DENSITY AND MORPHOLOGY OF IMPACT CRATERS ON TESSERA TERRAIN, VENUS

Densities of impact craters on tessera, which is complex ridged terrai n of tectonic origin, and on the remainder of the planet, which is mos tly 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 o f the planet. This means that the tessera crater retention age is larg er than the age of the plains. This is in agreement with the well know n 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 b y a difficulty in recognizing the small craters on rough tessera terra in. The absence of recognizable tectonic deformation in most of the la rge on-tessera craters means that during the period of crater emplacem ent most of the studied tesserae were tectonically stable and did not undergo noticeable degree of deformation.