This photo was taken at Mars closest approach, minutes before Mariner IV slipped behind the planet while transmitting towards Earth. In the moments as it disappeared behind the planet, the transmitted signal, as viewed from Earth, passed through the thin Martian atmosphere, from its upper reaches down to the surface, executing history's first planetary atmospheric and ionospheric radio occultation experiments. Minutes later as the signal reappeared on the other side of the planet, the first rising occultation was observed from Earth.