PREPARING FOR SAMPLE RETURN: COORDINATED ANALYSIS OF EXPERIMENTALLY SPACE WEATHERED CARBONACEOUS CHONDRITES

Over the next five years, the OSIRIS-REx and Hayabusa2 missions should deliver returned samples to Earth from the surfaces two primitive, organic-rich asteroids. Grains returned by these missions will likely have experienced a process known as space weathering during their time on the surfaces of their parent asteroids. Driven by solar wind irradiation and micrometeorite impacts, space weathering alters the morphology, microstructure, chemical composition, and optical properties of surface materials on airless bodies. Understanding how space weathering processes effect hydrated, carbonaceous materials is important for interpreting both remote sensing observations made during asteroid encounters, and for the eventual analyses of returned samples.

In order to understand space weathering of carbonaceous materials in advance of sample return, we can perform experiments to simulate these processes in the laboratory with analog materials. Here we perform progressive laser irradiation to simulate micrometeorite impacts of the CM2 Murchison meteorite. We measure changes in the optical properties, and the microstructure and chemistry of both organic and inorganic components of the meteorite. Reflectance spectra (0.35-2.5 µm) show overall darkening and bluing of the sample with laser irradiation. µL²MS measurements of the organic functional chemistry indicate simulated impact events act to fragment and depolymerize the macromolecular organic compounds in the meteorite, and increase the concentration of aromatics. We characterized changes in the microstructure and chemistry of olivine, sulfide and matrix phases using transmission electron microscopy (TEM). Those results show the presence of melt and vapor textures, including amorphous surface coatings, vesicles, and nanoparticles that are predominantly Fe-Ni-Sulfide in composition. These experiments underline the important role that Fe-bearing sulfides play in the space weathering of asteroidal regoliths, and inform on the observations we expect to make with the OSIRIS-REx and Hayabusa2 missions.