 The names, originally spelled "Phobus" and "Deimus" respectively, were suggested by Henry Madan (1838–1901), science master at Eton College, based on Greek mythology, in which Phobos is a companion to the god Ares.Phobos has dimensions of , and retains too little mass to be rounded under its own gravity.Phobos does not have an atmosphere due to its low mass and low gravity.