means In geology, the thick layer of hot, slowly-flowing rock between Earth's crust and core; more broadly, any covering layer or a cloak draped over the shoulders.

from From Latin 'mantellum,' meaning a cloak or covering, which entered Old English as 'mentel' and was reinforced by Old French 'mantel.' The image is wonderfully literal: the planet wears its mantle the way a person wears a cloak — a warm layer wrapped around the inner self. The same root gives us 'mantlepiece' (which once draped over a fire) and the verb 'to mantle,' meaning to cover or envelop.