To continue our history...
Two days after the launch of Mariner 4, the Soviets launched Zond 2. However, during some maneuvering, Zond 2 lost contact with Earth on May 5, 1965. Operating on limited power, most probably due to a solar panel not having deployed properly, the spacecraft continued on and is believed to have flown by Mars on August 6.
During the next launch window, in spring 1969, four spacecraft lifted off for Mars. The two Soviet craft, Mars 1969A and Mars 1969B, were both lost during the launch. The third stage of the Proton rocket carrying 1969A exploded, causing the rocket and spacecraft debris to be strewn over the Altai mountains; likewise, the first stage of 1969B's Proton rocket malfunctioned and crashed to the ground 41 seconds after liftoff, approximately 3 km from the launch site.
The American spacecraft were Mariner 6 and 7 (Mariner 5 was sent to Venus). This was a dual mission: Mariner 6 was launched 31 days before Mariner 7; however, because Mariner 7 traveled a tighter path to Mars, it flew by Mars only four days after Mariner 6. The mission's goals were to study the surface and atmosphere of Mars during close flybys to establish the basis for future investigations, particularly those relevant to the search for extraterrestrial life, and to demonstrate and develop technologies required for future Mars missions and other long-duration missions far from the Sun. Mariner 6 also had the objective of providing experience and data which would be useful in programming the Mariner 7 encounter.
On July 29, 1969, less than a week before the closest approach, JPL lost contact with Mariner 7. They regained the signal via the backup low-gain antenna and were able to start using the high gain antenna again shortly after Mariner 6's close encounter. It was later determined that a battery onboard Mariner 7 had exploded. Based on the observations made by Mariner 6, Mariner 7 was reprogrammed in flight to take further observations of areas of interest and actually returned more pictures than Mariner 6, despite the explosion. Both Mariners took a total of 198 photos, 143 pictures of Mars as the two spacecraft approached the planet, and 55 close-up pictures. Mariner 6 flew past the equatorial region of Mars, while Mariner 7 flew over the southern polar region.
Mariner 6 and 7 revealed cratered deserts, as well as depressions with no craters, huge concentrically terraced impact regions, and collapsed ridges. However, both spacecraft missed the giant northern volcanoes and Valles Marineris, the equatorial grand canyon, discovered later. Their approach pictures did, however, photograph about 20% of the planet's surface (compared with 1% of the planet's surface by Mariner 4), showing the dark features long seen from Earth, but none of the canals mistakenly observed by ground-based astronomers.
The above image, number 6N21, was taken by Mariner 6. The dark area to the left is Sinus Sabaeus and the lighter area is Deucalionis Regio. A careful crater count in both regions shows that there is no significant difference in the crater distribtions between the dark and light areas.