1. Original caption: "Global mosaic made from 102 Viking 1 Orbiter images of Mars, taken on orbit 1,334, 1980 Feb22. The images are projected into point perspective, representing what a viewer would see from a spacecraft at a vertical altitude of 2,500 km. At the center is the huge canyon Valles Marineris. Note the channels running up (north) from the central and eastern portions of Valles Marineris to the area at upper right, Chryse Planitia. At the left is the Tharsis Montes (3 giant shield volcanoes of the Tharsis region: the southernmost one is hard to see), and to the south is ancient, heavily impacted terrain. Viking 1 Orbiter image MG07S078-334SP." (Slightly edited.)

  2. Valles Marineris is the largest canyon (or more accurately system of canyons) on Mars by far. It extends over 4000 km (almost a quarter way around the equatorial region), reaches to about 7 km deep, and is up to 200 km wide (see Wikipedia: Valles Marineris). It is considered to be a rift valley system.

  3. Some impact craters, channels (probably mainly Martian outflow channels), and Martian dark regions can also be seen. The Martian dark regions are regions of exposed volcanic basalt. The regions are exposed in being NOT heavily covered by Martian dust (see Wikipedia: Syrtis Major Planum) which is reddish due to iron oxide mineral content i.e., rust). The old rusty dust of Mars.