Orbiting spacecraft provide an important platform from which to observe planetary objects. Here over 100 images from one of the Viking Orbiters have been combined digitally (within a computer) to show one hemisphere of Mars in much greater detail than that available even from HST. The distinctive color (once again enhanced in this image) is the result of oxidized compounds of iron that make up the surface materials. The largest structural feature on the planet is visible in this view, covering over one-half the center of the disk. The Valles Marineris canyon system (named after the Mariner 9 spacecraft that first imaged this feature in 1972) spans almost 4000 kilometers (2400 miles) near the martian equator, comparable to the width of the continental United States. Dark windblown material has accumulated within some of the canyons and large craters on the adjacent plains. Three dark spots visible along the left limb are the summits of the Tharsis Montes, enormous volcanos that each are much larger than any single volcano on Earth.
Viking mosaic (NASA Education Lithograph HQL-318).