The Diacria quadrangle lies within the north circumpolar lowlands of Mars (Carr and others, 1973; Scott and Carr, 1978). The dominant structural and physiographic features are low, relatively smooth plains (Arcadia Planitia, Vastitas Borealis) that form the northern two-thirds of the quadrangle. The plains rise to the south to form the north edge of the Amazonis Planitia and to the southeast toward two major volcanic provinces of Mars, Olympus Mons and Alba Patera. The aureole deposits of these knobby terrain occurs in the southwestern part of the quadrangle and rises toward the Elysium volcanic province that lies 1600 km to the west. Original mapping of this quadrangle was from Mariner 9 pictures. The mapping was revised after high-resolution Viking Orbiter pictures were acquired. The Viking pictures, where available, were used to determine crater statistics and the nature and detail of stratigraphic units. Many Marine 9 pictures were of poor quality because of hazes and cloud layers over most of the quadrangle, particularly the north half. Many Viking picture also contained haze and clouds in these areas.
