Caloris basin on Mercury. Image is a combination of Mariner 10 and Messenger images. Basin diameter is estimated as ~1600 km (blue circle). Credits: NASA/Johns Hopkins Univ. Applied Physics Laboratory/Carnegie Inst./Brown Univ. ( 

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... impacts and atmospheric entry of celestial bodies is the regular natural process operating daily in the Solar System. Impacts at the solid planetary surfaces create footprints of the process – impact craters. Study of impact cratering provides useful tools for terrestrial geology and comparative planetology of the Solar System. The invited presentation reviews selected new results and discuss current problems of impact crater investigations Beyond the small community of pure impact-minded researches, data and understanding of impact cratering are used in several ways: (i) study of behaviour and transformation of condensed matter at high pressure and temperature (physics, geochemistry), (ii) study of cratering processes (mechanics, material science), (iii) study of individual impact craters (mainly – terrestrial, rarely – extraterrestrial) as geological objects with an impact origin and long following evolution (geology, geophysics), and (iv) study of cratering populations on planetary surfaces to compare model ages in places where no samples have been returned (comparative planetology). The specific of these studies is that rarely knowledge of a single discipline only allows researches to make a progress. Typically one should combine a set of very different data and skills to make sensible conclusions. In recent years new data have been obtained and analyzed my many teams. Besides a lot of new field data about terrestrial craters one should outline the Bosumtwi drilling ISDP project (Fig.1). Two drill holes at the central uplift delivered very interesting core samples (see [1] for review). Together with a wide program of geophysical study it deliver to society the new set of data about shock compression of a new kind of target – metasedimentary rocks (possibly shocked in the water-saturated state). New set of shock metamorphism features is now under investigation. A chain of scientific orbiters around Mars (MGS [2], Odyssey [3], MEX [4], MRO [5]) delivering stereo and colour images of the surface and impact craters with the resolution down to 25 cm per pixel (HiRise camera [6]). Now we see impact craters slightly larger than 1 m in diameter. The finding is that about half of new impact craters formed in the last 7 years are clusters [2, 7]. After the proper analysis crater cluster on Mars promise us new constraints on density and strength of small celestial projectiles never reached Earth surface. The presence of small crater clusters in a populations need to elaborate new approach to Mars/Moon comparison, as the same projectiles create single craters on the Moon. Hence, we need a model to convert each Martian clustered impact into an equivalent small lunar crater. With a preliminary correction new small craters allow us to verify theoretical interplanetary impact rate ratios used for crater chronology calibration [8] Messenger's flyby resulted in the imaging of new Mercury areas, never seen before. As one could assume, these areas are densely pitted with impact craters [9, 10]. Besides new craters the new view of Caloris basin is now available for future analysis in parallel to giant impact basins on other planets and moons. Impact basins are believed to be able to make a window into past epochs and in planetary interiors. Numerical modelling nowadays is a useful tool for analysis of experimental and observational data especially in a study of dynamic natural processes. Sometime naïve modelling put forward unexpected problems which may be resolved only with multidisciplinary approach. For example, impact structures similar in size to Caloris basin (Fig. 3) may be modelled with relatively simple hydrocodes [11, 12]. Using modern models of Mercury interior structure the vertical impact with 30 kms-1 impact velocity produce in 10,000 seconds the circular structure of ~1800 km (comparable with Caloris size (Fig. 4). Note that the model predicts the huge zone of melting in the hot early Mercury which penetrates the iron core. Inside the melting zone new crust and new mantle should be formed via a standard magmatic evolution of the impact “hot spot”. It is a long way ahead to construct the bridge between modelling (Fig. 4) and observations (Fig. 3). What is underneath giant impact basins? One of ways to improve modelling is the usage of better equations of state. In modelling of relatively small impact events porosity is still under investigation. The novel implementation of porosity collapse model into a hydrocode [13, 14] promises many new results on impact melting and scaling laws – one more still vital problem in the impact cratering. Study of impact cratering in ice-reach upper Martian crust put forward problems of permafrost description in hydrocodes. New hydrocode-friendly EOS’es for H 2 0 have been compiled and implemented into ice/rock layers and mixtures behaviour [15, 16]. Conclusion. During recent years a large volume of observational data has been accumulated. The scientific deciphering of these data promises new important results for terrestrial geology and comparative planetology. Acknowledgement. Author is supported by RFBR (grant #08-05-00908- а ) and partially by two NASA PGG projects (PI’s H. J. Melosh and E. ...

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... impacts and atmospheric entry of celestial bodies is the regular natural process operating daily in the Solar System. Impacts at the solid planetary surfaces create footprints of the process – impact craters. Study of impact cratering provides useful tools for terrestrial geology and comparative planetology of the Solar System. The invited presentation reviews selected new results and discuss current problems of impact crater investigations Beyond the small community of pure impact-minded researches, data and understanding of impact cratering are used in several ways: (i) study of behaviour and transformation of condensed matter at high pressure and temperature (physics, geochemistry), (ii) study of cratering processes (mechanics, material science), (iii) study of individual impact craters (mainly – terrestrial, rarely – extraterrestrial) as geological objects with an impact origin and long following evolution (geology, geophysics), and (iv) study of cratering populations on planetary surfaces to compare model ages in places where no samples have been returned (comparative planetology). The specific of these studies is that rarely knowledge of a single discipline only allows researches to make a progress. Typically one should combine a set of very different data and skills to make sensible conclusions. In recent years new data have been obtained and analyzed my many teams. Besides a lot of new field data about terrestrial craters one should outline the Bosumtwi drilling ISDP project (Fig.1). Two drill holes at the central uplift delivered very interesting core samples (see [1] for review). Together with a wide program of geophysical study it deliver to society the new set of data about shock compression of a new kind of target – metasedimentary rocks (possibly shocked in the water-saturated state). New set of shock metamorphism features is now under investigation. A chain of scientific orbiters around Mars (MGS [2], Odyssey [3], MEX [4], MRO [5]) delivering stereo and colour images of the surface and impact craters with the resolution down to 25 cm per pixel (HiRise camera [6]). Now we see impact craters slightly larger than 1 m in diameter. The finding is that about half of new impact craters formed in the last 7 years are clusters [2, 7]. After the proper analysis crater cluster on Mars promise us new constraints on density and strength of small celestial projectiles never reached Earth surface. The presence of small crater clusters in a populations need to elaborate new approach to Mars/Moon comparison, as the same projectiles create single craters on the Moon. Hence, we need a model to convert each Martian clustered impact into an equivalent small lunar crater. With a preliminary correction new small craters allow us to verify theoretical interplanetary impact rate ratios used for crater chronology calibration [8] Messenger's flyby resulted in the imaging of new Mercury areas, never seen before. As one could assume, these areas are densely pitted with impact craters [9, 10]. Besides new craters the new view of Caloris basin is now available for future analysis in parallel to giant impact basins on other planets and moons. Impact basins are believed to be able to make a window into past epochs and in planetary interiors. Numerical modelling nowadays is a useful tool for analysis of experimental and observational data especially in a study of dynamic natural processes. Sometime naïve modelling put forward unexpected problems which may be resolved only with multidisciplinary approach. For example, impact structures similar in size to Caloris basin (Fig. 3) may be modelled with relatively simple hydrocodes [11, 12]. Using modern models of Mercury interior structure the vertical impact with 30 kms-1 impact velocity produce in 10,000 seconds the circular structure of ~1800 km (comparable with Caloris size (Fig. 4). Note that the model predicts the huge zone of melting in the hot early Mercury which penetrates the iron core. Inside the melting zone new crust and new mantle should be formed via a standard magmatic evolution of the impact “hot spot”. It is a long way ahead to construct the bridge between modelling (Fig. 4) and observations (Fig. 3). What is underneath giant impact basins? One of ways to improve modelling is the usage of better equations of state. In modelling of relatively small impact events porosity is still under investigation. The novel implementation of porosity collapse model into a hydrocode [13, 14] promises many new results on impact melting and scaling laws – one more still vital problem in the impact cratering. Study of impact cratering in ice-reach upper Martian crust put forward problems of permafrost description in hydrocodes. New hydrocode-friendly EOS’es for H 2 0 have been compiled and implemented into ice/rock layers and mixtures behaviour [15, 16]. Conclusion. During recent years a large volume of observational data has been accumulated. The scientific deciphering of these data promises new important results for terrestrial geology and comparative planetology. Acknowledgement. Author is supported by RFBR (grant #08-05-00908- а ) and partially by two NASA PGG projects (PI’s H. J. Melosh and E. ...