonaceous Chondrites Asteroid Science 2019 (LPI Contrib. No. 2189)
• Cronin J.R. & Pizzarello S. Enantiomeric Excesses in Meteoritic Amino Acids Science 14Feb 1997 vol 275 Issue 5302 pp. 951-955.
• Shanos G.T. Carbon in the Murchison CM2 Carbonaceous Chondrite Meteorite! Vol. 5 no. 3 August 1999 pp 37-40.
