Crater Size Distributions and Impact Probabilities on Earth from Lunar, Terrestrial Planeta, and Asteroid Cratering Data

The terrestrial planets, the Earth's Moon, Mars,Venus, Mercury and the asteroids Gaspra and Ida showstriking similarities in their production crater size distribution characteristics. The investigation of hte crater populations in Gaspra and Ida yield information on the crater size distribution in the source region of the bodieslargely responsibe for cratering the inner solar system planets. Comparison of these data especially with the lunar impact record, which is our most reliable data base, confirms the complex shape of the crater size distribution curve (standarddistibution), and lends strong support to the idea of a common population of bodies impacting the inner planets and largely stemming from the asteroid belt. The steepening of the production crater size distribution at sizes D>= 1 km is confirmed to be due to the characteristics of the primary impactor production size distribution and not to an admixture of objects from secondary cratering processes. The impact hazard for craters, or objects, is assessed for the Earth and the other terrestrial planets relative to the lunar case through application of the lunar production size-frequency distribution which is well known for the crater diameter range 10m>D>1000km. current or past impact rates can be calculated for any size of crater or projectile and are given for specific crater sizes and respective projektile sizes as well as for projectile energies. The impact hazard for projectiles, e.g., capableof forming 1-km craters on solid surfacesof the Earth (if the atomsphere were absent)is assessed for the present and results in a production rate of one crater every 1600 yr for the surface area of the whole Earth, or every 6000 yr for the area of the continents; for 100-km craters, e.g. the corresponding number is one event every 27 Myr for the whole Earth.