Exploration of Mars
The Viking program launched "Viking 1" and "Viking 2" spacecraft to Mars in 1975; The program consisted of two orbiters and two landers – these were the first two spacecraft to successfully land and operate on Mars.
The primary scientific objectives of the lander mission were to search for biosignatures and observe meteorologic, seismic and magnetic properties of Mars. The results of the biological experiments on board the Viking landers remain inconclusive, with a reanalysis of the Viking data published in 2012 suggesting signs of microbial life on Mars.
The Viking orbiters revealed that large floods of water carved deep valleys, eroded grooves into bedrock, and traveled thousands of kilometers. Areas of branched streams, in the southern hemisphere, suggest that rain once fell.
Mars Exploration Rover
NASA's Mars Exploration Rover (MER) mission is an ongoing robotic space mission involving two Mars rovers, Spirit and Opportunity, exploring the planet Mars. It began in 2003 with the launch of the two rovers: MER-A "Spirit" and MER-B "Opportunity"—to explore the Martian surface and geology; both landed on Mars at separate locations in January 2004. Both rovers far outlived their planned missions of 90 Martian solar days: MER-A "Spirit" was active until 2010, while MER-B "Opportunity" was active until 2018, and holds the record for the longest distance driven by any off-Earth wheeled vehicle.