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 Scout Program
The first robotic spacecraft in this program was "Phoenix", a lander originally intended for the canceled Mars Surveyor mission but adapted to the new mission. "Phoenix" was launched on August 4, 2007, and landed on the icy northern polar region of the planet on May 25, 2008. "Phoenix" was designed to search for environments suitable for microbial life on Mars, and to research the history of water there. The 90-day primary mission was successful and the overall mission was concluded on November 10, 2008, after engineers were unable to contact the craft. The lander last made a brief communication with Earth on November 2, 2008.