Mathematics – Logic
Scientific paper
Apr 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja......627w&link_type=abstract
EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #627
Mathematics
Logic
Scientific paper
Global stratigraphic schemes for planetary bodies are usually based on the most common resurfacing process: the impacts of planetesimals which remain as craters or crater-related features on planetary surfaces. Through this random cratering process, counting of the accumulated number of impact craters on planetary surface units offers a valuable procedure in understanding the chronostratigraphy of a certain object. Recent ground-based observational data indicate a wavy (deviating from a simple power law) size-frequency distribution (SFD) for main belt and near-Earth asteroid populations. Neukum proposed a non-power law SFD for the lunar cratering record (Neukum and Ivanov, 1994). This SFD has kept its shape over a period of more than 4 Gyr. The lunar crater SFD is used to estimate the crater-generating family of projectiles from the lunar crater production function since the imagery of the Moon is the most complete and best investigated among the terrestrial planets. Two lunar crater production functions (PF's) were proposed by W. Hartmann (HPF) and G. Neukum (NPF) (Hartmann and Neukum, 2001). The maximum discrepancy between the HPF and NPF is roughly a factor of 3 for crater diameters near D=6 km. For diameters D<1 km and in the diameter range of 30 to 100 km both functions give similar results. The application of cratering scaling laws allows to estimate the SFD of projectiles from the lunar crater SFD as well as to transfer the lunar impact crater SFD to other terrestrial bodies: Mercury, Venus, Earth or Mars. The crater size-frequency measurements for various geologic units allow us to estimate an absolute age of these surfaces. The major conclusion is that terrestrial planets are most probably bombarded by a single projectile source, i.e. bodies derived from the asteroid belt. The current understanding of Mars's geologic history is based mainly on crater size-frequency measurements carried out on high-resolution Viking imagery and on an impact chronology model for Mars estimated from observations of planet-crossing asteroids and scaled by the lunar production function, which is the best investigated among terrestrial body surfaces. The similarity to crater size-frequency distributions found in the inner solar system suggests a similar origin of the projectiles, probably mainly stemming from the asteroid belt, and the impact rate Mars has had a lunar-like decay with time. Under this assumption, absolute ages may be derived making use of the idea of the "marker horizon", i. e. formation of the youngest basins about 3.8 Gyr ago. A good test is to date the large impact basins with the derived Martian production function. As we know for the Moon, the large basins were produced no later than about 3.9 Gyr ago and the situation should be similar for Mars. Heavily cratered terrain on Mars was created between about 4.2 to 3.8 billion years ago during the period of the heavy bombardment that is recorded on most of the solid bodies of the Solar System whose surfaces subsequently have not been extensively modified. For the interpretation of the ages of Martian basins we remapped the ejecta blankets of about 20 impact basins (most of them larger than 250 km) on Viking imagery. Additional information for the interpretation of important geological units was obtained using MOLA topographic data. Preliminary results for the basins with detectable ejecta blankets are within the expected range of 3.7 - 4.1 Gyr. This implies that like on the Moon the formation of basins ceased around 3.8 Gyr ago. This is consistent with the applied chronology model. Here we will present these results. Hartmann and Neukum, 2001, In: Chronology and evolution of Mars, Space Sci. Rev. 96, 165-194. Neukum and Ivanov, 1994, In: Hazards due to Asteroids and Comets, Arizona Press, 359-416.
Gerhard Neukum
Werner Stephanie C.
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