Boulder populations and orientation trends on asteroid Ryugu: implications for rubble-pile surface processes

Abstract

Asteroid (162173) Ryugu is a spinning-top shaped, rubble-pile C-type asteroid with a surface dominated by boulders spanning a broad size-range. Using Hayabusa2 ONC-T mosaics for mapping, we present the most extensive global boulder dataset to date, representing an eleven-fold increase over the earliest survey and greater completeness than later AI-based estimates. Crucially, this work provides the first global boulder orientation dataset. The cumulative size-frequency distribution follows a power-law slope of − 2.665 ± 0.066 (boulders ≥ 3 m), indicating its fragmented nature. Boulder density is lower along the equatorial ridge, consistent with regolith accumulation during earlier YORP-driven spin-up and poleward migration of blocks during the present spin-down phase. Orientation analyses reveal systematic hemispheric trends in what we define as slope-breaker zones (i.e., regions of high slope differentials), with NW–SE alignment in the northern hemisphere and NE–SW in the southern hemisphere, but absent at the equatorial ridge and poles. We interpret these as reorientations that developed during downslope boulder migration, as despite ongoing spin-down, the current spin rate remains high enough to drive such motion. In addition, fresh craters exhibit locally heightened boulder densities, recording ongoing resurfacing by impact events that exhume buried subsurface boulders and segregate them from finer regolith via seismic shaking induced granular convection. Together, these results yield new insights into rubble-pile surface evolution, rotational dynamics, and impact-driven resurfacing, with boulder orientations providing critical evidence of spin-related downslope transport and establish a framework for comparative asteroid studies.