Why are Deimos and Phobos nonspherical?

Deimos and Phobos are classified as nonspherical because they lack sufficient mass for their gravity to pull them into a rounded shape. In celestial bodies, gravity works to give them a spherical form, as this is the shape that allows for the most efficient distribution of mass under its own gravity. However, Deimos and Phobos are relatively small—Phobos is around 22 kilometers across, and Deimos is about 12 kilometers across—making their gravitational pull weak.

As a result of their low mass, the gravitational forces acting on these moons are not strong enough to overcome the structural integrity of the material they consist of, which allows them to maintain irregular shapes rather than being shaped into spheres. Thus, their nonspherical nature is directly tied to their insufficient mass, which leads to lower gravitational force. This is in contrast to larger moons and planets, where greater mass allows for a more rounded shape due to stronger gravitational forces.