Preliminary analysis of an expanded corona database for Venus

We have conducted an extensive survey of the Magellan data to reassess the population of coronae. We identify a new type of coronae, here referred to as 'Type 2 coronae', having the same basic morphology as previously identif ied coronae (Type 1), but lacking a significant (> 50%) annulus of closely spaced fractures. 106 Type 2 coronae are included in the new database givin g a total of 515 coronae on Venus. The characteristics of the expanded popu lation of Type I coronae are similar to those of the previously described p opulation, but the Type 2 coronae are smaller, tend to have relatively flat interiors surrounded by a topographic rim, and are more likely to be found isolated from other features. Our observations, in particular the morpholo gy and setting of the Type 2 coronae, may provide supporting evidence for t he existence of a depleted mantle layer under the venusian plains.