Mariner 6 and 7 find more craters
Two flybys returned 201 pictures and pinned the atmosphere at 6 to 7 millibars, but both happened to cross the oldest, dullest terrain on Mars.
Mariner 6 passed 3,431 kilometres above Mars at 05:19:07 UT on 31 July 1969, ten days after Neil Armstrong stepped onto the Moon. The two spacecraft — Mariner 6 launched 25 February 1969, Mariner 7 on 27 March — carried far better cameras than Mariner 4 plus infrared and ultraviolet spectrometers. Between them they returned 201 images, 49 far-encounter and 26 close-up frames from Mariner 6 and 93 and 33 from Mariner 7, and about 800 million bits of data, against the 5.2 million bits Mariner 4 returned four years earlier.
On 30 July, with the encounter days away, the tracking station at Johannesburg lost the signal from Mariner 7's high-gain antenna. The spacecraft had been knocked out of alignment, most likely by a rupturing onboard battery, though a micrometeoroid strike was never ruled out. Controllers got it back on the low-gain antenna and then onto the high-gain again, but fifteen telemetry channels were gone for good. They used the extra days to reprogram it, moving more of its close-up sequence over the south polar cap after what Mariner 6 had seen there. Closest approach came on 5 August at about 3,430 kilometres.
The infrared spectrometer showed the south polar cap was mostly frozen carbon dioxide rather than water ice — dry ice, at around -125°C. Radio occultation put surface pressure between 6 and 7 millibars, consistent with Mariner 4. Hellas, an enormous bright circular feature, came out almost blank, with no craters and no detail. The mission report called it a basin rimmed by higher ground. The absence of craters was real, not haze.
Both trajectories crossed old cratered ground, but covered about twenty times more surface than Mariner 4. The verdict drawn from the two hundred frames — a dead, cratered world with a trace of atmosphere — was wrong, and held for just over two more years.