Previous massive dust storms were observed by NASA orbiters like Mariner 9 in 1971, Viking I in 1977, and by Mars Global Surveyor in 2001. The Mariner 9 dust storm experience was in particular remarkable, since upon arrival at Mars the probe transmitted only images that did not show any surface features – the planet was completely hulled in a dust storm. Only the summit of the 22 km high Olympus Mons peaked out of the clouds. When the storm calmed down, Mariner 9 mapped Mars globally for the first time.