What aspect of Phobos and Deimos affects their shape?

Phobos and Deimos, the two moons of Mars, have an irregular shape primarily due to their relatively small size and low mass, which are influenced by their gravitational characteristics. Unlike larger celestial bodies, such as Earth or the larger moons in the solar system, Phobos and Deimos do not have enough gravitational force to pull themselves into a more rounded shape. Instead, they retain a more potato-like appearance.

This lack of sufficient gravitational influence means that they cannot overcome structural stresses and forces to achieve a spherical shape, which is typically seen in larger celestial bodies where gravity is strong enough to shape them. The irregularities in their shapes are a direct result of their small size and the limited gravitational impact they exert, which is insufficient to mold them into a round form.

The other factors mentioned, such as orbital rotation, size comparison to Io, and the composition of surface materials, do not significantly contribute to their shape. Orbital rotation refers to how they move around Mars, but does not affect their physical form. Comparing their size to Io, one of Jupiter's larger moons, is not relevant since the size and gravitational forces of Io are vastly different. The composition of their surface materials may influence their geological characteristics, but it does not determine their overall shape