¹K.H. McDermott, ¹M.C. Price, ¹M. Cole, ¹M.J. Burchell,
¹Centre for Astrophysics and Planetary Science, School of Physical Sciences, University of Kent, Canterbury, Kent, CT2 7NH

During hypervelocity impact (> a few km s-1) the resulting cratering and/or disruption of the target body often outweighs interest on the outcome of the projectile material, with the majority of projectiles assumed to be vaporised. However, on Earth, fragments, often metallic, have been recovered from impact sites, meaning that metallic projectile fragments may survive a hypervelocity impact and still exist within the wall, floor and/or ejecta of the impact crater post-impact. The discovery of the remnant impactor composition within the craters of asteroids, planets and comets could provide further information regarding the impact history of a body. Accordingly, we study in the laboratory the survivability of 1 and 2 mm diameter copper projectiles fired onto ice at speeds between 1.00 – 7.05 km s-1. The projectile was recovered intact at speeds up to 1.50 km s-1, with no ductile deformation, but some surface pitting was observed. At 2.39 km s-1, the projectile showed increasing ductile deformation and broke into two parts. Above velocities of 2.60 km s-1 increasing numbers of projectile fragments were identified post impact, with the mean size of the fragments decreasing with increasing impact velocity. The decrease in size also corresponds with an increase in the number of projectile fragments recovered, as with increasing shock pressure the projectile material is more intensely disrupted, producing smaller and more numerous fragments. The damage to the projectile is divided into four classes with increasing speed and shock pressure: (1) minimal damage, (2) ductile deformation, start of break up, (3) increasing fragmentation, and (4) complete fragmentation. The implications of such behaviour is considered for specific examples of impacts of metallic impactors onto Solar System bodies, including LCROSS impacting the Moon, iron meteorites onto Mars and NASA’s “Deep Impact” mission where a spacecraft impacted a comet.

Reference
McDermott KH, Price MC, Cole M, Burchell MJ (2016) Survivability of copper projectiles during hypervelocity impacts in porous ice: A laboratory investigation of the survivability of projectiles impacting comets or other bodies. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.12.037]
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¹Déborah Chavrit, ¹Manuel A. Moreira, ^1,2Frédéric Moynier
¹Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ Paris Diderot, CNRS, F-75005 Paris, France
²Institut Universitaire de France, Paris, France

Sediments contain interplanetary dust particles (IDPs) carrying extraterrestrial noble gases, such as 3He, which have previously been used to estimate the IDP accretion flux over time and the duration of past environmental events. However, due to its high diffusivity, He can be lost by diffusion either due to frictional heating during entry in the atmosphere, or once it has been incorporated in the sediments. Therefore the absolute values of 3He IDP fluxes cannot be known. Due to its lower diffusivity, Ne is less likely to be lost by diffusion than He and can potentially provide an absolute IDP flux value. Here, we studied the Ne and He isotopic composition of 21 sediments of different ages (3 to 38 Myr, 56 Myr and 183 Myr) in order to better constrain the retention of 3He in such deposits. The samples are carbonates from 2 sites of the Integrated Ocean Drilling Program (IODP), which previously showed evidence of detectable extraterrestrial 3He, and from the Sancerre core in the Paris basin. The 3He/4He, 20Ne/22Ne and 21Ne/22Ne ratios of decarbonated residues vary respectively from 0.09×10−60.09×10−6 to 76.5×10−676.5×10−6, 9.54±0.089.54±0.08 to 11.30±0.6011.30±0.60 and from 0.0295±0.00010.0295±0.0001 to 0.0344±0.00030.0344±0.0003. These isotopic compositions can be explained by a mixing between two terrestrial components (atmosphere and radiogenic He and nucleogenic Ne present in the terrigenous fractions) and an extraterrestrial component. The linear relationship between 20Ne/22Ne and 3He/22Ne ratios shows that the extraterrestrial component has a unique composition and is similar to the He and Ne composition of implanted solar wind. This composition is different from the individual stratospheric IDPs for which the Ne and He isotopic compositions have been measured. We suggest that this difference is due to a bias in the sampling of the individual IDPs previously analyzed toward the largest ones that are more likely to lose He during entry in the atmosphere. Our data further constrains the size of the majority of the IDPs to be less than 10 μm10 μm in diameter. In addition, the constant 3He/22Ne ratio of the extraterrestrial component present in the samples, which is similar to the implanted solar wind composition, suggests that no diffusive loss of 3He occurred in the atmosphere or on the seafloor. Thus, neglecting any non-fractionating He and Ne loss by weathering and/or alteration of the host phases on the seafloor, the extraterrestrial 3He and 20Ne fluxes between 3 to 38 Myr ago are respectively View the MathML source0.2±0.1×10−12 cm3cm−2kyr−1 and View the MathML source0.2±0.1×10−11 cm3cm−2kyr−1. During the sharp increases of the late Eocene and late Miocene, the IDP 3He and 20Ne fluxes reach values up to five times higher.

Reference
Chavrit D, Moreira MA, Moynier F (2016) Estimation of the extraterrestrial 3He and 20Ne fluxes on Earth from He and Ne systematics in marine sediments. Earth and Planetary Science Letters 436, 10–18 Link to Article [doi:10.1016/j.epsl.2015.12.030]
Copyright Elsevier