DENSITY AND MORPHOLOGY OF IMPACT CRATERS ON TESSERA TERRAIN, VENUS

Densities of impact craters on tessera, which is complex ridged terrain of tectonic origin, and on the remainder of the planet, which is mostly volcanic plains, were studied using Magellan images for about 96% of the surface of Venus. The density of large (D>16 km) impact craters on tessera is higher by a factor of about 1.4 than on the remainder of the planet. This means that the tessera crater retention age is larger than the age of the plains. This is in agreement with the well known fact that tessera is embayed by the surrounding volcanic plains. The density of small (D<16 km) impact craters on tessera is lower than on the remainder of the planet, evidently an observational bias caused by a difficulty in recognizing the small craters on rough tessera terrain. The absence of recognizable tectonic deformation in most of the large on-tessera craters means that during the period of crater emplacement most of the studied tesserae were tectonically stable and did not undergo noticeable degree of deformation.