Shattered Dirt: Surface Fracture of Granular Asteroids

Introduction: The hypothesis explored here is that asteroid 433 Eros, a relatively large (~33 km x 13 km x 13 km) rocky near-Earth asteroid, is structurally similar to the upper meters of the Moon – that it is globally a dry rocky dust-rich regolith, beaten apart by eons of cosmic impacts, but under 1/300 th the gravity. According to this view, the fractures expressed on the surface of Eros – as well as Phobos, Ida and other asteroidal bodies – are not cracks in hard rock, but are fissures in the mildly cohesive soil which comprises their bulk. Fissures form in the outer tens of meters in response to impacts (or possibly tides in the case of Phobos) and fade away by reverberation and gardening from later small impacts. At depth, where overburden exceeds cohesion, deformation is continual. We are driven to this conclusion by the physics of rock fracture. The conclusion is supported by the expected behavior of powdery soils in microgravity. But the puzzle of why the fissures have pits, and why they occur in parallel, may not be explained until we have a better look at these very interesting small worlds. The problem with strength: Eros has been regarded as a fractured monolith or " shatter pile " , which is a rocky body with structural integrity across global scales, and whose components are jointed but not jumbled [e.g. 1, 2]. But contemporary models of fragmentation and disruption [3] predict that any large rocky asteroid will be transformed into a jumble of dust, gravel, talus and boulders on a timescale much shorter than its catastrophic disruption lifetime. So there is a