Sodium-rich Particle as Evidence of Recent and Intense Micrometeoroid Bombardment on Asteroid Ryugu

Abstract The Hayabusa2 spacecraft returned regolith particles of the near-Earth asteroid Ryugu. We investigated two of these particles and found differences in space weathering by-products, including chemical abundance, redox state, and surface morphology. The more weathered particle shows widespread evidence of microimpacts and, within its outermost surface layers (<10 nm), a pronounced enrichment in sodium (a volatile element), likely vapor deposited during micrometeoroid impacts. The preservation of this ultrathin volatile-rich layer in the harsh space environment suggests that Ryugu’s surface experienced an intense micrometeoroid bombardment, possibly within the past ∼1000 yr. These findings underscore the role of meteoroid streams and associated outbursts in driving the space weathering of planetary surfaces.