Secondary Cratering and Age Determination on Mars

Introduction: The discovery of a secondary-crater strewn field generated by the 10-km crater Zunil [1] stirred up a discussion of what is the real shape of the primary production crater size-frequency distribution and if age determination based on craters in the smaller-crater size range is possible. The main point of the discussion is whether or not the steep branch (below about 1 km diameter) is due to secondary or primary cratering. Here, we present crater counts inside and outside the Zunil strewn field as well as a discussion based on empirical data of the implications on the crater size-frequency distribution if secondary cratering occurs. Crater Size-Frequency Measurements at Zunil: Based on HRSC and MOC imaging data we were able to measure the size-frequency distributions (SFD) of Zunil’s secondaries and the underlying primary crater distribution. The secondary crater field around Zunil shows typical secondary crater characteristics, e.g. clustering, and appearing as dark-haloed pits. Other craters not showing these features and appearing randomly distributed are considered as primaries. The resulting distribution for the primary crater population follows the predicted shape of the crater SFD for Mars [2, 3], which has been observed also in the asteroid belt, the projectile source region [e.g. 8]. The cumulative distribution is close to a N ~ D distribution, while the secondary distribution clearly shows a steeper distribution behavior (N ~ D) for the larger secondary crater range, and which has been observed elsewhere [4, 5]. 640 secondary craters of the Zunil strewn field have been counted in the HRSC image (orbit 1152) in an area of 5900 km at a distance of 300 km from Zunil. The SFD of counted craters is shown in cumulative form in Fig. 1 (primaries-left, secondaries-right) and as an R-plot in Fig. 2. In Fig. 1 we compare the measured SFDs with the crater production function derived in [2, 3, 6] for a best-fit model crater age of about 14 Ma. The secondary crater SFD is obviously dissimilar from the assumed production function. The flattening of the SFD, observed below D2 ~ 70 m, is not due to unresolved craters being well above the HRSC image resolution of 12.5 m/pixel; cratering equilibrium is not reached either. The computer model of McEwen et al. [1] also predicts flattening of the SFD for secondary craters being shifted to 10 times smaller diameters. Fig. 2 shows the R-plot for the Zunil secondaries and three measurements of close secondary crater fields [9] on Mars. For comparison, R-plot isochrones for 10 Ma and 1 Ma are given [2, 3]. The age of the Cerberus plains, commonly interpreted as very young, is found to be 14 Ma when applying the Hartmann/Neukum cratering chronology model [6] to the primary crater count. The misinterpretation in age, if unwittingly counting all craters, would be less than a factor of two. Even such an error that might occur if no caution is taken and one measured in the middle of a strewn field of secondaries is not an argument against the method of age determination using crater counts.