Ariel has the brightest surface of Uranus’ moons, but none of them reflect more than 1/3 of the sunlight that shines on them.
Scientists believe this is because their surfaces have been darkened by a carbonaceous material. Notably, Ariel is much brighter when in opposition (when the observer is directly between the moon and the Sun).
This indicates that its surface is porous and may cast shadows that decrease reflectivity when illuminated at certain angles.