Understand Space Weathering of Carbonaceous Asteroids through the Analysis of Hayabusa2 Samples and Experimental Analogs

The surfaces of airless bodies are continually subjected to solar wind irradiation and micrometeoroid impacts, driving changes to the microstructure, chemistry, and spectral properties of regolith grains through a process known as space weathering. Studies of space weathering are critical for building our understanding of how airless bodies evolve throughout the history of the solar system and for interpreting remote sensing data collected by orbital spacecraft. In particular, our understanding of how space weathering affects the primitive, organic- and volatile-rich surfaces of carbonaceous chondrites is at an early stage. In order to better understand how these processes affect carbonaceous materials, the Japan Aerospace Exploration Agency (JAXA)’s Hayabusa2 mission successfully returned over 5 g of sample from asteroid Ryugu to Earth in December 2020. These samples marked the first return of material from a carbonaceous asteroid and enabled the investigation of the structural and chemical characteristics resulting from space weathering on the surface of Ryugu. In addition to the analysis of returned samples, experiments simulating space weathering of carbonaceous materials have been performed. These experimental analogs indicate that samples returned by Hayabusa2 have experienced both solar wind irradiation and micrometeoroid impacts on the surface of Ryugu. However, the majority of microstructural and chemical space weathering features including the presence of melt layers, vesicles, and the composition of nanoparticles (predominantly Fe-Ni-S) distributed throughout the melt layers are all characteristics remarkably similar to laser irradiation experiments simulating progressive micrometeoroid bombardment of carbonaceous meteorites. Interestingly, the analysis of returned samples from asteroid Itokawa indicated that solar wind irradiation was the predominant space weathering mechanism operating on small bodies. However, our analyses of Ryugu samples indicate that micrometeoroid bombardment may play a more significant role in the space weathering of asteroids than previously thought. In addition, prior studies have shown that the effects of nano- and micro-phase sulfide particles on the spectral properties of airless surfaces are not uniform or well-understood and may play an important role in the interpretation of remote sensing observations made for asteroid Ryugu. Together, laboratory experiments and returned sample analyses indicate that space weathering of carbonaceous materials is a complex phenomenon which requires coordinated efforts to understand.