Using Small Impact Craters to Date Surfaces on Mars: Successful Test and New Opportunities

Large (km-scale) craters have been used for some years to date broad Martian surfaces, giving results subsequently supported by Martian meteorites – a mix of geologically young and old ages [1, 2]. More recently, small (10-20 m) primary craters have been seen forming on Mars [3], and the reported formation rate agrees with our crater-dating system [4, 5, 6]. This opens the door to dating of small (km.-scale) geologic formations on Mars, but potential problems abound. McEwen et al. developed a useful test to evaluate whether small craters give ages consistent with data from the larger craters [7]. They argued that fresh-looking, Zunil-style Martian ray craters are the youngest or near-youngest craters in their size ranges. Thus, counts of small craters, superimposed on these " Zunils, " should give ages comparable to the expected formation intervals of the host " Zunils, " typically 1 to a few My. McEwen et al., however, found few or no small superposed craters in MOC images, and concluded that crater chronometry systems are in error by factors of 700 to 2000 at these scales. At the International Space Science Institute in Bern, we studied the three craters tested by McEwen et al. and 5 other craters, using newer HiRISE imagery. In every case we find that deca-meter-scale craters do exist and give the expected order of magnitude ages (from a few 10 5 y to a few 10 6 y) for the youngest host craters in each size bin [8], in both the Hartmann and Neu-kum isochron systems [1, 2]. This successful test of crater chronometry at decameter scale encourages use of small craters to date small-scale Martian