Coronae on Venus range from 60 to over 2000 km across and are characterized by a complex range of morphologies. The annuli around coronae range from about 10 to 150 km across and have tectonic features ranging from extensional to compressional to a combination of both. Topographically, coronae are domes, plateaus, plateaus with interior lows, and rimmed depressions. A subset of features classified here as coronae corresponds to depressions and is interpreted to consist of large‐scale calderas. A number of features have been identified with many of the basic characteristics of coronae (similar interior deformation, associations with volcanism, high topography) but lacking a distinct tectonic annulus. These features tend to be somewhat smaller than coronae and may represent “failed” coronae or coronae in an early stage of evolution. The size distribution of coronae and coronalike features with maximum widths greater than about 250 km is well represented by a power law of the form N(D) = kD−α, where N is the number of coronae with maximum widths greater than D (km) and α = 3.05. The spatial distribution of coronae is not random; the features are concentrated in a few groups and along several chains. Coronae are similar in many morphologic characteristics to major volcanic shield structures and volcanic rises such as Western Eistla Regio. The largest corona, Artemis, is actually larger than several volcanic rises on Venus. Coronae and volcanic rises appear to be surface manifestations of mantle plumes. There is no evidence of any systematic variation in age along chains of coronae as occurs in hotspot chains on Earth. Instead, a number of multiple and overlapping coronae may indicate limited movement of the surface above a hotspot or mantle plume. The morphology and size distribution of coronae, highlands, and major shields suggest that mantle upwelling on Venus operates either on several spatial scales, with coronae representing smaller‐scale upflows and major volcanic rises representing larger convective upwellings, or on several temporal scales, with coronae representing shorter duration upflows and major volcanic rises representing long‐term upwellings.

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ABSTRACT

The Phanerozoic paleontological record is marked by several biological extinction events. One of them, at the Cretaceous/Tertiary (K/T) boundary, was responsible for the demise of about 50% of genera and 75% of species, including the dinosaurs. These drastic and abrupt changes in the development of life on Earth puzzled paleontologists in the past. Many a cause was put forward to account for them, amongst them climate changes, disease, or overspecialization. About 20 years ago, Alvarez et al. [1980] discovered a high iridium concentration in an Italian K/T boundary clay layer.They proposed that the iridium was derived from an extra‐terrestrial impact 65 Ma ago, and that the impact was the cause for the K/T boundary extinctions. The iridium layer was subsequently found at K/T boundary locations worldwide. Further evidence for a K/T impact came from the discovery of shocked quartz, nano‐diamonds, glass spherules, and nickel‐rich spinels in microkrystites in the iridium‐rich layer. There was evidence for an impact event, but no crater.