A GLOBAL SURVEY OF VENUS' CIRCULAR LOWS (CLS) A SUBSET OF CORONAE

About 500 quasi-circular (60-1100 km) tectonomagmatic features called coronae occur in chains (68%), clusters, (21%) and as isolated lowland features (11%) across Venus. Coronae are widely accepted as the surface expression of endogenic diapirs, although other hypotheses have been proposed, including formation as volcanic caldera and by exogenic impactors. Coronae display a wide range of topographic, geomorphic, and geologic characteristics, typically taken as recording different stages of evolution. We suggest these differences may, instead, reflect multiple genetic processes for this ‘family' of features. We are conducted a global survey of a subset of coronae, circular lows, CLs. Negative, basinal topography distinguish CLs from other coronae. Previous geologic mapping of ~10 CLs is consistent with formation related to bolide impact. ~60 potential CLs (75-375 km; ~125 km median) are identified and analyzed using NASA Magellan SAR images, altimetry data, and synthetic stereo images of GIS-registered VMap quadrangles (V-1 to V-62). For each CL we determine: location, tectonic setting, host terrain, shape (ellipticity), diameter, topographic profiles, annuli character (width, degree spread, structural character), presence and extent of radial fractures, location and extent of flows, presence of rim, and interior features; other criteria may emerge during the course of the global survey. Locations and characteristics are compiled using ArcGlobe GIS™ allowing global comparison of CLs with one another, global altimetry, tectonic provinces, impact craters, deformation belts, corona-chasma chains, volcanic rises, crustal plateaus, and outcrops of ribbon-tessera terrain (possible collapsed crustal plateaus). Areally CLs are relatively equally distributed. Topographically CLs favor the lowlands, although they avoid the deepest lowland basins. Seven CLs occur in within the highlands: three in Lakshmi Planum, one each in Atla and Ovda Regiones, and two in western Ovda Regio. Generally CLs avoid chasma-corona chains and volcanic rises. Six CLs may occur within the region between Atla Regio and Atahensik Corona, although outward from the local chasm-corona chain; these features might, however, reflect regional topographic complexity, rather than specific morphological features.