Deciphering the Evolution of Extraterrestrial Organic Matter and Volatiles: An Experimental and Analytical Approach to Prebiotic Chemistry Across Diverse Asteroidal Environments

Volatile-rich, carbonaceous asteroids are primitive small bodies containing abundant water in hydrous minerals (~70–90 vol.%) and ~3–5 wt.% carbon, predominantly as organic matter (OM). These asteroids are thought to have played an important role in delivering biorelevant constituents to the terrestrial planets, including early Earth. They preserve evidence of parent body processing, including low-temperature (<150 °C) aqueous alteration and, in some cases,
post-hydration thermal metamorphism (>300 °C), as demonstrated by the carbonaceous chondrite meteorites derived from these bodies. These events are known to have modified the organic and volatile inventories of these primitive asteroids; however, constraints on the nature and extent of their physical and chemical evolution remain poorly understood. This thesis investigates the thermal and aqueous processing histories of volatile-rich, carbonaceous asteroids through the study of CI, CM, and CY carbonaceous chondrites, and
samples from the carbon-rich asteroid (162173) Ryugu returned by JAXA’s Hayabusa2 mission. It also considers implications for other chemically and mineralogically similar systems, including icy moons and dwarf planets. Controlled laboratory experiments combined with in situ, multiscale analyses are used to assess OM evolution under conditions relevant to carbonaceous bodies and related environments. The aims are to: (1) constrain the thermal histories of Stage II (300–500 °C) and Stage III (500–750 °C) heated CM chondrites and assess links to CY chondrites; (2) compare surface and subsurface grains from Ryugu for evidence of high-temperature processing or primitive character; and (3) investigate organic–mineral interactions during alkaline (pH 10), low-temperature (150 °C) hydrothermal alteration in CI-like asteroids (e.g., Ryugu and Bennu) and related systems (e.g., Enceladus). Results indicate that Stage II and III CM chondrites are consistent with impact heating over a duration of 1–10 h at ~400 °C and ~700 °C, respectively. Nanoscale analyses suggest Ryugu did not undergo wide-scale high-temperature processing, although surface irradiation may have degraded OM. Alteration experiments show that Mg-rich clays sequester newly synthesised N-rich OM, with carbonate catalysing organic synthesis at CI abundances but inhibiting it at higher concentrations. Mineralogy strongly controls organic retention, producing inventories representative of bodies such as Enceladus and Ceres. Collectively, this advances our knowledge of OM synthesis, evolution, and survivability in the early Solar System, providing insight into planetary habitability and the processes that preceded life on Earth.