Fourth planet from the Sun. During Earth's prehistoric times, Mars was the home of the Ice Warriors (OO). The Fendahl attacked Mars, but was driven away, presumably by the Ice Warriors, and moved on to Earth (4X). When their planet began to die, the Martians sent Varga and a scouting expedition to Eath, but they were frozen inside a glacier and never returned (OO). Because of this, or possibly as a consequence of the Fendahl's devastating attack, the Martians decided to leave their dying world and emigrated to another part of the galaxy.
The god-like Osirian Horus later chose a relatively deserted corner of Mars to build a pyramid housing the Eye of Horus, which somehow kept his brother Sutekh prisoner on Earth (4G).
During the late 20th Century, Mankind sent the Probe spaceships to Mars. Probes 6 and 7 made contact with peaceful, radioactive aliens who had elected to inhabit the planet. After the alien Ambassadors were kidnapped and used by General Carrington, the aliens left the Solar System (CCC).
In the 21st Century, a fleet of the original Martians returned to the Solar System and tried to take over Earth by using the "Seeds of Death". They were defeated thanks to the intervention of the Second Doctor (XX).
In the far future, Mars was terraformed by Mankind. Bret Vyon was born on Colony 16 on Mars (V). Much later, under Usurian influence, a segment of Humanity temporarily moved to Mars, then on to Pluto, after the depleting Earth's natural ressources (4W).