Primitive astromaterials (i.e., primitive meteorites, interplanetary dust particles, and micrometeorites) preserve the only surviving samples of the fundamental building blocks of our solar system, thus providing a unique window into its origins and evolution. Meteorites originate from the asteroid belt and represent the most commonly studied of these astromaterials due to their comparatively large sizes. In contrast, interplanetary dust particles (cosmic dust collected in the stratosphere) and micrometeorites (the <10% of cosmic dust particles that survive atmospheric entry) sample a vast range of rocky bodies throughout the solar system, e.g., asteroids from the asteroid belt, and comets and asteroids from the Kuiper Belt and Oort Cloud.

Unmelted micrometeorites retain pre-atmospheric records of their parent bodies. Laboratory studies show that many micrometeorites have textural, geochemical, and mineralogical affinities with known classes of primitive meteorites, indicating a similar origin, i.e., from C-type (carbonaceous) asteroids. However, some micrometeorites show sufficient differences, such as variations in oxygen (O) isotopic compositions, to indicate they are fragments of C-type asteroids not represented within current meteorite classes.

A team of scientists led by Martin Suttle at the University of Pisa, Italy, studied the interior structures using micro-CT and oxygen isotopic compositions by mass spectrometry of micrometeorites. Comparisons between observed textures and known carbonaceous chondrites (primitive meteorites) indicated that two micrometeorites were from a ¹⁶O-poor, hydrated, C-type asteroid with close petrographic and isotopic affinities to the CO-CM clan of carbonaceous chondrites. Results indicated a link to either CY chondrites, assuming an expansion of the known O isotopic range of this chondrite group, or a new carbonaceous chondrite group. The authors propose a similar origin for previously studied “group 4” melted micrometeorites (cosmic spherules) based on similar oxygen isotopic compositions. This discovery expands the known petrologic and isotopic diversity of the asteroid belt and our understanding of processes and environmental conditions in the early solar system. READ MORE