 Stress fractures calculated for this model line up with the grooves on Phobos.The model is supported with the discovery that some of the grooves are younger than others, implying that the process that produces the grooves is ongoing.Given Phobos's irregular shape and assuming that it is a pile of rubble (specifically a Mohr–Coulomb body), it will eventually break up when it reaches approximately 2.1 Mars radii.