Life and the Waters of Mars
The search for life on Mars has been an inspiration for centuries. In the 18th century astronomer William Herschel believed that there was active life on the planet, with a viable atmosphere that included clouds. This continued throughout the 19th century, with Richard A. Proctor 'mapping' seas, islands, and inlets, and then Giovanni Schiaparelli mapping what were to become known as the 'canals' on Mars, as described by Percival Lowell.
The romantic notion of life on Mars continued throughout the 20th century, though as the years progressed further observations led to the belief that the planet was now lifeless, with only the canals as proof that intelligent beings had indeed lived there. Then, in 1976, the Viking 1 orbiter beamed back an image in the Cydonia region that startlingly resembled a face, leading many to think that the final proof had indeed been found! However, several return visits during the past decade have presented higher-definition imagery revealing that sadly this was merely an illusion.
Of course, for life to have existed, water too needs to have been present. The earlier 'observations' of clouds, seas, and of course canals had all suggested that it was abundant on Mars, at least at some point, and missions such as the Mariner 9 space orbiter, which reached Mars in November 1971, continued to support that theory with what appeared to be dry river beds. It wasn't until the Viking landings and their images of a barren landscape that the idea of finding life-supporting water on the planet dwindled.
However, the past decade has generated renewed optimism for finding evidence of life, with the polar icecaps identified as containing huge ice deposits. Herschel's original ponderings on clouds are even vindicated with observations of cloud formations caused by evaporation in sunlight discovered by the Opportunity mission in 2004.
Mars and Doctor Who
Being a close planetary neighbour of such interest, it was inevitable that the programme would utilise its potential for drama. The Martians first 'invade' the series during The Ice Warriors, as the discovery of an "armoured giant" in a glacier on Earth leads to the revitalisation of an isolated group of Warriors from the Red Planet. In The Seeds of Death the Doctor fought off an attempt by the aggressive, reptilian-like species to transform Earth into a new home in the mid-21st century. However, by the 25th century the Martians had joined the Galactic Federation and were (mostly) devoted to peace in the galaxy. (The Curse of Peladon/The Monster of Peladon)
During the British space programme's manned landings on Mars, General Carrington developed paranoid delusions after meeting another visiting race on the planet from outside the galaxy. Tricking ambassadors to travel to Earth on the next mission, he kidnapped them and attempted to use their radioactive nature as a weapon to "prove" they actually intended to invade Earth. It was only through the Doctor's negotiations that the Earth avoided being plunged into an intergalactic war! (The Ambassadors of Death)
The Doctor was later forced to travel to Mars itself during the titular Pyramids of Mars as the Osiran Sutekh used him to take his servants to where they could stop the transmissions that imprisoned him on Earth. Many years later Sarah Jane Smith, who had accompanied the Doctor to Mars, had her computer Mr Smith block live transmissions from a NASA mission on the planet before the aforementioned pyramid would be discovered! (The Vault of Secrets)
In the 21st Century series of Doctor Who, the evidence of water on Mars became an inspiration for the 2009 special The Waters of Mars, where the Doctor discovers how 'water' was an entity in its own right that had ultimately subjugated and caused the extinction of other life on the planet.