Molecular distribution and ¹³C isotope composition of volatile organic compounds in the Murchison and Sutter's Mill carbonaceous chondrites

Volatile organic compounds (VOCs) are carbon-containing chemicals that may evaporate rapidly at room temperature and standard pressure. Such organic compounds can be preserved inside carbonaceous chondrite matrices. However, unlike meteoritic soluble organic matter (SOM) and insoluble organic matter (IOM), VOCs are typically lost (at least in part) during sample processing (meteorite crushing) and exposure to terrestrial atmosphere and/or solvents. Like SOM and IOM, VOCs can provide valuable insights into the chemical inventory of the meteorite parent body and even the presolar cloud from which our solar system formed, as well as the composition and processes that occurred during the early formation of our solar system and the asteroidal stage. Thus, in this work, we designed and built an instrument that allowed us to access the VOCs present in samples of the carbonaceous chondrites Murchison and Sutter's Mill after mineral disaggregation by means of freeze–thaw cycling. We simultaneously evaluated the abundances and compound-specific ¹³C-distributions of the volatiles evolving after meteorite powdering at ~20, 60, and 100°C. Carbon monoxide (CO) and methane (CH₄) were released from these meteorites as the most abundant VOCs. They were combusted together for analysis and showed positive δ¹³C values, indicative of their extraterrestrial origins. Carbon dioxide (CO₂) was also an abundant VOC in both meteorites, and its isotopic values suggest that it was mainly formed from dissolved carbonates in the samples. We also detected aldehydes, ketones, and aromatic compounds in low amounts. Contrary to Murchison, which mostly yielded VOCs with positive δ¹³C values, Sutter's Mill yielded VOCs with negative δ¹³C values. The less enriched ¹³C isotope composition of the VOCs detected in Sutter's Mill suggest that they are either terrestrial contaminants, such as VOCs in compressed gas dusters and common laboratory solvents, or compounds disconnected from interstellar sources and/or formed through parent body processing. Understanding the relative abundances and determining the molecular distributions and isotopic compositions of free meteoritic VOCs are key in assessing their extraterrestrial origins and those of chondritic SOM and IOM. Our newly developed technique will be valuable in the study of the samples brought to the Earth from carbonaceous asteroid Bennu by NASA's OSIRIS-REx mission.