Twenty-five years ago, when the Viking Landers touched down on the surface of Mars, images of our planetary neighbourwere beamed back which astounded the world. But did they find life? Nasa said "no" but perhaps they were too hasty in their denial. Leo Enright looks at the past, present and future of Martian exploration and the quest to find life on the Red Planet.
Producer Pam Rutherford. E-MAIL: scirad@bbc.co.uk