The (not as) unexpected absence of amino acids in heated CI chondrites

Ivuna meteorite

Amino acids are used in biology to make proteins. As such, they are essential for life as we know it. Amino acids of abiotic origin have also been found in meteorites, including seven of the eight different groups of carbonaceous chondrites, a subset of meteorites that contain up to 5 weight-percent carbon. Thus, the general assumption when carbonaceous chondrites are analyzed, then, is that indigenous amino acids will be found. Through the analysis of amino acid abundances and distribution in meteorites, our understanding of how these compounds could have been formed and how likely they are to be found throughout the solar system has been greatly improved.

Because amino acids are so widespread among carbonaceous chondrites, it is important to understand the range of conditions that allow amino acid synthesis to occur, and what conditions are inhospitable to these molecules, either by preventing their formation to begin with or leading to their rapid destruction. One way of identifying conditions that are favorable or disfavorable for amino acid formation and survival is to compare the amino acid distributions of meteorites that are chemically similar but experienced different parent body conditions (e.g., more or less heating, water activity, etc.).

A previous blog post on this subject discussed samples of the Sutter’s Mill meteorite, a CM2 chondrite that fell in Caloma, California in 2012. Because most CM2 chondrites contain indigenous amino acids, the Sutter’s Mill stones were expected to contain amino acids as well. This expectation was not borne out, however. Unlike most CM2 chondrites that experienced relatively low temperature aqueous alteration, the Sutter’s Mill meteorites had been heated to temperatures of 400 °C and above, in some cases. These observations, coupled with laboratory experiments by others that showed rapid degradation of amino acids in water at temperatures above 150 °C, led to the hypothesis that elevated parent body temperatures are not hospitable for amino acids.

We (Aaron Burton, NASA JSC, along with researchers from the NASA Goddard Space Flight Center and River Hill High School) observed a similar absence of amino acids in CI chondrites that had experienced parent body heating. More typical CI chondrites such as Orgueil and Ivuna experienced parent body temperatures 150 °C, and contain appreciable levels of amino acids. The meteorites analyzed in this study, Yamato 86029 and Yamato 980115 experienced temperatures of up to 600 °C in addition to the aqueous alteration that is ubiquitous in CI chondrites. Again, there is a correlation between heating and an absence of amino acids as we observed with the Sutter’s Mill meteorites, supporting the hypothesis that the combination of parent body heating and water activity either prevents the formation or leads to the destruction of amino acids in meteorite parent bodies.

These findings help us to place limits on the stability of amino acids, and inform us about whether or not we should expect to find amino acids and potentially other molecules of biological importance, on various planetary bodies in space.

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