The U.S. exploration of Mars began on November 5 1964 with the launch of Mariner 3. It was an unfortunate beginning to Mars exploration as shortly after launch, a plastic shroud designed to protect the craft failed to separate and the spacecraft went into orbit around the Sun.

Mariner 4 quickly followed on November 28, after an intense project to construct a stronger shroud. This was the first success, and after slight course adjustments, the craft shot past Mars on July 16, 1965. Unfortunately, only 30minutes of pictures could be taken (22 exposures) before Mars disappeared into the distance and Mariner 4 was left to orbit around the Sun. Radio contact has been lost and gained several times since then, and the spacecraft (also carrying many experiments) has sent back some important results. The photographs showed little contrast, but many craters were observed. The Martian surface was consequently compared to that of the Moon. This is now known to be a very unrealistic comparison.

Mariners 6-7 were scheduled for launch in 1969, and the first successfully left for Mars on February 24. Mariner 7 was launched a month later. Both crafts were heavier than their predecessors and carried much more scientific equipment. On August 1, Mariner 6 successfully took 33 pictures of Mars over 20 hours. A second sequence of 25 pictures was taken at close range as the craft travelled around the equatorial region of Mars. Once this was completed, Mariner 6's mission was done. Mariner 7 had a far more eventful journey, and contact was temporarily lost as the craft apparently spun out of control. After long hours, contact was regained and eventually, on August 2, a series of 91 pictures were taken at long range. After reprogramming, a second series of 33 pictures were taken at close range. The images were very misleading, and left astronomers believing that Mars had few if any Earth like forms and did not undergo any of the internal processes that our home planet does. Also, the terrain was seen as almost totally cratered in a similar way to the Moon, and it appeared as though Mars had never had an atmosphere like Earth's. Finally, it was inferred that water did not play any important role in modifying the Martian surface. Today, we know all of these assumptions to be wrong, but many other spacecraft had to travel there before we could see this.

Results from the spectrometer aboard Mariner6-7 proved the previous observations of a CO2 (carbon dioxide) rich atmosphere. However, a number of peaks also suggested the presence of CH4 (methane) and NH3 (ammonia) which, in the Earth's atmosphere, are produced by biological processes. This was probably the strongest indication of life on Mars ever seen, and it sparked off much excitement.