Large impact craters and basins on Venus, with implications for ring mechanics on the terrestrial planets

Seventy-two unequivocal peak-ring craters and four structures that are interpreted to be multiringed basins are identified on Venus from Earth-based Arecibo, Venera 15/16, and Magellan radar images. These ringed craters are relatively pristine, and so they serve as an important new data set that will further understanding of the mechanics of ringed crater formation as well as the structural and rheological properties of the Venusian surface. They are also the most direct analogues for craters formed on the Earth in Proterozoic and Phanerozoic time, such as Sudbury and Chicxulub. For peak-ring craters on Venus, crater-rim to inner peak-ring diameter, or ring, ratios decrease with increasing crater diameter; the ratios do not follow √2 spacing. The morphology of peak-ring craters, the decrease in ring ratios with increasing crater size, and the general size-morphology progression from complex central-peak to peak-ring crater, including some with both an inner ring and a central peak or peaks, on Venus and the other terrestrial planets suggest a similar process of peak-ring formation: hydrodynamic downward and outward collapse of an unstable central peak to form a ring. The four largest ringed structures on Venus-Klenova, Lise Meitner, Mead, and Isabella-are structurally and morphologically more similar to the Orientale Basin on the Moon, and are probably true multiringed basins. Although the four are smaller than Orientale we suggest that the higher gravity and temperature gradients on Venus, compared with that of the Moon when its basins formed, compensate for their smaller scale and allow a crustal or mantle asthenosphere to form and inward viscous flow to create substantial radial stress in the overlying lithosphere. This stress initiates circumferential normal faulting and outer ring formation.