Mariner 10 was the first spacecraft to visit Mercury in the mid-1970s. During the Mariner 10 mission, scientists saw Mercury's heavily cratered surface for the first time. Each approach Mariner 10 made to the planet only revealed the same side, so only 45 percent of the planet was mapped during this mission. Here NASA mapped a color composite of the planet, "formed to highlight differences in opaque minerals (such as ilmenite), iron content, and soil maturity."