Mariner crater on Mars, coloured Mariner 4 image. The Mariner crater is seen at centre, containing two smaller craters (lower centre). Mariner 4 was launched on 28th November 1964 and on 14th July 1965 it passed Mars at a distance of 9846 kilometres. It took 21 images, covering about 1% of the surface, revealing that Mars was heavily cratered and very dry. Mariner 4 also discovered that Mars had no radiation belts or magnetic field, indicating that the planet did not have a metallic core, and found that the carbon dioxide atmosphere was very thin. This image was taken at a distance of 12600 kilometres from Mars.