SECONDARY CRATERS AND RAYS OF TYCHO

Recent work has described large numbers of secondary craters formed by ejecta from hypervelocity impacts on Europa (Bierhaus et al, in press) and the Zunil crater on Mars (McEwen et al, 2005) suggesting that secondaries dominate crater statistics at small diameters. This calls into question ages based on counting small craters, which depend on the assumption that the craters are independent, random events. Obvious secondary craters around a primary impact are routinely excluded from crater counts, but distant secondaries may not be recognized and could skew both counts and the standard production functions. Spallation, the primary mechanism for high-velocity ejecta (Melosh, 1984), is suppressed by a thick regolith cover and the Zunil region and Europa have little regolith, so perhaps these counts are atypical. This idea can be tested by examining the secondary production of Tycho Crater, which formed in the heavily brecciated lunar highlands.

Tycho Crater is a young (~109 Ma) impact crater, 85 km in diameter, with a system of bright rays formed by the impact of ejecta extending as far as 2000 km. We mapped out the areal coverage of Tycho rays from Clementine color maps and counted craters in portions of the rays covered by high-resolution Apollo images. These rays are roughly saturated with craters at diameters of ~100-300 m, indicating at least ~10⁶ secondaries in this size range over the entire ray system. At very small diameters the crater density on the rays drops below saturation and resembles the population on the continuous ejecta blanket of Tycho (i.e., post-Tycho cratering). A debris surge from the impact of larger secondaries may have removed any small secondary craters and refreshed the surface. In spite of the thick regolith, Tycho formed large numbers of secondary craters, many of them far from the primary impact. This supports the argument that distant secondaries are common from many large craters, and thus may globally dominate the small-crater population.

Bierhaus, E. B., Chapman C. R. and Merline W. J. Secondary craters on Europa and implications for cratered surfaces. Nature, in press. McEwen, A. S. et al, 2005. The rayed crater Zunil and interpretations of small impact craters on Mars. Icarus 176: 351-381. Melosh H. J. 1984. Impact ejection, spallation, and the origin of meteorites. Icarus 59: 234-260.