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.
Exploration of Mars
In January 2004, the NASA twin Mars Exploration Rovers named "Spirit" (MER-A) and "Opportunity" (MER-B) landed on the surface of Mars. Both have met and exceeded all their science objectives. Among the most significant scientific returns has been conclusive evidence that liquid water existed at some time in the past at both landing sites. Martian dust devils and windstorms have occasionally cleaned both rovers' solar panels, and thus increased their lifespan. "Spirit" rover (MER-A) was active until 2010, when it stopped sending data because it got stuck in a sand dune and was unable to reorient itself to recharge its batteries.