The next NASA mission to Mars consisted of a pair of flyby
probes, Mariner 6, launched in February of 1969, and Mariner 7, launched in
March of 1969. Mariner 6 passed by Mars at a distance of 3431 km on July 31st, 1969. Mariner 7
passed by Mars at a distance of 3430 km on August 5th, 1969.
The two Mariner probes beamed back a number of images back
to Earth that showed that Mars, unlike in the images sent by Mariner 4, was
very much unlike the Moon. While once again no canals or signs of life were
found, the images showed a volcano, plains without impact craters, and areas of
chaotic hills. The Martian south pole was found to be comprised almost entirely
of frozen carbon dioxide. The surface pressure of the Martian atmosphere was
measured to by between 6 and 7 millibars. The spectrometers showed that Mars
was very cold (-123°C at the south pole), and that Mars' thin atmosphere was
almost all carbon dioxide. The spacecraft instruments measured UV and IR
emissions and radio refractivity of the Martian atmosphere, Radio science
refined the measurements of Mars’ mass, radius, and shape.
Mariner 9
Mariner 9 was launched in May of 1971 and in November of
1971 became the first artificial satellite to orbit another planet. A planet-wide dust storm which obscured all of Mars’ features delayed the mapping
mission for about a month. Mariner 9 provided the first global map of the
Martian surface, providing 7329 images. These included the first detailed views
of the martian volcanoes, including Olympus Mons, 600 kilometers across at its
base and 25 kilometers tall, Valles Marineris, a canyon up to 100 kilometers
wide and 10 kilometers deep that would reach from Los Angeles to New York, the
polar caps, and the satellites Phobos and Deimos. Data returned by Mariner 9
provided the first evidence that Mars had rivers and lakes at one time. The
spacecraft gathered data on the atmospheric composition, density, pressure, and
temperature and also the surface composition, temperature, gravity, and
topography of Mars.