Mariner 2, in December of 1962, was the first successful probe to flyby Venus. It confirmed that Venus's surface temperature was very hot (it read about 800 degrees F, which has been revised to 900 degrees F, hot enough to melt lead). This disproved the previous theory that there were oceans on Venus. It also confirmed that Venus was cloud-covered, and that its atmosphere was comprised mostly of carbon dioxide (CO₂).

Mariner 3 was destined for Mars on November 5, 1964, but when it was lost when its protective shield failed to eject. It was unable to use its solar panels to collect energy, because the shield failed to eject, so its batteries soon died out. (It is now in solar orbit).

Mariner 4 was the sister probe of Mariner 3. It reached Mars in 1965 and took the first twenty-one pictures ever taken close-up of Mars. It showed that Mars was heavily cratered and had a thinner atmosphere that previously thought. Most scientists concluded that Mars was dead geologically and biologically from these initial scans.

Mariner 6 and 7 flew to Mars in 1969, and they transmitted back pictures and data that they had collected. They also showed that Mars was uninteresting geologically. Mariner 7 also flew by the South Pole of Mars.

Mariner 8 was also destined for Mars, but it died on the launch pad.

Mariner 9 was the sister probe of Mariner 8, and in 1971, it became the first probe to orbit another planet. Mariner 9 also photographed the entire of Mars. It showed that Mars had huge volcanoes, deep canyon systems, and found evidence that water once flowed across Mars. Mariner 9 also took the first detailed close up images of Mars's two moons, Phobos and Deimos.

Courtesy of http://pds.jpl.nasa.gov (see Bibliography)

Mariner 10, after passing Venus and making the first ultraviolet images of its clouds and being the first to conduct a gravity assist to another planet, traveled to Mercury and made three fly-bys of it in 1974 and 1975, before its attitude fuel was exhausted. Mariner 10 showed that Mercury was heavily cratered, and that it looked just like the Moon. Mariner 10 also showed that Mercury's mass was much larger than it was previously thought to be. This seemed to indicate that Mercury's core is composed primarily of iron, and that it makes up about seventy-five per cent of the planet. Further support for this lies in the fact that Mercury was discovered to generate a magnetic field. Earth's magnetic field is formed by its liquid iron core, so scientists believe that this may generate Mercury's, too. Mariner 10 only photographed part of Mercury, but scientists believe that there is no reason to think that the remainder is any different.