Geologic problems in the northern plains of Mars

Viking pictures have made it possible to subdivide the few broad plains categories recognized on Mariner-9 pictures into smaller more definitive geologic units. Mapping problems arise in places where image resolution is sufficient to reveal the texture characteristics of individual units, but does not disclose the nature of their boundaries nor their stratigraphic relations. The mottled plains remain the most extensive undivided geologic unit in the northern Martian hemisphere; they may represent the northern remnant of the highlands, separated by crustal extension. Further resolution of the mottled plains unit may depend largely on digital enhancement of pictures in selected areas.