The surface strength of small rubble-pile asteroids, which are aggregates of unconsolidated material under microgravity, is poorly constrained but critical to understanding surface evolution and geologic history of the asteroid. Here we use images of an impact ejecta deposit and downslope avalanche adjacent to a 70-m-diameter impact crater on the rubble-pile asteroid (101955) Bennu to constrain the asteroid’s surface properties. We infer that the ejecta deposited near the crater must have been mobilized with velocities less than Bennu’s escape velocity (20 cm s^–1); such low velocities can be explained only if the effective strength of the local surface is exceedingly low, nominally ≤2 Pa. This value is four orders of magnitude below strength values commonly used for asteroid surfaces, but it is consistent with recent estimates of internal strength of rubble-pile asteroids and with the surface strength of another rubble-pile asteroid, Ryugu. We find a downslope avalanche indicating a surface composed of material readily mobilized by impacts and that has probably been renewed multiple times since Bennu’s initial assembly. Compared with stronger surfaces, very weak surfaces imply (1) more retention of material because of the low ejecta velocities and (2) lower crater-based age estimates—although the heterogeneous structure of rubble piles complicates interpretation.

小碎石堆小行星的表面强度是微重力下松散材料的聚集体，其表面强度受到的限制很差，但对于了解小行星的表面演化和地质历史至关重要。在这里，我们使用碎石堆小行星 (101955) Bennu 上直径 70 米的撞击坑附近的撞击喷射沉积物和下坡雪崩的图像来限制小行星的表面特性。我们推断沉积在陨石坑附近的喷射物一定是以小于 Bennu 逃逸速度 (20 cm s ^–1); 只有当局部表面的有效强度非常低（名义上≤2 Pa）时才能解释如此低的速度。这个值比通常用于小行星表面的强度值低四个数量级，但它与最近对内部强度的估计一致由碎石堆小行星组成，并具有另一颗碎石堆小行星 Ryugu 的表面强度。我们发现下坡雪崩表明表面由容易被撞击移动的材料组成，并且自 Bennu 最初组装以来可能已经多次更新。与更坚固的表面相比，非常薄弱的​​表面意味着（1）由于低喷射速度而更多地保留了材料，以及（2）基于火山口的年龄估计值更低——尽管碎石堆的异质结构使解释变得复杂。