Enigmatic Hills of the Mottled Plains, Mare Acidalium Quadrangle (MC-4), Mars

Interpretations of Mariner 9 and Viking images of the mottled plains in Mare Acidalium are present and discussed. Although the boundaries between all the units are gradational, four subdivisions were recognized within the mottled plains region. One of these subdivisions was designated hummocky mottled plains. The unit is characterized by many small (1 km), irregularly spaced dark hills surrounded by brighter material. One very poor-quality, high-resolution Viking strip (528B01-04) indicates that some of these dark hills have circular summit depressions. Possible interpretations of origin of these hills include: cinder cones, volcanic domes, maar volcanoes, pseudocraters, impact craters, or pingos. The uncratered hills or knobs may be igneous plugs, volcanic necks, or the dark remnants of a hummocky, cratered surface protruding through a relatively bright eolian blanket. The presence, however, of the small dark hills on the flow of the large (130 km diameter) C(2) crater Lomonosov, suggests that similar dark hills elsewhere also may be secondary.