Apparently none of the Mariner missions were definitive.
http://nssdc.gsfc.nasa.gov/planetary/mars/mariner.html
"The results of the Mariner 9 mission paved the way for the Viking program. Confirmation of atmospheric pressure allowed engineers to design the Viking landers for a safe descent, and the fantastic images and data retrieved fueled the fire for further Mars exploration. The possibility of life on Mars was an irresistable mystery and, on August 20, 1975, the Viking program was launched with a variety of experiments... including experiments designed to search for signs of Martian life."