The planet Mars.
[Greek Ares, the war god equivalent to the Roman god Mars after whom the planet is named.]
Most terms containing areo- are direct equivalents to ones in geo- that relate to Earth. Several are late nineteenth-century coinages resulting from increased interest in the planet by astronomers: areographer, a person who studies the surface features of Mars; areology, the science of the physical structure of the planet. Others are relatively recent inventions resulting from exploration of the planet by space probes or deriving from science fiction: areosynchronous refers to a spacecraft in orbit around Mars that stays above the same spot on its surface, and areobotany is the study of (hypothetical) Martian vegetation, its areoflora.