GEOLOGIC HISTORY OF THE MEAD IMPACT BASIN, VENUS

The geologic history of the Mead impact basin on Venus, a basin simila r in size to Chicxulub (Mexico), may be a guide of what to expect from future exploration of Chicxulub. During the collapse phase of crater formation in the Mead basin, radar-bright impact melt material was dep osited as a topographically flat surface within a Large central area, burying the transient cavity rim and other underlying structures. The central area is not flat now and has been modified by viscous relaxati on and thermal cooling effects, Substantial parts of the ejecta deposi ts have been covered by postimpact volcanic flows that are not obvious without the topographic data. Previous global surveys of Venusian imp act craters, using only image data, may have underestimated the number of craters embayed by volcanism.