Aims. We provide a detailed quantitative analysis of isolated boulder fields situated in three different regions of comet 67P/Churyumov-Gerasimenko: Imhotep, Hapi, and Hatmehit. This is done to supply a useful method for analyzing the morphology of the boulders and to characterize the regions themselves. Methods. We used OSIRIS Narrow Angle Camera images with a spatial scale smaller than 2 m px −1 and analyzed the size-frequency distribution and the cumulative fractional area per boulder population. In addition, we correlated shape parameters, such as circularity and solidity, with both the spatial and the size-frequency distribution of the three populations. Results. We identified 11 811 boulders in the Imhotep, Hapi, and Hatmehit regions. We found that the Hatmehit and Imhotep areas show power indices in the range of −2.3/−2.7. These values could represent a transition between gravitational events caused by thermal weathering and sublimation, and material formed during collapses that has undergone sublimation. The Hapi area is characterized by a lower power index (−1.2/−1.7), suggesting that those boulders have a different origin. They can be the result of material formed during gravitational events and collapses that has undergone continuous fragmentation. We calculated the cumulative fractional area (CFA) in order to investigate how the area is covered by boulders as a function of their sizes. The Hatmehit and Imhotep regions show a CFA that is well fit by a power law. In contrast, the Hapi area does not show the same trend. We analyzed the fractal distributions, finding that the populations seem to be fractal at all dimensions, except for the Hapi distribution, which shows a possible fractal behavior for small dimensions only. Finally, the average values of the shape parameters reveal solid and roundish boulders in all populations we studied.
Because of the diurnal thermal cycle and the irregular shape of the nucleus, gas outflow of comet 67P/Churyumov–Gerasimenko could be highly anisotropic as indicated by the colliminated dust jet structures on the sunlit side. Based on the OSIRIS imaging observations of the outgassing effect, a simple model of surface sublimation can be constructed by taking into account the dependence on the solar insolation. With preliminary information on the time variability of the global gas production rate, a sequence of gas coma models can be generated at different epochs before and after perihelion. We also investigate different patterns of dust particle dynamics under the influences of nuclear rotation and gas drag. From these considerations, a consistent picture of the spatial distribution of dusty materials across the surface of comet 67P as it moves around the perihelion can be developed. It is found that because of the redeposition of the ejected dust from the Southern hemisphere to the Northern hemisphere during the southern summer season the Hapi region could gain up to 0.4 m while the Wosret region would lose up to 1.8 m of dust mantle per orbit.
The identification of meteorite parent bodies provides the context for understanding planetesimal formation and evolution as well as the key Solar System events they have witnessed. However, identifying such links has proven challenging and some appear ambiguous. Here, we identify that the family of asteroid fragments whose largest member is (161) Athor is the unique source of the rare EL enstatite chondrite meteorites, the closest meteorites to Earth in terms of their isotopic ratios. The Athor family was created by the collisional fragmentation of a parent body 3 Gyr ago in the inner main belt. We calculate that the diameter of the Athor family progenitor was 64 km in diameter, much smaller than the putative size of the EL original planetesimal. Therefore, we deduce that the EL planetesimal that accreted in the terrestrial planet region underwent a first catastrophic collision in that region, and one of its fragments suffered a more recent catastrophic collision in the main belt, generating the current source of the EL meteorites.
