The adjective “mercurial” can apply to several senses of “mercury.” For example, it can describe a relation to the element mercury, a metal in liquid form at room temperature. That chemical was named after the planet Mercury, so when “Mercurial” is used to describe things relative to the planet, it is capitalized. But when the adjective is applied to a person, it’s befitting someone who is often quick-tempered and prone to sudden changes in mood.