科研之友
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摘要
On average, secondary impact craters are expected to deepen and become more symmetric as impact velocity (v(i)) increases with downrange distance (L). We have used high-resolution topography (1-2m/pixel) to characterize the morphometry of secondary craters as a function of L for several well-preserved primary craters on Mars. The secondaries in this study (N = 2644) span a range of diameters (25m D400m) and estimated impact velocities (0.4km/s v(i)2km/s). The range of diameter-normalized rim-to-floor depth (d/D) broadens and reaches a ceiling of d/D approximate to 0.22 at L approximate to 280km (v(i)=1-1.2km/s), whereas average rim height shows little dependence on v(i) for the largest craters (h/D approximate to 0.02, D > 60m). Populations of secondaries that express the following morphometric asymmetries are confined to regions of differing radial extent: planform elongations (L<110-160km), taller downrange rims (L < 280km), and cavities that are deeper uprange (L<450-500km). Populations of secondaries with lopsided ejecta were found to extend to at least L approximate to 700km. Impact hydrocode simulations with iSALE-2D for strong, intact projectile and target materials predict a ceiling for d/D versus L whose trend is consistent with our measurements. This study illuminates the morphometric transition from subsonic to hypervelocity cratering and describes the initial state of secondary crater populations. This has applications to understanding the chronology of planetary surfaces and the long-term evolution of small crater populations.
- 出版日期2017-8
