Mars is the fourth planet from the Sun in our Solar System. When Dr. Manhattan realizes, in late 1985, that he may have been inadvertently responsible for the cancer deaths of several friends, he leaves for the solitude of Mars. It is later revealed that he was framed for the deaths.
He builds an enormous crystalline clockwork ship in which to travel through the Martian atmosphere. After contemplating his own history and the future of mankind, he returns briefly to Earth to take Silk Spectre with him, where she'll convince him that human life is worth saving in the face of nuclear war.
He later permits the ship to fall and be destroyed in Galle crater, on the eastern rim of the huge impact basin Argyre Planitia.
Galle is often known as the "happy face crater" because the illusion of a smiley is created by a semicircular mountain range and two smaller craters within the main crater; it was first photographed by Viking Orbiter 1. The crater's distinctive topography evokes the smiley button worn by The Comedian.
While life in Mars' soil or water has been a matter of research and debate for years, Dr. Manhattan is definitely positive that (in the Watchmen universe) life does not exist at all on Mars. Manhattan contrasts the "uselessness" of life contrasting the chaos of Earth against the serene beauty of Mars.