At 5:12 a.m. PDT on July 20, 1976, mission controllers at NASA's Jet Propulsion Laboratory erupted in cheers as they learned the Viking 1 lander had survived its descent through Mars's thin atmosphere. Forty minutes later, the lander's first image began appearing on their monitors - line by line, left to right - giving humanity its first look at Martian terrain from the surface.

Dr. Thomas Mutch, leader of the Viking lander imaging team, described the moment with poetic flair: "I studied the black screen, waiting for that narrow strip that will signal the first few lines of the first picture. And it appeared. A sliver of electronic magic. Areas of brightness and darkness. The picture begins to fill the screen. Rocks and sand are visible and - finally at the far right - one of the spacecraft foot pads, a symbolic artifact that stamps our accomplishment with the sign of reality. Time and time again I repeat, 'It's incredible.'"

Fifty years later, Viking 1 remains NASA's first robot to explore Mars's surface and kick off the search for extraterrestrial life. Six weeks later, its twin Viking 2 landed on a different region, while two orbiters kept snapping photos and relaying messages back to Earth. NASA's ongoing Mars missions owe a debt to this pioneering lander - and to the rocks it so memorably photographed.