NASA had several Mars flybys before Mariner 9. Those flybys showed that Mars had a cratered surface, so some scientists thought that Mars looked just like the moon. Then Mariner 9 blew their minds.
When Mariner 9 got to Mars, it arrived in the middle of a dust storm. It saw some tall structures sticking through the clouds. When the dust settled, images from Mariner 9 revealed that these structures were the tops of huge volcanoes.
Mariner 9 worked for one year in orbit. When it ran out of gas, NASA shut it off. But the spacecraft remains one of the most famous Mars voyagers of all time.
On This Day in Space Archive!
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