TERRESTRIAL IMPACT CRATERS - THEIR SPATIAL AND TEMPORAL DISTRIBUTION AND IMPACTING BODIES

The terrestrial impact record contains currently similar to 145 struct ures and includes the morphological crater types observed on the other terrestrial planets. It has, however, been severely modified by terre strial geologic processes and is biased towards young (less than or eq ual to 200 Ma) and large (greater than or equal to 20 km) impact struc tures on relatively well-studied cratonic areas. Nevertheless, the gro und-truth data available from terrestrial impact structures have provi ded important constraints for the current understanding of cratering p rocesses. If the known sample of impact structures is restricted to a subsample in which it is believed that all structures greater than or equal to 20 km in diameter (D) have been discovered, the estimated ter restrial cratering rate is 5.5 +/- 2.7 x 10(-15) km(-2) a(-1) for D gr eater than or equal to 20 km. This rate estimate is equivalent to that based on astronomical observations of Earth-crossing bodies. These ra tes are a factor of two higher, however, than the estimated post-mare cratering rate on the moon but the large uncertainties preclude defini tive conclusions as to tile significance of this observation. Statemen ts regarding a periodicity in the terrestrial cratering record based o n time-series analyses of crater ages are considered unjustified, base d on statistical arguments and the large uncertainties attached to man y crater age estimates. Trace element and isotopic analyses of general ly siderophile group elements in impact lithologies, particularly impa ct melt rocks, have provided the basis for the identification of impac ting body compositions at a number of structures. These range from met eoritic class, e.g., C-1 chondrite, to tentative identifications, e.g. , stone?, depending on the quality and quantity of analytical data. Th e majority of the identifications indicate chondritic impacting bodies , particularly with respect to the larger impact structures. This may indicate an increasing role for cometary impacts at larger diameters; although, the data base is limited and some identifications are equivo cal. To realize the full potential of the terrestrial impact record to constrain the character of the impact flux, it will be necessary to u ndertake additional and systematic isotopic and trace element analyses of impact lithologies at well-characterized terrestrial impact struct ures.