EFFECTS OF THE VENUSIAN ATMOSPHERE ON INCOMING METEOROIDS AND THE IMPACT CRATER POPULATION

The dense atmosphere on Venus prevents craters smaller than about 2 km in diameter from forming and also causes formation of several crater fields and multiple-floored craters (collectively referred to as multi ple impacts). A model has been constructed that simulates the behavior of a meteoroid in a dense planetary atmosphere. This model was then c ombined with an assumed flux of incoming meteoroids in an effort to re produce the size-frequency distribution of impact craters and several aspects of the population of crater fields and multiple-floored crater s on Venus. The modeling indicates that it is plausible that the obser ved rollover in the size-frequency curve for Venus is due entirely to atmospheric effects on incoming meteoroids. However, there must be sub stantial variation in the density and behavior of incoming meteoroids in the atmosphere, Lower-density meteoroids must be less likely to sur vive atmospheric passage than simple density differences can account f or. Consequently, it is likely that the percentage of craters formed b y high density meteoroids is very high at small crater diameters, and this percentage decreases substantially with increasing crater diamete r. Overall, high-density meteoroids created a disproportionately large percentage of the impact craters on Venus. Also, our results indicate that a process such as meteoroid flattening or atmospheric explosion of meteoroids must be invoked to prevent craters smaller than the obse rved minimum diameter (2 km) from forming. In terms of using the size- frequency distribution to age-date the surface, the model indicates th at the observed population has at least 75% of the craters over 32 km in diameter that would be expected on an atmosphereless Venus; thus, t his part of the curve is most suitable for comparison with the calibra ted curves for the Moon. Separation of meteoroid fragments by aerodyna mic drag alone is not adequate to explain either the number of multipl e impacts or the dispersion of individual craters in multiple impacts. A large transverse velocity must be imparted on the fragments at brea kup. This requires that meteoroids that form multiple impacts must hav e enough strength to resist breakup until sufficient pressure is built up on the leading edge of the meteoroid. It is likely that most multi ple impacts were formed by asteroidal, and not cometary, meteoroids. ( C) 1994 Academic Press, Inc.