MULTIRINGED IMPACT CRATERS ON VENUS - AN OVERVIEW FROM ARECIBO AND VENERA IMAGES AND INITIAL MAGELLAN DATA

Examination of Earth-based Arecibo and Venera 15 16 radar images of Venus reveals the presence of unequivocal peak-ring and larger ringed impact craters. We identify 15 peak-ring craters defined by a distinct radar-bright outer rim and an inner ring of concentrically arranged peaks. Ejecta blankets are generally radar-bright (rough), well-defined, and exhibit both symmetrical and asymmetrical forms in plan. The ratio of crater diameter to peak-ring diameter ranges from ∼ 3.5 to 2 with increasing crater diameter, and Magellan data indicate the existence of even larger ratios at the central-peak to peak-ring transition. The overall morphology of Venusian peak-ring craters is similar to that of peak-ring craters on other terrestrial planets. Effective viscosities of the target rock, estimated from Melosh's model of crater collapse, are proportional to crater diameter, indicating that the scale dependence of viscosity may dominate gravity in the formation of peak-ring craters. Two larger ringed craters, Klenova and Lise Meitner, are different in morphology from the peak-ring forms. They are each characterized by two well-defined, radar-bright rings that we interpret to represent scarp/mountains similar to the Cordillera and Outer Rook of Orientale on the Moon. Also, Klenova exhibits an inner ring of peaks that is similar to Orientale's Inner Rook, and Meitner may display an additional partial scarp-like outer ring. Diameter ratios between the major rings of these two large craters are ∼1.6, close to the √2 often suggested for unequivocal lunar multiringed basins. We interpret Klenova and Meitner to be true multiranged impact basins. We argue that a third large multiringed structure, Mead, is seen in Magellan images. It is characterized by two well-developed rings that may also represent fault scarps similar to the Cordillera and Outer Rook rings. The ring-diameter ratio for Mead is variable but ∼1.4. Ejecta blankets for Meitner and Mead are distinct, but their appearance in radar images is suppressed relative to that of peak-ring crater ejecta. Although multiringed basins on Venus occur at much smaller diameters than on the other terrestrial planets (except perhaps the Earth), effective viscosities are low enough at depth to allow inward asthenospheric flow, which is the model of Melosh and McKinnon can cause circumferential faulting, block rotation, and ring formation. © 1992.