The unexpected absence of amino acids in the Sutter’s Mill meteorite

Some of the more than 70 fragments of the Sutter’s Mill meteorite that have been collected to date.

Amino acids are the building blocks of proteins, the molecular machines that are necessary to speed up chemical reactions enough to make  life possible. The discovery of amino acids of extraterrestrial origin in the Murchison meteorite in the 1970s, coupled with the  abiotic formation of amino acids in the Miller-Urey spark discharge experiments, provided compelling evidence that the building blocks of life were likely readily available throughout the Solar System when life was starting.

Recently, Aaron Burton (now at the NASA Johnson Space Center) and researchers at the NASA Goddard Space Flight Center, SETI, and UC-Davis performed amino acid analyses of several fragments of the Sutter’s Mill (SM) meteorite, which fell in Caloma, California in April 2012. The findings of these analyses were published in the journal Meteoritics & Planetary Science.  Like the Murchison meteorite, the SM meteorite is classified as a CM2. It was thus expected that it would have similar amino acid abundances and distributions  as the  Murchison. Instead, the SM meteorite samples were nearly devoid of indigenous amino acids. While both meteorites experienced aqueous alteration (the ‘2’ of CM2 indicates mild to moderate aqueous alteration), the SM meteorite parent body also experienced heating at temperatures of 150 °C to 400 °C or higher. Because laboratory experiments have shown that amino acids degrade at temperatures above 160 °C in aqueous environments in the presence of minerals, the most likely explanation for the missing amino acids is that they were destroyed during this heating process. This finding helps place an upper limit on the temperatures that life’s building blocks can withstand, both in meteorite parent bodies as well as hydrothermal environments on the early Earth.

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