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

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 similar to 1000 yr. These findings underscore the role of meteoroid streams and associated outbursts in driving the space weathering of planetary surfaces.