Topographic constraints on impact crater morphology on Venus from high-resolution stereo synthetic aperture radar digital elevation models

High-resolution digital elevation models (DEMS) derived from Magellan Left-Left Stereo synthetic aperture radar data of Venus for a set of impact craters ranging in rim diameter from 5 to 300 km exhibit depths broadly as expected from theory but with significant departures for both large and small craters. In craters larger than 38 km diameter, rim-floor depth becomes independent of diameter with a mean of about 900 m. Most craters smaller than 18 km diameter are also found to be unexpectedly shallow. We discuss the accuracy and principal sources of error in our DEMs and conclude that this shallowness is real. This shallowness of small craters shows that midsize bolides (200-1000 M diameter) fragment in Venus' atmosphere and disperse over an area 10 to 20 times the original bolide diameter. We identify these craters as a new class of impact feature, the Compound Crater, which may correspond to the Knobby Base morphological class defined by others.