NEW DATED CRATERS ON MARS: THE CURRENT CRATERING RATE

More than 400 new impact sites have been found on Mars. They are recognized by the appearance of new low-albedo extended blast zones in before-and-after images. HiRISE then investigates to confirm a recent impact origin, measure the crater diameters, and monitor changes in the fresh impact sites over time.

We measure the current martian cratering rate using a subset of 110 of these new craters with Context camera (CTX) before and after images. The observational bias to the dusty areas of Mars is minimized by scaling the number of new impacts to areas with repeat CTX coverage and a minimum of dust cover. We also performed a spatial randomness analysis using a Monte Carlo approach. The results indicate that the detected population is not in fact random within dust-covered areas, thus dark blast zones are not uniformly created or detected everywhere in dusty regions. The resulting cumulative impact rate is 1.8×10-6 craters/km²/yr with effective crater diameters ≥3.9 m; correcting for non-randomness increases this by a minimum factor of 1.7x. Our measured production function falls below models by Neukum et al. [2001]/Ivanov [2001] and Hartmann [2005] by ~4x, and has a shallower slope (smaller exponent in the power-law fit to the size-frequency distribution).

Reasons for the discrepancy could include inclusion of distant unrecognized secondaries in the models; inaccurately modeled atmospheric fragmentation, ablation, deceleration; mistaking clustered craters for separate impacts; short-term fluctuations in impact rate; target property effects; or other uncertainties in the models. All of these issues imply that craters <~50 m diameter should not be used for dating. Our current impact rate statistics provide the best empirical isochrons for the youngest surfaces on Mars, but they still include uncertainties of a factor of ~4.