We review previously published and newly obtained crater size-frequency distributions in the inner solar system. These data indicate that the Moon and the ter- restrial planets have been bombarded by two populations o...We review previously published and newly obtained crater size-frequency distributions in the inner solar system. These data indicate that the Moon and the ter- restrial planets have been bombarded by two populations of objects. Population 1, dominating at early times, had nearly the same size distribution as the present-day asteroid belt, and produced heavily cratered surfaces with a complex, multi-sloped crater size-frequency distribution. Population 2, dominating since about 3.8-3.7 Gyr, had the same size distribution as near-Earth objects （NEOs） and a much lower im- pact flux, and produced a crater size distribution characterized by a differential -3 single-slope power law in the crater diameter range 0.02 km to 100 km. Taken to- gether with the results from a large body of work on age-dating of lunar and meteorite samples and theoretical work in solar system dynamics, a plausible interpretation of these data is as follows. The NEO population is the source of Population 2 and it has been in near-steady state over the past ～ 3.7-3.8 Gyr; these objects are derived from the main asteroid belt by size-dependent non-gravitational effects that favor the ejection of smaller asteroids. However, Population 1 was composed of main belt as- teroids ejected from their source region in a size-independent manner, possibly by means of gravitational resonance sweeping during orbit migration of giant planets; this caused the so-called Late Heavy Bombardment （LHB）. The LHB began some time before ～3.9 Gyr, peaked and declined rapidly over the next ～ 100 to 300 Myr, and possibly more slowly from about 3.8-3.7 Gyr to ～2 Gyr. A third crater population （Population S） consisted of secondary impact craters that can dominate the cratering record at small diameters.展开更多
Low-inclination near-earth asteroid （NEA） （297274） 1996 SK, which isalso classified as a potentially hazardous asteroid, has a highly eccentric orbit. Itwas studied by multi-wavelength photometry within the framew...Low-inclination near-earth asteroid （NEA） （297274） 1996 SK, which isalso classified as a potentially hazardous asteroid, has a highly eccentric orbit. Itwas studied by multi-wavelength photometry within the framework of an NEA colorsurvey at Lulin Observatory. Here, we report the finding of large color variationacross the surface of （297274） 1996 SK within one asteroidal rotation period of4.656 ± 0.122 hours and classify it as an S-type asteroid according to its averagecolors ofB - V = 0.767 ± 0.033, V - R = 0.482 ± 0.021, V - I = 0.801 ± 0.025and the corresponding relative reflectance spectrum. These results might be indica-tive of differential space weathering or compositional inhomogeneity in the surfacematerials.展开更多
文摘We review previously published and newly obtained crater size-frequency distributions in the inner solar system. These data indicate that the Moon and the ter- restrial planets have been bombarded by two populations of objects. Population 1, dominating at early times, had nearly the same size distribution as the present-day asteroid belt, and produced heavily cratered surfaces with a complex, multi-sloped crater size-frequency distribution. Population 2, dominating since about 3.8-3.7 Gyr, had the same size distribution as near-Earth objects （NEOs） and a much lower im- pact flux, and produced a crater size distribution characterized by a differential -3 single-slope power law in the crater diameter range 0.02 km to 100 km. Taken to- gether with the results from a large body of work on age-dating of lunar and meteorite samples and theoretical work in solar system dynamics, a plausible interpretation of these data is as follows. The NEO population is the source of Population 2 and it has been in near-steady state over the past ～ 3.7-3.8 Gyr; these objects are derived from the main asteroid belt by size-dependent non-gravitational effects that favor the ejection of smaller asteroids. However, Population 1 was composed of main belt as- teroids ejected from their source region in a size-independent manner, possibly by means of gravitational resonance sweeping during orbit migration of giant planets; this caused the so-called Late Heavy Bombardment （LHB）. The LHB began some time before ～3.9 Gyr, peaked and declined rapidly over the next ～ 100 to 300 Myr, and possibly more slowly from about 3.8-3.7 Gyr to ～2 Gyr. A third crater population （Population S） consisted of secondary impact craters that can dominate the cratering record at small diameters.
基金partially supported by Project 019/2010/A2 of the Macao Science and Technology Development Fund:MSAR No.0166Taiwan Ministry of Education under the Aim for Top University Program NCULulin Observatory is operated by the Institute of Astronomy,National Central University,Taiwan,under grant NSC 96-2752-M-008-011-PAE
文摘Low-inclination near-earth asteroid （NEA） （297274） 1996 SK, which isalso classified as a potentially hazardous asteroid, has a highly eccentric orbit. Itwas studied by multi-wavelength photometry within the framework of an NEA colorsurvey at Lulin Observatory. Here, we report the finding of large color variationacross the surface of （297274） 1996 SK within one asteroidal rotation period of4.656 ± 0.122 hours and classify it as an S-type asteroid according to its averagecolors ofB - V = 0.767 ± 0.033, V - R = 0.482 ± 0.021, V - I = 0.801 ± 0.025and the corresponding relative reflectance spectrum. These results might be indica-tive of differential space weathering or compositional inhomogeneity in the surfacematerials.
