It arrived during a dust storm that covered almost the whole planet, so at first only the tops of the giant volcanoes could be seen. When the dust cleared months later, Mariner 9 mapped about 85 percent of the surface.
Its photos showed Olympus Mons, the vast canyon system later named Valles Marineris after the probe, and dry riverbeds suggesting that water once flowed on Mars. Two Soviet probes reached orbit only weeks later.
Source: Wikipedia