On asteroids, fractures develop due to stresses driven by diurnal temperature variations at spatial scales ranging from sub-millimetres to metres. However, the timescales of such rock fracturing by thermal fatigue are poorly constrained by observations. Here we analyse images of the asteroid (101955) Bennu obtained by the Origins, Spectral Interpretation, Resource Identification and Security-Regolith Explorer (OSIRIS-REx) mission and show that metre-scale fractures on the boulders exposed at the surface have a preferential meridional orientation, consistent with cracking induced by diurnal temperature variations. Using an analytical model of fracture propagation, we suggest that fractures the length of those on Bennu’s boulders can be produced in 10⁴–10⁵ years. This is a comparable or shorter timescale than mass movement processes that act to expose fresh surfaces and reorient boulders and any preferential direction signature. We propose that boulder surface fracturing happens rapidly compared with the lifetime in near-Earth space of Bennu and other carbonaceous asteroids. The damage due to this space-weathering process has consequences for the material properties of these asteroids, with implications for the preservation of the primordial signature acquired during the accretional phases in the protoplanetary disk of our solar system.

在小行星上，由于空间尺度从亚毫米到米的昼夜温度变化驱动的应力，裂缝会发展。然而，这种由热疲劳引起的岩石破裂的时间尺度很少受到观测的限制。在这里，我们分析了由起源、光谱解释、资源识别和安全-风化层探测器 (OSIRIS-REx) 任务获得的小行星 (101955) Bennu 的图像，并表明暴露在表面的巨石上的米级裂缝具有优先经向取向，与昼夜温度变化引起的开裂一致。使用裂缝扩展的分析模型，我们建议可以在 10 ⁴ –10 ^{5中产生与 Bennu 巨石上的裂缝长度相同的裂缝}年。与暴露新鲜表面和重新定向巨石和任何优先方向特征的质量运动过程相比，这是一个可比或更短的时间尺度。我们提出，与 Bennu 和其他碳质小行星在近地空间中的寿命相比，巨石表面破裂发生得更快。这种太空风化过程造成的损害会对这些小行星的物质特性产生影响，这对保存在我们太阳系原行星盘的吸积阶段获得的原始特征具有影响。