 In November 2018, astronomers concluded that the many grooves on Phobos were caused by boulders ejected from the asteroid impact that created Stickney and rolled around on the surface of the moon.An alternative theory is that the grooves are stretch marks caused by tidal forces.Images and models indicate that Phobos may be a rubble pile held together by a thin crust, and that it is being torn apart by tidal interactions.