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The Thermal Evolution of Post-Impact Planetesimals

Introduction: Short-lived radionuclides undoubtedly played an important role in the heating and meta-morphism of planetesimals and meteorite parent bodies [e.g. 1]. However, the role that planetesimal impacts and collisions have had on the thermal evolution of small bodies is less certain. Quantifying the amount of heat deposited in an impact and its subsequent transport through a planetesimal is critical for understanding the chemical evolution of primitive bodies as well as the planets that accreted them. Keil et al. [2] carried out a series of SPH simulations to examine the amount of heat deposited during the collision of planetesimals. It was found that the volume of heated material in a given collision was small compared to the original volumes of the planetesimals. Further the material which was heated to the highest temperatures (and in some cases, melted) was largely ejected from the largest bodies that survived the collisions, and thus lost to space. Keil et al [2] thus concluded that impacts could not have produced enough heat to explain the large-scale metamor-phisim amount of chondrite parent bodies or the igne-ous processes seen in other meteorite parent bodies. Recently, we demonstrated that the amount of heating that would occur in planetesimal collisons would depend sensitively on the porosity of the planetesimals [3], with higher porosities generally leading to higher post-impact temperatures. Here we explore how shock-heated material in such collisions evolved dynamically (what fraction was retained versus ejected) and how the heat in the material that was retained was redistributed throughout the planetesimal. Porous Planetesimal Collisions: As a starting point, we have simulated the collision of a 1 km planetesimal into a 10 km planetesimal, each with a porosity of 50% with the iSALE hydrocode. The results of such a collision is shown in Figure 1, with contours of density (left) and temperature (right) indicating the extent to which materials were processed as a result of the collision. The smaller planetesimal excavates a cavity approximately 3 km deep in the larger one. At the bottom of the crater, a layer ~500 m in thickness remains in which the solids are heated and compressed. Maximum temperatures and densities (T~2000 K, ρ~3.3 g/cm 3 , ϕ=0%) are reached at the surface of the crater, falling off with distance into the planetesimal.